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  • 1. Abrikosov, I. A.
    et al.
    Kissavos, A. E.
    Liot, F.
    Alling, B.
    Simak, S. I.
    Peil, O.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Competition between magnetic structures in the Fe rich fcc FeNi alloys2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 76, no 1Article in journal (Refereed)
    Abstract [en]

    We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.

  • 2. Abrikosov, I. A.
    et al.
    Ponomareva, A. V.
    Barannikova, S. A.
    Hellman, O.
    Vekilova, O.Yu.
    Simak, S. I.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Multiscale approach to theoretical simulations of materials for nuclear energy applications: Fe-Cr and Zr-based alloys2013In: Advances in materials for nuclear energy: symposium held November 25-30, Boston, Massachusetts, U.S.A., Materials Research Society, 2013, p. 3-14Conference paper (Refereed)
    Abstract [en]

    We review basic ideas behind state-of-the-art techniques for first-principles theoretical simulations of the phase stabilities and properties of alloys. We concentrate on methods that allow for an efficient treatment of compositional and thermal disorder effects. In particular, we present novel approach to evaluate free energy for strongly anharmonic systems. Theoretical tools are then employed in studies of two materials systems relevant for nuclear energy applications: Fe-Cr and Zr-based alloys. In particular, we investigate the effect of hydrostatic pressure and multicomponent alloying on the mixing enthalpy of Fe-Cr alloys, and show that in the ferromagnetic state both of them reduce the alloy stability at low Cr concentration. For Zr-Nb alloys, we demonstrate how microscopic parameters calculated from first-principles can be used in higher-level models.

  • 3. Adit Maark, T.
    et al.
    Hussain, T.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Structural, electronic and thermodynamic properties of Al- and Si-doped α-, γ-, and β-MgH2: Density functional and hybrid density functional calculations2012In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 37, no 11, p. 9112-9122Article in journal (Refereed)
    Abstract [en]

    In this work, we present a detailed study of Al- and Si-doped α-, γ-, and β-MgH 2 phases using the gradient corrected density functional GGA-PBE and the hybrid Hartree-Fock density functionals PBE0 and HSE06 within the framework of generalized Kohn-Sham density functional theory (DFT) using a plane-wave basis set. We investigate the structural, electronic, and thermodynamical properties of these compounds with regard to their hydrogen storage effectiveness. PBE0 and HSE06 predict cell parameters and bond lengths that are in good agreement with the GGA-PBE calculations and previously known experimental results. As expected smaller band gaps (E gs) are predicted by GGA-PBE for the pure magnesium hydride phases. PBE0 overcomes the deficiencies of DFT in treating these materials better than HSE06 and yields E gs that compare even better with previous GW calculations. Both the hybrid functionals increase the E gs of the Al-doped magnesium hydrides by much less magnitudes than of the Si-doped phases. This difference is interpreted in terms of charge density distributions. Best H 2 adsorption energies (ΔH ads) are computed by HSE06 while GGA-PBE significantly overestimates them. Si-doped α- and β-MgH 2 exhibited the least negative ΔH ads in close proximity to the H 2 binding energy range of -0.21 to -0.41 eV ideal for practical H 2 storage transportation applications.

  • 4.
    Ahuja, Rajeev
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Sa
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje Örje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    High pressure structural transitions in Cm metal2006In: Mater Res Soc Symp Proc, 2006, p. 247-254Conference paper (Refereed)
    Abstract [en]

    The high pressure behaviour of Cm metal is investigated theoretically using ab initio electronic structure methods. Our calculations reproduce the structural phase transitions which are obsereved in recent experiment performed by Heatman et al. [Science 309, 110 (2005)]. Calculated transition pressures are in reasonable agreement with the experimental values. Calculations performed for an antiferromagnetic state is essential to reproduce the stability of Cm-III phase. Thus, the stability of the Cm-III phase is related to the spin polarization of its 5f electrons.

  • 5.
    Ahuja, Rajeev
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Sa
    Luo, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    High-pressure structural transitions in Cm and Am0.5Cm0.5 binary alloy2006In: High Pressure Research, ISSN 0895-7959, E-ISSN 1477-2299, Vol. 26, no 4, p. 377-381Article in journal (Refereed)
    Abstract [en]

    The high-pressure behaviour of Cm and Am0.5Cm0.5 binary alloy is investigated theoretically using ab initio electronic structure methods. Our calculations reproduce the structural phase transitions, which are observed in recent experiment performed by Heathman et al. [S. Heathman, R.G. Haire, I Le Bihan et al., Science 309 110 (2005)] and Lindbaum et al. [A. Lindbaum, S. Heathman, T. Le Bihan et al., J. Phys: Condens. Matter 15 S2297 (2003)]. Calculated transition pressures are in reasonable agreement with the experimental values. Calculations performed for an antiferromagnetic state are essential to reproduce the stability of Cm-III phase.

  • 6. Airiskallio, E.
    et al.
    Nurmi, E.
    Heinonen, M. H.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, M. P. J.
    Pitkanen, H.
    Alatalo, M.
    Kollar, J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    High temperature oxidation of Fe-Al and Fe-Cr-Al alloys: The role of Cr as a chemically active element2010In: Corrosion Science, ISSN 0010-938X, E-ISSN 1879-0496, Vol. 52, no 10, p. 3394-3404Article in journal (Refereed)
    Abstract [en]

    Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the alpha-Al2O3 film which is formed on the surface provided temperature is above 900 degrees C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales. (C) 2010 Elsevier Ltd. All rights reserved.

  • 7. Airiskallio, E.
    et al.
    Nurmi, E.
    Heinonen, M. H.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, M. P. J.
    Pitkanen, H.
    Alatalo, M.
    Kollar, J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Third element effect in the surface zone of Fe-Cr-Al alloys2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 3Article in journal (Refereed)
    Abstract [en]

    The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.

  • 8. Airiskallio, E.
    et al.
    Nurmi, E.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, M. P. J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Magnetic origin of the chemical balance in alloyed Fe-Cr stainless steels: First-principles and Ising model study2014In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 92, p. 135-140Article in journal (Refereed)
    Abstract [en]

    Iron-chromium is the base material for most of the stainless steel grades. Recently, new insights into the origins of fundamental physical and chemical characteristics of Fe-Cr based alloys have been achieved. Some of the new results are quite unexpected and call for further investigations. The present study focuses on the magnetic contribution in the atomic driving forces related to the chemical composition in Fe-Cr when alloyed with Al, Ti, V, Mn, Co, Ni, and Mo. Using the ab initio exact muffin-tin orbitals method combined with an Ising-type spin model, we demonstrate that the magnetic moment of the solute atoms with the induced changes in the magnetic moments of the host atoms form the main factor in determining the mixing energy and chemical potentials of low-Cr Fe-Cr based alloys. The results obtained in the present work are related to the designing and tuning of the microstructure and corrosion protection of low-Cr steels.

  • 9. Airiskallio, E.
    et al.
    Nurmi, E.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, Marko P. J.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Tuning the surface chemistry of Fe-Cr by V doping2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 15Article in journal (Refereed)
    Abstract [en]

    The reversal of the magnitudes of the bulk and surface chemical-potential differences induces the outburst of Cr on the otherwise pure Fe surface of Fe-Cr alloys. This threshold value for the Cr content is about 10 at. %. It is found that vanadium addition to Fe-Cr shifts the Cr threshold to a substantially lower value suggesting V having a positive effect on the corrosion resistance of low Cr steels. The obtained shift in the Cr threshold is shown to be connected to the change in volume of the alloy.

  • 10. Alatalo, M.
    et al.
    Pitkänen, H.
    Ropo, M.
    Kokko, K.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Modeling of steels and steel surfaces using quantum mechanical first principles methods2013In: Physical and Numerical Simulation of Materials Processing VII: selected, peer reviewed papers from the 7th International Conference on Physical and Numerical Simulation of Materials Processing (ICPNS'13), June 16-19, 2013, Oulu, Finland, Trans Tech Publications Inc., 2013, Vol. 762, p. 445-450Conference paper (Refereed)
    Abstract [en]

    We describe recent progress in first principles materials modelling applied to iron alloys. First principles methods in general have proven to be an effective way of describing atomic level phenomena in solids. When applied to alloys with chemical disorder, however, the widely used supercell methods turn out to be impractical due to the vast variety of different possible configurations. This problem can be overcome using the coherent potential approximation (CPA), which enables the description of a multicomponent alloy in terms of an effective medium constructed in such a way that it represents, on the average, the scattering properties of the alloy. A bulk alloy, in the case of substitutional random alloys, can thus be described with a single atom while a slab is needed to describe surfaces. The exact muffin-tin orbitals (EMTO) method provides a first principles method that can be combined with the CPA in order to describe steels and other multicomponent alloys. We describe the EMTO-CPA method and provide examples of both bulk and surface properties that can be modelled with this method.

  • 11. Alling, B.
    et al.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Abrikosov, I. A.
    Effect of thermal expansion, electronic excitations, and disorder on the Curie temperature of Ni1-xCuxMnSb alloys2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 13Article in journal (Refereed)
    Abstract [en]

    We demonstrate the importance of thermal effects such as temperature-induced electronic, magnetic and vibrational excitations, as well as structural defects in the first-principles calculations of the magnetic critical temperature of complex alloys using half-Heusler Ni1-xCuxMnSb alloys as a case study. The thermal lattice expansion and one-electron excitations have been accounted for self-consistently in the Curie temperature calculations. In the Ni-rich region, electronic excitations, thermal expansion, and structural defects substantially decrease the calculated Curie temperature. At the same time, some defects are shown to increase T-C in Cu-rich samples.

  • 12. Alling, B.
    et al.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Karimi, A.
    Hultman, L.
    Abrikosov, I. A.
    Unified cluster expansion method applied to the configurational thermodynamics of cubic Ti1-xAlxN2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 10, p. 104203-Article in journal (Refereed)
    Abstract [en]

    We study the thermodynamics of cubic Ti1-xAlxN using a unified cluster expansion approach for the alloy problem. The purely configurational part of the alloy Hamiltonian is expanded in terms of concentration- and volume-dependent effective cluster interactions. By separate expansions of the chemical fixed lattice, and local lattice relaxation terms of the ordering energies, we demonstrate how the screened generalized perturbation method can be fruitfully combined with a concentration-dependent Connolly-Williams cluster expansion method. Utilizing the obtained Hamiltonian in Monte Carlo simulations we access the free energy of Ti1-xAlxN alloys and construct the isostructural phase diagram. The results show striking similarities with the previously obtained mean-field results: The metastable c-TiAlN is subject to coherent spinodal decomposition over a larger part of the concentration range, e.g., from x >= 0.33 at 2000 K.

  • 13. Alling, B.
    et al.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Karimi, A.
    Peil, O. E.
    Simak, S. I.
    Hultman, L.
    Abrikosov, I. A.
    Mixing and decomposition thermodynamics of c-Ti1-xAlxN from first-principles calculations2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 4Article in journal (Refereed)
    Abstract [en]

    We describe an efficient first-principles method that can be used to calculate mixing enthalpies of transition metal nitrides with B1 structure and substitutional disorder at the metal sublattice. The technique is based on the density functional theory. The independent sublattice model is suggested for the treatment of disorder-induced local lattice relaxation effects. It supplements the description of the substitutional disorder within the coherent potential approximation. We demonstrate the excellent accuracy of the method by comparison with calculations performed by means of the projector augumented wave method on supercells constructed as special quasirandom structures. At the same time, the efficiency of the technique allows for total energy calculations on a very fine mesh of concentrations which enables a reliable calculation of the second concentration derivative of the alloy total energy. This is a first step towards first-principles predictions of concentrations and temperature intervals where the alloy decomposition proceeds via the spinodal mechanism. We thus calculate electronic structure, lattice parameter, and mixing enthalpies of the quasibinary alloy c-Ti1-xAlxN. The lattice parameter follows Vegard's law at low fractions of AlN but deviates increasingly with increasing Al content. We show that the asymmetry of the mixing enthalpy and its second concentration derivative is associated with substantial variations of the electronic structure with alloy composition. The phase diagram is constructed within the mean-field approximation.

  • 14. Almeida, Roseley
    et al.
    Banerjee, Amitava
    Chakraborty, Sudip
    Almeida, Jailton
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Theoretical Evidence behind Bifunctional Catalytic Activity in Pristine and Functionalized Al2C Monolayers2018In: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 19, no 1, p. 148-152Article in journal (Refereed)
    Abstract [en]

    First principles electronic structure calculations based on the density functional theory (DFT) framework are performed to investigate hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on two-dimensional Al2C monolayers. In addition to the pristine Al2C monolayer, monolayers doped with Nitrogen (N), Phosphorous (P), Boron (B), and Sulphur (S) are also investigated. After determining the individual adsorption energy of hydrogen and oxygen on the different functionalized Al2C monolayers, the adsorption free energies are predicted for each of the functionalized monolayers in order to assess their suitability for HER or OER. The density of states and optical absorption spectra calculations along with the work function of the functionalized Al2C monolayers enable us to gain a profound understanding of the electronic structure for the individual system and their relation to the water splitting mechanism.

  • 15.
    Al-Zoubi, Noura
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Describing Interstitials in Close-packed Lattices: First-principles Study2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Describing interstitial atoms in intermetallics or simple mono-atomic close-packed metals is a straightforward procedure in common full-potential calculations. One establishes a sufficiently large supercell, introduces the interstitial impurity and performs the electronic structure and total energy calculation. Real systems, however, are rarely mono-atomic or ordered metals. In most of the cases, the matrix is a random or quasirandom mixture of several chemically and/or magnetically distinct components. Because of that a proper computational tool should incorporate advanced alloy theory and at the same time have sufficiently high accuracy to describe interstitial positions in close-packed solids. The purpose of the present thesis is to make a step towards solving this fundamental problem in computational materials science. To this end, in the first part of the thesis a prestudy on some selected metals and compounds was presented, and in the second part tools were applied to investigate the effect of interstitial carbon on the structural properties of steels.

    For the prestudy, the equation of state for the selected Al, Cu and Rh was investigated in two equivalent phases: in conventional face-centered-cubic lattice (fcc, str-I) and in a face-centered-cubic lattice with one atomic and three interstitial empty potentialwells per primitive cell (str-II). A proper basis set of the exact muffin-tin orbitals as well as a proper potential sphere radius were established by calculating the equilibrium Wigner-Seitz radius and bulk modulus of the above elements in str-I and str-II using the exact muffin-tin orbitals (EMTO) first-principle density functional method. It was found that for Al spd orbitals are sufficient to describe the equilibrium bulk properties in both structures, while for str-II Rh and Cu at least five orbitals (spdfg) are needed to get accurate equilibrium volume and bulk modulus. Furthermore, it was shown that in general, for the str-II type of structure (close-packed structure with interstitials) the optimized overlapping muffin-tin potential in combination with spdfg orbitals ensures well converged bulk properties.

    As an application of the above work in alloys, (i) the chemical reaction between hydrogen H2 molecule and ScAl1−xMgx (0≤x≤0.3) random alloys, (ii) the phase stability of the hydrogenated alloys in different structures and (iii) the hydrogen absorption/desorption temperatures were studied by calculating the Gibbs energy for the components of the reaction. Experimental and theoretical studies by Sahlberg et al . showed that the ScAl0.8Mg0.2 compound with CsCl structure absorbs hydrogen by decomposing into ScH2 with CaF2 structure and fcc Al0.8Mg0.2. This reaction was found to be very fast, even without adding catalyst, and fully reversible. The theoretical hydrogen absorption/desorption temperatures agree well with the experimental values. On the other hand, the stability field of the hydrogenated alloys was found to be strongly depends on Mg content and on the microstructure of the hydrogenated alloys. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with the Mg concentration.

    The second part of the thesis focused on steel materials with special emphases on the effect of interstitial carbon. Steels are considered to be one of the most important engineering materials. They are mainly composed of iron and carbon. Other alloying elements in steel are introduced to get specific properties like microstructure, corrosion resistance, hardness, brittleness, etc. In order to describe the effect of carbon interstitial in iron alloys, it is important to know how the substitutional alloying elements affect the softness and some other properties of iron alloys. For that reason, the alloying effects on the energetic and magnetic structure of paramagnetic Fe0.85Cr0.1M0.05 (M = Cr, Mn, Fe, Co and Ni) alloys along the tetragonal distortion path connecting the body centered cubic (bcc) and the face centered cubic (fcc) phases were investigated. It was shown that Cr stabilizes bcc phase and increases the energy barrier (relative to bcc phase) between fcc and bcc phases. Cobalt and Ni stabilize fcc structure. Cobalt increases whereas Ni slightly decreases the energy barrier relative to fcc structure. Manganese and iron have negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. The local magnetic moments on Fe atoms have maximum values at bcc phase and minimum values at fcc phase. Cobalt atoms possess local magnetic moments only for tetragonal lattices with c/a < 1.30, and the Mn magnetic moments have almost constant value along the Bain path.

    The tetragonality of Fe-C martensite was discovered in 1928. Early experimental works showed that the tetragonality of Fe-C is linearly depends on C content. However, Later many observations indicated that the tetragonality of martensite is influenced also by alloying and interstitial carbon distributions. Very few ab initio studies focus on investigating the tetragonality of Fe-C based alloys. In this thesis the interstitial carbon in ferromagnetic Fe-based alloys and it is impact on the tetragonal lattice ratio of Fe matrix as well as the alloying effect on the tetragonality of Fe-C system were investigated. It was found that the ferromagnetic Fe-C system with C content ∼ 1.3 wt. % has a body-centered tetragonal (bct) structure with c/a ∼ 1.07. Alloying has an impact on the tetragonality; adding 5% Al, Co or Ni enhances while 5% Cr addition decreases the tetragonal lattice ratio.

    The electronic structure and total energy calculations from this thesis are based on firstprinciples exact muffin-tin orbitals method. The chemical and magnetic disorder was treated using coherent-potential approximation and the paramagnetic phase was modeled by the disordered local magnetic moments approach. Some test calculations involved also full-potential tools as implemented in Vienna ab-initio simulation package (VASP).

  • 16.
    Al-Zoubi, Noura
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Describing interstitials in close-packed lattices: first-principles study2010Licentiate thesis, comprehensive summary (Other academic)
  • 17.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Nilson, G.
    Uddeholms AB, Hagfors, Sweden.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    The bain path of paramagnetic Fe-Cr based alloys2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 1, p. 013708-Article in journal (Refereed)
    Abstract [en]

    Employing the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, we calculated the total energy and local magnetic moments of paramagnetic Fe-Cr-M (M = Cr, Mn, Fe, Co, Ni) alloys along the tetragonal distortion (Bain) path connecting the body centered cubic (bcc) and the face centered cubic (fcc) structures. The paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all alloys, the local magnetic moments on Fe atoms decrease from the maximum value corresponding to the bcc phase toward the minimum value realized for the fcc phase. Cobalt atoms have non-vanishing local magnetic moments only for tetragonal lattices with c/a < 1.30, whereas the local magnetic moments of Mn show weak crystal structure dependence. We find that Cr stabilizes the bcc lattice and increases the energy barrier as going from the bcc toward the fcc phase. Both Co and Ni favor the fcc lattice and decrease the energy barrier relative to the bcc phase. On the other hand, the tetragonal distortion around the fcc phase is facilitated by Cr and to a somewhat lesser extent also by Ni, but strongly impeded by Co. Manganese has negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. Our findings on the alloying induced softening or hardening of Fe-Cr based alloys against tetragonal distortions are important for understanding the interstitial driven martensitic transformations in alloy steels.

  • 18. Al-Zoubi, Noura
    et al.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schonecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Uppsala, Sweden .
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Uppsala, Sweden; Wigner Research Center for Physics, Budapest, Hungary .
    Influence of manganese on the bulk properties of Fe-Cr-Mn alloys: a first-principles study2014In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 89, no 12, p. 125702-Article in journal (Refereed)
    Abstract [en]

    We investigate the effect of manganese on lattice stability and magnetic moments of paramagnetic Fe-Cr-Mn steel alloys along the Bain path connecting the body-centered cubic (bcc) and face-centered cubic (fcc) structures. The calculations are carried out using the ab initio exact muffin-tin orbital method, in combination with the coherent potential approximation, and the paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all Fe-Cr-Mn alloys considered here, the local magnetic moments on Fe atoms have the minimum values for the fcc structure and the maximum values for the bcc structure, whereas the local magnetic moments on Mn have almost the same value along the constant-volume Bain path. Our results show that Mn addition to paramagnetic Fe-Cr solid solution stabilizes the bcc structure. However, when considering the paramagnetic fcc phase relative to the ferromagnetic bcc ground state, then Mn turns out to be a clear fcc stabilizer, in line with observations.

  • 19.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Completeness of the exact muffin-tin orbitals: application to hydrogenated alloys2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 4Article in journal (Refereed)
    Abstract [en]

    We investigate the basis set convergence of the exact muffin-tin orbitals by monitoring the equation of state for Al, Cu, and Rh calculated in the conventional face-centered-cubic lattice (str-I) and in a face-centered-cubic lattice with one atomic and three empty sites per primitive cell (str-II). We demonstrate that three (spd) muffin-tin orbitals are sufficient to describe Al in both structures, but for str-II Cu and Rh at least five (spdfg) orbitals are needed to get converged equilibrium Wigner-Seitz radius (within <= 0.8%) and bulk modulus (<= 3.3%). We ascribe this slow convergence to the nearly spherical densities localized around the Cu and Rh atoms, which create strongly asymmetric charge distributions within the nearest cells around the empty sites. The potential sphere radius dependence of the theoretical results for structure str-II is discussed. It is shown that a properly optimized overlapping muffin-tin potential in combination with the spdfg basis yields acceptable errors in the equilibrium bulk properties. The basis set convergence is also shown on hydrogenated Sc and Sc-based alloys.

  • 20.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Influence of Magnesium on hydrogenated ScAl1-xMgx alloys: a theoretical study2011In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 50, no 10, p. 2848-2853Article in journal (Refereed)
    Abstract [en]

    Ab initio total energy calculations, based on the projector augmented wave method and the exact mu±n-tin orbitals method in combination with the coherent-potential approximation, are used to examine the effect of magnesium on hydrogen absorption/desorption temperature and phase stability of hydrogenated ScAl1-xMgx (0 ≤ x ≤ 0:3) alloys. According to the  experiments, ScAl1-xMgx adopts the CsCl structure, and upon hydrogen absorption it decomposes into ScH2 with CaF2 structure and Al-Mg with face centered cubic structure. Here we demonstrate that the stability field of the hydrogenated alloys depends sensitively on Mg content and on the microstructure of the decomposed system. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with Mg concentration.

  • 21.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Medvedeva, A.
    Andersson, J.
    Nilson, G.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Tetragonality of carbon-doped ferromagnetic iron alloys: A first-principles study2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 1, p. 014112-Article in journal (Refereed)
    Abstract [en]

    Using density-functional theory in combination with the exact muffin-tin orbital (EMTO) method and coherent potential approximation, we investigate the alloying effect on the tetragonality of Fe-C solid solution forming the basis of steels. In order to assess the accuracy of our approach, first we perform a detailed study of the performance of the EMTO method for the Fe(16)C(1) binary system by comparing the EMTO results to those obtained using the projector augmented wave method. In the second step, we introduce different substitutional alloying elements (Al, Cr, Co, Ni) into the Fe matrix and study their impact on the structural parameters. We demonstrate that a small amount of Al, Co, and Ni enhances the tetragonal lattice ratio of Fe(16)C(1) whereas Cr leaves the ratio almost unchanged. The obtained trends are correlated with the single-crystal elastic parameters calculated for carbon-free alloys.

  • 22. Amft, Martin
    et al.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Skorodumova, Natalia V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Influence of the cluster dimensionality on the binding behavior of CO and O(2) on Au(13)2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 136, no 2, p. 024312-Article in journal (Refereed)
    Abstract [en]

    We present an ab initio density functional theory study of the binding behavior of CO and O(2) molecules to two-and three-dimensional isomers of Au(13) in order to investigate the potential catalytic activity of this cluster towards low-temperature CO oxidation. First, we scanned the potential energy surface of Au(13) and studied the effect of spin-orbit coupling on the relative stabilities of the 21 isomers we identified. While spin-orbit coupling increases the stability of the three-dimensional more than the two-dimensional isomers, the ground state structure at 0 K remains planar. Second, we systematically studied the binding of CO and O(2) molecules onto the planar and three-dimensional structures lowest in energy. We find that the isomer dimensionality has little effect on the binding of CO to Au(13). O(2), on the other hand, binds significantly to the three-dimensional isomer only. The simultaneous binding of multiple CO molecules decreases the binding energy per molecule. Still, the CO binding remains stronger than the O(2) binding. We did not find a synergetic effect due to the co-adsorption of both molecular species. On the three-dimensional isomer, we find O(2) dissociation to be exothermic with an dissociation barrier of 1.44 eV.

  • 23.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles based calculation of binary and multicomponent phase diagrams for titanium carbonitride2008In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 32, no 3, p. 543-565Article in journal (Refereed)
    Abstract [en]

    In this paper we have used a combined first principles and Calphad approach to calculate phase diagrams in the titanium-carbon-nitrogen system, with particular focus on the vacancy-induced ordering of the substoichiometric carbonitride phase, TiCxNy (x + y <= 1). Results from earlier Monte Carlo simulations of the low-temperature binary phase diagrams are used in order to formulate sublattice models for TiCxNy within the compound energy formalism (CEF) that are capable of describing both the low temperature ordered and the high-temperature disordered state. We parameterize these models using first-principles calculations and then we demonstrate how they can be merged with thermodynamic descriptions of the remaining Ti-C-N phases that are derived within the Calphad method by fitting model parameters to experimental data. We also discuss structural and electronic properties of the ordered end-member compounds, as well as short range order effects in the TiCxNy phase.

  • 24.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Thermodynamics of structural vacancies in titanium monoxide from first principles calculations2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 14, p. 144101-Article in journal (Refereed)
    Abstract [en]

    The structure, stability and electronic properties of the low oxygen oxides of titanium, TiOx with 1/3 <= x <= 3/2, have been studied by means of accurate first-principles calculations. In both stoichiometric and nonstoichiometric TiO there are large fractions of vacant lattice sites. These so-called structural vacancies are essential for understanding the properties and phase stability of titanium oxides. Structures with an ordered arrangement of vacancies were treated with a plane wave pseudo-potential method, while calculations for structures with disordered vacancies were performed within the framework of the Korringa-Kohn-Rostoker Green's function technique. The relaxed structural parameters in general compare well with experimental data, though some discrepancies exist for stoichiometric TiO in the ideal B1 structure, i.e., without any vacancies. The equation of state as well as the elastic properties are also derived. A monoclinic, vacancy-containing, structure of stoichiometric TiO is confirmed to be stable at low temperature and pressure. Experimentally a transition from a stoichiometric cubic structure with disordered vacancies to the ideal B1 structure without any vacancies has been observed at high pressure. It is discussed how this experimental observation relates to the present theoretical results for defect-containing and defect-free TiO.

  • 25.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simak, S. I.
    Skorodumova, N. V.
    Abrikosov, I. A.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Modeling of Ce2, Ce2O3 ,and CeO2-x in the LDA+U formalism2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 75, no 3, p. 035109-Article in journal (Refereed)
    Abstract [sv]

    The electronic structure and thermodynamic properties of CeO2 and Ce2O3 have been studied from first principles by the all-electron projector-augmented-wave (PAW) method, as implemented in the ab initio total-energy and molecular-dynamics program VASP (Vienna ab initio simulation package). The local density approximation (LDA)+U formalism has been used to account for the strong on-site Coulomb repulsion among the localized Ce 4f electrons. We discuss how the properties of CeO2 and Ce2O3 are affected by the choice of U as well as the choice of exchange-correlation potential, i.e., the local density approximation or the generalized gradient approximation. Further, reduction of CeO2, leading to formation of Ce2O3 and CeO2-x, and its dependence on U and exchange-correlation potential have been studied in detail. Our results show that by choosing an appropriate U it is possible to consistently describe structural, thermodynamic, and electronic properties of CeO2, Ce2O3, and CeO2-x, which enables modeling of redox processes involving ceria-based materials.

  • 26.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simak, S. I.
    Skorodumova, N. V.
    Abrikosov, I. A.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Redox properties of CeO2-MO2 MO2 (M=Ti, Zr, Hf or Th) solid solutions from first principles calculations2007In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 90, no 3, p. 031909-Article in journal (Refereed)
    Abstract [en]

    The authors have used density functional theory calculations to investigate how the redox thermodynamics and kinetics of CeO2 are influenced by forming solid solutions with TiO2, ZrO2, HfO2, and ThO2. Reduction is facilitated by dissolving TiO2 (largest improvement), HfO2, or ZrO2 (least improvement), while ThO2 makes reduction slightly more difficult. The migration barrier is much lower in the neighborhood of a Ti (largest decrease), Hf, or Zr (least decrease), while the binding energy of solute ions and vacancies increases in the same sequence. They rationalize the properties of ceria solid solutions in terms of defect cluster relaxations.

  • 27.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simak, S. I.
    Skorodumova, N. V.
    Abrikosov, I. A.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Theoretical study of CeO(2) doped with tetravalent ions2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 76, no 7, p. 174119-Article in journal (Refereed)
    Abstract [en]

    We have used density functional theory calculations within the LDA+U formulation to investigate how small amounts of dissolved SiO2, GeO2, SnO2, or PbO2 affect the redox thermodynamics of ceria (CeO2). Compared to pure ceria, reduction is facilitated and the reducibility increases in the sequence of CeO2-SnO2, CeO2-GeO2, and CeO2-SiO2, which correlates with the decrease of the ionic radii of the solutes. For low solute concentrations, there is an inverse relation between high reducibility and the solution energy of tetravalent solutes. CeO2-PbO2 is unique in the sense that the initial reduction occurs by Pb(IV)double right arrow Pb(II) instead of the usual Ce(IV)double right arrow Ce(III) reaction. Among the investigated ceria compounds, CeO2-PbO2 has the lowest reduction energy and rather low solution energy. We have studied how the solution and reduction energies depend on the concentration of Si, Ge, Sn, Pb, Ti, Zr, Hf, and Th solute ions. While the solution energy increases monotonously with concentration, the reduction energy first decreases, as compared to pure ceria (except for Th, which exhibits a small increase), and with further increase of solute concentration, it either remains almost constant (Zr, Hf, and Th) or slightly increases (Ti, Si, Ge, and Sn).

  • 28.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Simak, Sergei I.
    Skorodumova, Natalia V.
    Abrikosov, Igor A.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Optimization of ionic conductivity in doped ceria2006In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 103, no 10, p. 3518-3521Article in journal (Refereed)
    Abstract [en]

    Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present a quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy-dopant interactions, represented by association (binding) energies of vacancy-dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately.

  • 29.
    Andersson, Margareta
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Appelberg, Jesper
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Tilliander, Anders
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Nakajima, Keiji
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Shibata, Hiroyuki
    Tohoku University, Institute of Multidisciplinary Research for Advanced Materials.
    Kitamura, Shinya
    Tohoku University, Institute of Multidisciplinary Research for Advanced Materials.
    Jonsson, Lage
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Jönsson, Pär
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Process Metallurgy.
    Some Aspects on Grain Refining Additions with Focus on Clogging during Casting2006In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 46, no 6, p. 814-823Article in journal (Refereed)
    Abstract [en]

    Some ideas of how to study optimum conditions for implementation of grain refining in liquid steel processing with focus on how to avoid clogging are discussed. It is assumed that the inclusions most beneficial for grain refining are known from studies by physical metallurgists. The challenge for a process metallurgist is how to provide a homogeneous distribution of grain refiners at the onset of solidification. Four different ways of providing information to succeed with this are discussed. Thermodynamic modeling can be used to predict what additions to make to create potential grain refiners, if relevant thermodynamic data is available. Mathematical fluid-flow modeling can be used to study where to add potential grain refiners. It is discussed that the tundish is the most appropriate reactor to add grain refiners, since enough time is given to a complete mixing of the grain refiner into the steel before the steel enters the mold. By using the scanning laser microscopy technique it is possible to study which potential grain refiners has the lowest attraction forces between each other. This is important in order to minimise growth of inclusions when they collide during transport in the tundish, which can lead to the formation of larger inclusions that do not serve as useful grain refiners. Finally, it is suggested that laboratory experiments are carried out in order to study the tendency for nozzle clogging, before the use of grain refiners is tested in industrial scale.

  • 30. Anversa, Jonas
    et al.
    Chakraborty, Sudip
    Piquini, Paulo
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    High pressure driven superconducting critical temperature tuning in Sb2Se3 topological insulator2016In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 108, no 21, article id 212601Article in journal (Refereed)
    Abstract [en]

    In this letter, we are reporting the change of superconducting critical temperature in Sb2Se3 topological insulator under the influence of an external hydrostatic pressure based on first principles electronic structure calculations coupled with Migdal-Eliashberg model. Experimentally, it was shown previously that Sb2Se3 was undergoing through a transition to a superconducting phase when subjected to a compressive pressure. Our results show that the critical temperature increases up to 6.15K under the pressure unto 40GPa and, subsequently, drops down until 70 GPa. Throughout this pressure range, the system is preserving the initial Pnma symmetry without any structural transformation. Our results suggest that the possible relevant mechanism behind the superconductivity in Sb2Se3 is primarily the electron-phonon coupling.

  • 31. Arapan, Sergiu
    et al.
    Skorodumova, Natalia V.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Determination of the Structural Parameters of an Incommensurate Phase from First Principles: The Case of Sc-II2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 102, no 8, p. 085701-Article in journal (Refereed)
    Abstract [en]

    We propose a procedure to accurately describe the structural parameters of an incommensurate phase using ab initio methods by approximating it with a set of analogous commensurate supercells. We apply this approach to obtain the structural parameters of the Sc-II phase, which has recently been identified as a complex incommensurate structure similar to Sr-V. The calculated incommensurate ratio gamma, lattice parameters, and Wyckoff positions of Sc-II are in excellent agreement with the available experimental data. Our results show that gamma increases with pressure up to 60 GPa approaching but never reaching the commensurate value 4/3. Hence calculations do not confirm the prediction made based on the reanalyzing of experimental data. When pressure exceeds 70 GPa, gamma shows a sharp decrease that might be considered as the precursor of a new structural phase transition.

  • 32. Araujo, C. M.
    et al.
    Blomqvist, A.
    Scheicher, R. H.
    Chen, P.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Superionicity in the hydrogen storage material Li2NH: Molecular dynamics simulations2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 17Article in journal (Refereed)
    Abstract [en]

    We have employed ab initio molecular dynamics simulations in an attempt to study a temperature-induced order-disorder structural phase transformation that occurs in Li2NH at about 385 K. A structural phase transition was observed by us in the temperature range 300-400 K, in good agreement with experiment. This transition is associated with a melting of the cation sublattice (Li+), giving rise to a superionic phase, which in turn is accompanied by an order-disorder transition of the N-H bond orientation. The results obtained here can contribute to a better understanding of the hydrogen storage reactions involving Li2NH and furthermore broaden its possible technological applications toward batteries and fuel cells.

  • 33. Araujo, C. Moyses
    et al.
    Nagar, Sandeep
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ramzan, Muhammad
    Shukla, R.
    Jayakumar, O. D.
    Tyagi, A. K.
    Liu, Yi-Sheng
    Chen, Jeng-Lung
    Glans, Per-Anders
    Chang, Chinglin
    Blomqvist, Andreas
    Lizarraga, Raquel
    Holmström, Erik
    Belova, Lyubov
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Guo, Jinghua
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites2014In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, p. 4686-Article in journal (Refereed)
    Abstract [en]

    We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect.

  • 34.
    Araujo, C. Moysés
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Kapilashrami, Mukes
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Jun, Xu
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Jayakumar, Onattu D.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Nagar, Sandeep
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Wu, Yan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Århammar, Cecilia
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Belova, Lyubov
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Gehring, Gillian A.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Room temperature ferromagnetism in pristine MgO thin films2010In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 96, no 23Article in journal (Refereed)
    Abstract [en]

    Robust ferromagnetic ordering at, and well above room temperature is observed in pure transparent MgO thin films (<170 nm thick) deposited by three different techniques. Careful study of the wide scan x-ray photoelectron spectroscopy rule out the possible presence of any magnetic contaminants. In the magnetron sputtered films, we observe magnetic phase transitions as a function of film thickness. The maximum saturation magnetization of 5.7 emu/cm(3) is measured on a 170 nm thick film. The films above 500 nm are found to be diamagnetic. Ab initio calculations suggest that the ferromagnetism is mediated by cation vacancies.

  • 35. Araujo, Rafael B.
    et al.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala universitet, Sweden.
    Evaluating bulk Nb2O2F3 for Li-battery electrode applications2016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 5, p. 3530-3535Article in journal (Refereed)
    Abstract [en]

    This investigation has the primary objective of elucidating the lithium intercalation process in the crystal structure of a new niobium oxyfluoride compound Nb2O2F3. The framework of the density functional theory was applied in a generalized gradient approximation together with the hybrid functional method. It is revealed that lithium atoms intercalate in this material in a maximum concentration of one Li atom per formula unit forming LiNb2O2F3. Moreover, octahedral positions in between the layers of Nb-O-F appear as the Li preferred occupancy resulting in a structural volume expansion of only 5%. Electronic structure evolution with the insertion of lithium displays a transformation from semi-conductor to metal when half of the lithium atoms are added. This transformation occurs due to a symmetry break induced by the transition from the + 8 to + 7 oxidation state of half of the Nb2 dimers. Then, after full lithiation the symmetry is recovered and the material becomes a semiconductor again with a band gap amounting to 1 eV. The evaluated average deintercalation potential reaches 1.29 V vs. Li/Li+ with activation energy for lithium ion migration of 0.79 eV. The computed low potential of the redox reaction Nb-2(8+) to Nb-2(7+) includes niobium oxyfluoride in the map of possible materials for the anode application of Li-ion batteries.

  • 36. Araujo, Rafael B.
    et al.
    Banerjee, Amitava
    Ahujati, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Divulging the Hidden Capacity and Sodiation Kinetics of NaxC6Cl4O2: A High Voltage Organic Cathode for Sodium Rechargeable Batteries2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 26, p. 14027-14036Article in journal (Refereed)
    Abstract [en]

    In the current emerging sustainable organic battery field, quinones are seen as one of the prime candidates for application in rechargeable battery electrodes. Recently, C6Cl4O2, a modified quinone, has been proposed as a high voltage organic cathode. However, the sodium insertion mechanism behind the cell reaction remained unclear due to the nescience of the right crystal structure. Here, the framework of the density functional theory (DFT) together with an evolutionary algorithm was employed to elucidate the crystal structures of the compounds NaxC6Cl4O2 (x = 0.5, 1.0, 1.5 and 2). Along with the usefulness of PBE functional to reflect the experimental potential, also the importance of the hybrid functional to divulge the hidden theoretical capacity is evaluated. We showed that the experimentally observed lower specific capacity is a result of the great stabilization of the intermediate phase Na1.5C6Cl4O2. The calculated activation barriers for the ionic hops, are 0.68, 0.40, and 0.31 eV, respectively, for NaC6Cl4O2, Na1.5C6Cl4O2, and Na2C6Cl4O2. These results indicate that the kinetic process must not be a limiting factor upon Na insertion. Finally, the correct prediction of the specific capacity has confirmed that the theoretical strategy used, employing evolutionary simulations together with the hybrid functional framework, can rightly model the thermodynamic process in organic electrode compounds.

  • 37. Araujo, Rafael B.
    et al.
    Banerjee, Amitava
    Panigrahi, Puspamitra
    Yang, Li
    Sjodin, Martin
    Stromme, Maria
    Araujo, C. Moyses
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala Univ, Sweden.
    Assessing the electrochemical properties of polypyridine and polythiophene for prospective applications in sustainable organic batteries2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 4, p. 3307-3314Article in journal (Refereed)
    Abstract [en]

    Conducting polymers are being considered promising candidates for sustainable organic batteries mainly due to their fast electron transport properties and high recyclability. In this work, the key properties of polythiophene and polypyridine have been assessed through a combined theoretical and experimental study focusing on such applications. A theoretical protocol has been developed to calculate redox potentials in solution within the framework of the density functional theory and using continuous solvation models. Here, the evolution of the electrochemical properties of solvated oligomers as a function of the length of the chain is analyzed and then the polymer properties are estimated via linear regressions using ordinary least square. The predicted values were verified against our electrochemical experiments. This protocol can now be employed to screen a large database of compounds in order to identify organic electrodes with superior properties.

  • 38. Araujo, Rafael B.
    et al.
    Banerjee, Amitava
    Panigrahi, Puspamitra
    Yang, Li
    Stromme, Maria
    Sjodin, Martin
    Araujo, C. Moyses
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Designing strategies to tune reduction potential of organic molecules for sustainable high capacity battery application2017In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 5, no 9, p. 4430-4454Article in journal (Refereed)
    Abstract [en]

    Organic compounds evolve as a promising alternative to currently used inorganic materials in rechargeable batteries due to their low-cost, environmental friendliness and flexibility. One of the strategies to reach acceptable energy densities and to deal with the high solubility of known organic compounds is to combine small redox active molecules, acting as capacity carrying centres, with conducting polymers. Following this strategy, it is important to achieve redox matching between the chosen molecule and the polymer backbone. Here, a synergetic approach combining theory and experiment has been employed to investigate this strategy. The framework of the density functional theory connected with the reaction field method has been applied to predict the formal potential of 137 molecules and identify promising candidates for the referent application. The effects of including different ring types, e.g. fused rings or bonded rings, heteroatoms, and pi bonds, as well as carboxyl groups on the formal potential, have been rationalized. Finally, we have identified a number of molecules with acceptable theoretical capacities that show redox matching with thiophene-based conducting polymers which, hence, are suggested as pendent groups for the development of conducting redox polymer based electrode materials.

  • 39. Araujo, Rafael B.
    et al.
    Chakraborty, Sudip
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden .
    Unveiling the charge migration mechanism in Na2O2: implications for sodium-air batteries2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 12, p. 8203-8209Article in journal (Refereed)
    Abstract [en]

    Metal-air batteries have become promising candidates for modern energy storage due to their high theoretical energy density in comparison to other storage devices. The lower overpotential of Na compared with Li makes Na-air batteries more efficient in terms of battery lifetime. Additionally, the abundance of Na over Li is another advantage for Na batteries compared to Li batteries. Na2O2 is one of the main products of sodium-air battery reactions. The efficiency of air cells is always related to the charge transport mechanisms in the formed product. To unveil these diffusion mechanisms in one of the main products of the cell reaction Na-O-2 we systematically investigate the mobility of charge carriers as well as the electronic structural properties of sodium peroxide. The framework of the density functional theory based on hybrid functional approach is used to study the mobility of charge carriers and intrinsic defects in Na2O2. Our calculations reveal that the formation of small electron and hole polarons is preferentially occurring over the delocalized state in the crystal structure of Na2O2. The migration of these small polarons displays activation energies of about 0.92 eV and 0.32 eV for the electron and hole polarons respectively, while the analysis of the charged sodium vacancy mobility reveals an activation energy of about 0.5 eV. These results suggest that the charge transport in sodium peroxide would mainly occur through the diffusion of hole polarons.

  • 40. Araujo, Rafael B.
    et al.
    Chakraborty, Sudip
    Barpanda, Prabeer
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Sweden.
    Na2M2(SO4)(3) (M = Fe, Mn, Co and Ni): towards high-voltage sodium battery applications2016In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 14, p. 9658-9665Article in journal (Refereed)
    Abstract [en]

    Sodium-ion-based batteries have evolved as excellent alternatives to their lithium-ion-based counterparts due to the abundance, uniform geographical distribution and low price of Na resources. In the pursuit of sodium chemistry, recently the alluaudite framework Na2M2(SO4)(3) has been unveiled as a high-voltage sodium insertion system. In this context, the framework of density functional theory has been applied to systematically investigate the crystal structure evolution, density of states and charge transfer with sodium ions insertion, and the corresponding average redox potential, for Na2M2(SO4)(3) (M = Fe, Mn, Co and Ni). It is shown that full removal of sodium atoms from the Fe-based device is not a favorable process due to the 8% volume shrinkage. The imaginary frequencies obtained in the phonon dispersion also reflect this instability and the possible phase transition. This high volume change has not been observed in the cases of the Co- and Ni-based compounds. This is because the redox reaction assumes a different mechanism for each of the compounds investigated. For the polyanion with Fe, the removal of sodium ions induces a charge reorganization at the Fe centers. For the Mn case, the redox process induces a charge reorganization of the Mn centers with a small participation of the oxygen atoms. The Co and Ni compounds present a distinct trend with the redox reaction occurring with a strong participation of the oxygen sublattice, resulting in a very small volume change upon desodiation. Moreover, the average deintercalation potential for each of the compounds has been computed. The implications of our findings have been discussed both from the scientific perspective and in terms of technological aspects.

  • 41. Araujo, Rafael B.
    et al.
    Islam, M. S.
    Chakraborty, Sudip
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Predicting electrochemical properties and ionic diffusion in Na2+2xMn2-x(SO4)(3): crafting a promising high voltage cathode material2016In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 4, no 2, p. 451-457Article in journal (Refereed)
    Abstract [en]

    Sodium ion batteries have emerged as a good alternative to lithium based systems due to their low cost of production. In this scenario, the search for higher voltage, sodium cathodes results in a new promising alluaudite structure Na2+2xMn2-x(SO4)(3). The structural, electronic and Na diffusion properties along with defects have been reported in this investigation within the framework of density functional theory. A band gap of 3.61 eV has been computed and the average deintercalation potential is determined to be 4.11 V vs. Na/Na+. A low concentration of anti-site defects is predicted due to their high formation energy. The biggest issue for the ionic diffusion in the Na2+2xMn2-x(SO4)(3) crystal structure is revealed to be the effect of Mn vacancies increasing the activation energy of Na+ ions that hop along the [001] equilibrium positions. This effect leads to activation energies of almost the same high values for the ionic hop through the [010] direction characterizing a 2D like ionic diffusion mechanism in this system.

  • 42.
    Araujo, Rafael B.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Scheicher, R. H.
    De Almeida, J. S.
    Ferreira Da Silva, A.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Lithium transport investigation in LixFeSiO4: A promising cathode material2013In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 173, p. 9-13Article in journal (Refereed)
    Abstract [en]

    In this paper we investigate lithium mobility in both Li 2FeSiO4 and its half-lithiated state LiFeSiO4 considering an orthorhombic crystal structure. We find that the calculated activation energy of Li+ ions hopping between adjacent equilibrium sites predicts two least hindered diffusion pathways in both materials. One of them is along the [100] direction characterizing an ionic diffusion in a straight line and the other follows a zig-zag way between the Fe-Si-O layers. We also show that diffusion of Li+ ions in the half-lithiated structure follows the same behavior as in the lithiated structure. As a whole, the activation energies for the investigated compounds present a greater value compared with the activation energies in currently used materials such as LiFePO4. The results were calculated in the framework of density functional theory in conjunction with the climbing image nudged elastic band method. The Hubbard term was added to the Kohn-Sham Hamiltonian to overcome the delocalization problem of d electrons. Furthermore, the diffusion coefficients were calculated for both structures considering temperatures ranging from 300 to 700 K.

  • 43.
    Araujo, Rafael B.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Scheicher, Ralph H.
    de Almeida, J. S.
    Ferreira da Silva, A.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles investigation of Li ion diffusion in Li2FeSiO42013In: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 247, p. 8-14Article in journal (Refereed)
    Abstract [en]

    We have studied the Li-ion migration and the electrochemical performance of Li2FeSiO4 in the monoclinic crystal structure with P2(1) symmetry and the related delithiated system LiFeSiO4. For this purpose, the framework of the density functional theory within the generalized gradient approximation in conjunction with the climbing image nudged elastic band method was used. Addition of the Hubbard term was also considered in the Kohn-Sham Hamiltonian to better model the d electrons of the metal ions in this material. The calculated activation energies for Li ion migration are found to decrease by around 20% with the Hubbard term inclusion in the chosen diffusion pathways of Li2FeSiO4. Regarding the delithiated structure, the activation energies were found to be sensitive to the Hubbard term addition, however no general behavior such as in the lithiated structure was found. Furthermore, the diffusion coefficients were calculated considering temperatures of 300 K, 500 K, and 700 K.

  • 44. Arslanov, Temirlan R.
    et al.
    Mollaev, Akhmedbek Yu.
    Kamilov, Ibragimkhan K.
    Arslanov, Rasul K.
    Kilanski, Lukasz
    Minikaev, Roman
    Reszka, Anna
    Lopez-Moreno, Sinhue
    Romero, Aldo H.
    Ramzan, Muhammad
    Panigrahi, Puspamitra
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Trukhan, Vladimir M.
    Chatterji, Tapan
    Marenkin, Sergey F.
    Shoukavaya, Tatyana V.
    Pressure control of magnetic clusters in strongly inhomogeneous ferromagnetic chalcopyrites2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, p. 7720-Article in journal (Refereed)
    Abstract [en]

    Room-temperature ferromagnetism in Mn-doped chalcopyrites is a desire aspect when applying those materials to spin electronics. However, dominance of high Curie-temperatures due to cluster formation or inhomogeneities limited their consideration. Here we report how an external perturbation such as applied hydrostatic pressure in CdGeP2:Mn induces a two serial magnetic transitions from ferromagnet to non-magnet state at room temperature. This effect is related to the unconventional properties of created MnP magnetic clusters within the host material. Such behavior is also discussed in connection with ab initio density functional calculations, where the structural properties of MnP indicate magnetic transitions as function of pressure as observed experimentally. Our results point out new ways to obtain controlled response of embedded magnetic clusters.

  • 45. Asker, C.
    et al.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Abrikosov, I. A.
    Elastic constants and anisotropy in FeNi alloys at high pressures from first-principles calculations2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 21Article in journal (Refereed)
    Abstract [en]

    The single-crystal and polycrystalline elastic constants and the elastic anisotropy in face-centered cubic and hexagonal close-packed FeNi alloys have been investigated at ultrahigh pressures by means of first-principles calculations using the exact muffin-tin orbitals method and the coherent-potential approximation. Comparisons with earlier calculations for pure Fe and experimental results are presented and discussed. We show that Ni alloying into Fe increases slightly the density and has very little effect on bulk moduli. Moreover, the relative decrease in c(44) elastic constant is much stronger in the hcp phase than in the fcc one. It is found that the elastic anisotropy is higher for face-centered cubic than for the hexagonal close-packed structure of FeNi, even though the face-centered cubic phase has a higher degree of symmetry. The anisotropy in face-centered cubic structure decreases with increasing nickel concentration while a very weak increase is observed for the hexagonal close-packed structure.

  • 46. Astrath, N. G. C.
    et al.
    Bento, A. C.
    Baesso, M. L.
    da Silva, A. F.
    Ahuja, R.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhao, S.
    Granqvist, C. G.
    Thermal lens and photoacoustic spectroscopy to determine the thermo-optical properties of semiconductors2005In: Journal de Physique IV: Colloque, ISSN 1155-4339, E-ISSN 1764-7177, Vol. 125, p. 181-183Article in journal (Refereed)
    Abstract [en]

    In this work we used photoacoustic and thermal lens spectroscopy to study four different semiconductor samples: PbI2, 4HSiC, NiCrO and NiO. The results showed that the combination of these two techniques provided the values of the band gap energies and the thermal diffusivities.

  • 47. Astrath, N. G. C.
    et al.
    Bento, A. C.
    Baesso, M. L.
    da Silva, A. Ferreira
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Photoacoustic spectroscopy to determine the optical properties of thin film 4H-SiC2006In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 515, no 4, p. 2821-2823Article in journal (Refereed)
    Abstract [en]

    The optical transitions in a range of 1.5-5.2 eV of n-type 4H-SiC have been investigated experimentally by photoacoustic spectroscopy and theoretically by a full-potential linearized augmented plane wave method. From the absorption spectrum, we found the indirect optical bandgap at 3.2 eV and the direct transitions around 4.5 eV in very good agreement with what has been predicted by theoretical calculations.

  • 48. Astrath, N. G. C.
    et al.
    Sato, F.
    Pedrochi, F.
    Medina, A. N.
    Bento, A. C.
    Baesso, M. L.
    Persson, Clas
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    da Silva, A. Ferreira
    Band gap energy determination by photoacoustic spectroscopy under continuous light excitation2006In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 89, no 23Article in journal (Refereed)
    Abstract [en]

    In this work the authors used the photoacoustic spectroscopy under continuous light excitation to determine the optical band gap of semiconductors. The experiments were performed in lead iodide PbI2 and hexagonal silicon carbide 4H-SiC samples. The nonradiative relaxation processes are discussed in terms of the generated signal. A mechanism to describe the signal increase/decrease under the continuous excitation is presented. The results showed that the method was useful to locate the band gap directly from the optical absorption spectra.

  • 49. B. Araujo, R.
    et al.
    S. De Almeida, J.
    Ferreira Da Silva, A.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Insights in the electronic structure and redox reaction energy in LiFePO4 battery material from an accurate Tran-Blaha modified Becke Johnson potential2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 118, no 12, article id 125107Article in journal (Refereed)
    Abstract [en]

    The main goals of this paper are to investigate the accuracy of the Tran-Blaha modified Becke Johnson (TB-mBJ) potential to predict the electronic structure of lithium iron phosphate and the related redox reaction energy with the lithium deintercalation process. The computed electronic structures show that the TB-mBJ method is able to partially localize Fe-3d electrons in LiFePO<inf>4</inf> and FePO<inf>4</inf> which usually is a problem for the generalized gradient approximation (GGA) due to the self interaction error. The energy band gap is also improved by the TB-mBJ calculations in comparison with the GGA results. It turned out, however, that the redox reaction energy evaluated by the TB-mBJ technique is not in good agreement with the measured one. It is speculated that this disagreement in the computed redox energy and the experimental value is due to the lack of a formal expression to evaluate the exchange and correlation energy. Therefore, the TB-mBJ is an efficient method to improve the prediction of the electronic structures coming form the standard GGA functional in LiFePO<inf>4</inf> and FePO<inf>4</inf>. However, it does not appear to have the same efficiency for evaluating the redox reaction energies for the investigated system.

  • 50. Banerjee, A.
    et al.
    Araujo, R. B.
    Sjödin, M.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Identifying the tuning key of disproportionation redox reaction in terephthalate: A Li-based anode for sustainable organic batteries2018In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 47, p. 301-308Article in journal (Refereed)
    Abstract [en]

    The ever-increasing consumption of energy storage devices has pushed the scientific community to realize strategies toward organic electrodes with superior properties. This is owed to advantages such as economic viability and eco-friendliness. In this context, the family of conjugated dicarboxylates has emerged as an interesting candidate for the application as negative electrodes in advanced Li-ion batteries due to the revealed thermal stability, rate capability, high capacity and high cyclability. This work aims to rationalize the effects of small molecular modifications on the electrochemical properties of the terephthalate anode by means of first principles calculations. The crystal structure prediction of the investigated host compounds dilithium terephthalate (Li2TP) and diethyl terephthalate (Et2Li0TP) together with their crystal modification upon battery cycling enable us to calculate the potential profile of these materials. Distinct underlying mechanisms of the redox reactions were obtained where Li2TP comes with a disproportionation reaction while Et2Li0TP displays sequential redox reactions. This effect proved to be strongly correlated to the Li coordination number evolution upon the Li insertion into the host structures. Finally, the calculations of sublimation enthalpy inferred that polymerization techniques could easily be employed in Et2Li0TP as compared to Li2TP. Similar results are observed with methyl, propyl, and vinyl capped groups. That could be a strategy to enhance the properties of this compound placing it into the gallery of the new anode materials for state of art Li-batteries.

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