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  • 1.
    Forsblom, Mattias
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Atomistic simulations of lattice defects2005Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Mechanical properties of solids are governed by crystal imperfections. Computational materials science is largely concerned with the modelling of such defects, e.g. their formation, migration, and interaction energies. Atomistic simulations of systems containing lattice defects are inherently difficult because of the generally complicated geometrical structure of the defects, the need for large simulation cells, etc.

    In this thesis, the role of lattice defects in the mechanism behind homogeneous melting is demonstrated. Also, a generic calculational scheme for studying atomic vibrations close to extended defects (applied to a dislocation) has been considered. Furthermore, heat capacities in the solid and liquid phases of aluminium have been calculated, as well as various thermophysical defect properties.

    The work was carried out using classical atomistic simulations, mainly molecular dynamics, of aluminium and copper. The interatomic forces were modelled with effective interactions of the embedded-atom type.

    The main results of this thesis are the following:

    • The thermal fluctuation initiating melting is an aggregate typically with 6-7 interstitials and 3-4 vacancies.

    • In the initial stage of melting, no signs of a shear modulus melting mechanism, or the presence of line-like defects (dislocations), can be seen.

    • The typical time interval from when melting initiates to the time at which the liquid phase is fully developed is of the order of 1000τ, where the period τ corresponds to the maximum vibrational frequency in the solid.

    • The solid-liquid boundary advances at a pace comparable to that of thermal transport by vibrating atoms in the crystal at high temperatures.

    • The seemingly small anharmonic effect in the heat capacity of aluminium is caused by a partial cancellation of the low-order term linear in the temperature and anharmonic terms of higher order in the temperature.

    • The core region of an edge dislocation in face-centred cubic aluminium has compressed and expanded regions. The excess volume associated with the dislocation core is small, about 6 percent of the atomic volume, as a result of a partial cancellation between the volume changes of the compressed and expanded regions.

    • The compressed and expanded regions of the edge dislocation core give negative and positive contributions, respectively, to the excess vibrational entropy. The overall effect is a positive vibrational excess entropy of the dislocation core which is about 2kB per atomic repeat length along the dislocation core.

    • The atomic vibrations near the dislocation core are modelled by considering an atomic cluster with about 500-1000 atoms containing the core of dislocation, embedded in a large discrete, but relaxed, lattice of about 23 000 atoms. An atomic region that is four atomic layers thick and about 18 atomic diameters long in the direction parallel to the Burgers vector, accounts for most of the excess entropy.

    • The constant-pressure heat capacity of aluminium shows a minimum as a function of temperature in the liquid phase.

  • 2.
    Forsblom, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Heat capacity of liquid Al: Molecular dynamics simulations2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 13, p. 132204-Article in journal (Refereed)
    Abstract [en]

    The heat capacities at constant pressure, c(P), at constant volume and at fixed volume, and the isothermal bulk modulus, are calculated for liquid Al over a wide range of temperatures using molecular dynamics simulations with interactions due to Ercolessi and Adams and Mei and Davenport. c(P) has only a weak temperature dependence, with a shallow minimum that results from the opposing effects of a gradual loss of shear resistance and thermal expansion.

  • 3.
    Forsblom, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Homogeneous melting of superheated crystals: Molecular dynamics simulations2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 5, p. 054107-Article in journal (Refereed)
    Abstract [en]

    The homogeneous melting mechanism in a superheated fcc lattice is studied through molecular dynamics simulations, usually for about 20 000 atoms, with the Ercolessi and Adams interaction that represents aluminum. The periodic boundary conditions for the simulation cell suppress the usual surface-initiated melting at T-m=939 K, and the solid-to-liquid transition takes place at the temperature T-s=1.3T(m). By logging the position of each atom at every time step in the simulation, we can follow the melting process in detail at the atomic level. Thermal fluctuations close to T-s create interstitial-vacancy pairs, which occasionally separate into mobile interstitials and almost immobile vacancies. There is an attraction between two interstitials, with a calculated maximum interaction energy of about 0.7 eV. When three to four migrating interstitials have come close enough to form a bound aggregate of point defects, and a few thermally created interstitial-vacancy pairs have been added to the aggregate, such a defect configuration usually continues to grow irreversibly to the liquid state. For 20 000 atoms in the simulation cell, the growth process takes about 10(2)tau to be completed, where tau is the period of a typical atomic vibration in the solid phase. This melting mechanism involves fewer atoms in its crucial initial phase than has been suggested in other melting models. The elastic shear moduli c(44) and c(')=(c(11)-c(12))/2 were calculated as a function of temperature and were shown to be finite at the onset of melting.

  • 4.
    Forsblom, Mattias
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    How superheated crystals melt2005In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 4, no 5, p. 388-390Article in journal (Refereed)
    Abstract [en]

    The melting of superheated crystalline solids through the penetration of intense radiation within at a temperature above the equilibrium melting temperature was investigated. The atomistic simulations, relevant for aluminum, was used to show that the thermal fluctuation initiating melting is an aggregate typically with 6-7 interstitials and 3-4 vacancies. Vacancy-interstital pairs were created through thermal fluctuation and interstitial-vacancy pairs were created close to the interstitial aggregate. It was found that when a defect aggregate contains more than about 10 point defects, it usually grows rapidly and irreversibly.

  • 5.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Accuracy of models2008In: SCIENTIFIC MODELING AND SIMULATIONS, ISSN 1874-8554, Vol. 15, no 1-3, p. 41-57Article in journal (Refereed)
    Abstract [en]

    The ability of a mathematical model to accurately describe a phenomenon depends on how well the model incorporates all relevant aspects, how robust the model is with respect to its mathematical form, and with respect to the numerical values of input parameters. Some models are primarily intended to reproduce known data. In other cases the purpose is to make a prediction outside the range of knowledge, or to establish details in a physical mechanism. These aspects of modelling are discussed, with examples mainly taken from the field of materials science.

  • 6.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Characteristic quantities and dimensional analysis2008In: SCIENTIFIC MODELING AND SIMULATIONS, ISSN 1874-8554, Vol. 15, no 1-3, p. 21-39Article in journal (Refereed)
    Abstract [en]

    Phenomena in the physical sciences are described with quantities that have a numerical value and a dimension, i.e., a physical unit. Dimensional analysis is a powerful aspect of modeling and simulation. Characteristic quantities formed by a combination of model parameters can give new insights without detailed analytic or numerical calculations. Dimensional requirements lead to Buckingham's Pi theorem-a general mathematical structure of all models in physics. These aspects are illustrated with many examples of modeling, e. g., an elastic beam on supports, wave propagation on a liquid surface, the Lennard-Jones potential for the interaction between atoms, the Lindemann melting rule, and saturation phenomena in electrical and thermal conduction.

  • 7.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Editorial: A Timely Contribution to a Half-Century-Old Topic2014In: Physical Review X, ISSN 2160-3308, E-ISSN 2160-3308, Vol. 4, no 1, p. 010001-Article in journal (Refereed)
  • 8.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Extrapolative procedures in modelling and simulations: the role of instabilities2008In: SCIENTIFIC MODELING AND SIMULATIONS, ISSN 1874-8554, Vol. 15, no 1-3, p. 5-20Article in journal (Refereed)
    Abstract [en]

    In modelling and simulations there is a risk that one extrapolates into a region where the model is not valid. In this context instabilities are of particular interest, since they can arise without any precursors. This paper discusses instabilities encountered in the field of materials science, with emphasis on effects related to the vibrations of atoms. Examples deal with, i.a., common lattice structures being either metastable or mechanically unstable, negative elastic constants that imply an instability, unexpected variations in the composition dependence of elastic constants in alloys, and mechanisms governing the ultimate strength of perfect crystals.

  • 9.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Variation of elastic shear constants in transition metal alloys2005In: Complex Inorganic Solids: Structural, Stability, and Magnetic Properties of Alloys / [ed] Turchi, PEA; Gonis, A; Rajan, K; Meike, A, 2005, p. 295-305Conference paper (Refereed)
  • 10.
    Grimvall, Göran
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Magyari-Köpe, Blanka
    Ozolins, Vidvuds
    Persson, Kristin A.
    Lattice instabilities in metallic elements2012In: Reviews of Modern Physics, ISSN 0034-6861, E-ISSN 1539-0756, Vol. 84, no 2, p. 945-986Article in journal (Refereed)
    Abstract [en]

    Most metallic elements have a crystal structure that is either body-centered cubic (bcc), face-centered close packed, or hexagonal close packed. If the bcc lattice is the thermodynamically most stable structure, the close-packed structures usually are dynamically unstable, i.e., have elastic constants violating the Born stability conditions or, more generally, have phonons with imaginary frequencies. Conversely, the bcc lattice tends to be dynamically unstable if the equilibrium structure is close packed. This striking regularity essentially went unnoticed until ab initio total-energy calculations in the 1990s became accurate enough to model dynamical properties of solids in hypothetical lattice structures. After a review of stability criteria, thermodynamic functions in the vicinity of an instability, Bain paths, and how instabilities may arise or disappear when pressure, temperature, and/or chemical composition is varied are discussed. The role of dynamical instabilities in the ideal strength of solids and in metallurgical phase diagrams is then considered, and comments are made on amorphization, melting, and low-dimensional systems. The review concludes with extensive references to theoretical work on the stability properties of metallic elements.

  • 11. Moore, R. G.
    et al.
    Nascimento, V. B.
    Zhang, Jiandi
    Rundgren, John
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Jin, R.
    Mandrus, D.
    Plummer, E. W.
    Manifestations of broken symmetry: The surface phases of Ca2-xSrxRuO42008In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 100, no 6, p. 066102-Article in journal (Refereed)
    Abstract [en]

    The surface structural phases of Ca2-xSrxRuO4 are investigated using quantitative low energy electron diffraction. The broken symmetry at the surface enhances the structural instability against the RuO6 rotational distortion while diminishing the instability against the RuO6 tilt distortion occurring within the bulk crystal. As a result, suppressed structural and electronic surface phase transition temperatures are observed, including the appearance of an inherent Mott metal-to-insulator transition for x=0.1 and possible modifications of the surface quantum critical point near x(c)similar to 0.5.

  • 12. Nascimento, V. B.
    et al.
    Freeland, J. W.
    Saniz, R.
    Moore, R. G.
    Mazur, D.
    Liu, H.
    Pan, M. H.
    Rundgren, John
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Gray, K. E.
    Rosenberg, R. A.
    Zheng, H.
    Mitchell, J. F.
    Freeman, A. J.
    Veltruska, K.
    Plummer, E. W.
    Surface-Stabilized Nonferromagnetic Ordering of a Layered Ferromagnetic Manganite2009In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 103, no 22, p. 227201-Article in journal (Refereed)
    Abstract [en]

    An outstanding question regarding the probing or possible device applications of correlated electronic materials (CEMs) with layered structure is the extent to which their bulk and surface properties differ or not. The broken translational symmetry at the surface can lead to distinct functionality due to the charge, lattice, orbital, and spin coupling. Here we report on the case of bilayered manganites with hole doping levels corresponding to bulk ferromagnetic order. We find that, although the hole doping level is measured to be the same as in the bulk, the surface layer is not ferromagnetic. Further, our low-energy electron diffraction and x-ray measurements show that there is a c-axis collapse in the outermost layer. Bulk theoretical calculations reveal that, even at fixed doping level, the relaxation of the Jahn-Teller distortion at the surface is consistent with the stabilization of an A-type antiferromagnetic state.

  • 13. Palumbo, M.
    et al.
    Burton, B.
    Costa e Silva, A.
    Fultz, B.
    Grabowski, B.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Hallstedt, B.
    Hellman, O.
    Lindahl, Bonnie
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.
    Schneider, A.
    Turchi, P. E. A.
    Xiong, W.
    Thermodynamic modelling of crystalline unary phases2014In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 251, no 1, p. 14-32Article in journal (Refereed)
    Abstract [en]

    Progress in materials science through thermodynamic modelling may rest crucially on access to a database, such as that developed by Scientific Group Thermodata Europe (SGTE) around 1990. It gives the Gibbs energy G(T) of the elements in the form of series as a function of temperature, i.e. essentially a curve fitting to experimental data. In the light of progress in theoretical understanding and first-principles calculation methods, the possibility for an improved database description of the thermodynamics of the elements has become evident. It is the purpose of this paper to provide a framework for such work. Lattice vibrations, which usually give the major contribution to G(T), are treated in some detail with a discussion of neutron scattering studies of anharmonicity in aluminium, first-principles calculations including ab initio molecular dynamics (AIMD), and the strength and weakness of analytic model representations of data. Similarly, electronic contributions to G(T) are treated on the basis of the density of states N(E) for metals, with emphasis on effects at high T. Further, we consider G(T) below 300K, which is not covered by SGTE. Other parts in the paper discuss metastable and dynamically unstable lattices, G(T) in the region of superheated solids and the requirement on a database in the calculation of phase diagrams.

  • 14. Pentcheva, R.
    et al.
    Moritz, W.
    Rundgren, John
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Frank, S.
    Schrupp, D.
    Scheffler, M.
    A combined DFT/LEED-approach for complex oxide surface structure determination: Fe3O4(001)2008In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 602, no 7, p. 1299-1305Article in journal (Refereed)
    Abstract [en]

    A combination of density functional theory (DFT) calculations and low energy electron diffraction (LEED) analysis is used to determine the surface structure of Fe3O4 0 00). We find that the surface is rich in oxygen and the observed (root 2- x root 2-)R45 degrees reconstruction is a result of a Jahn-Teller distortion as established by recent DFT-calculations [R. Pentcheva, F. Wendler, H.L. Meyerheim, W. Moritz, N. Jedrecy, M. Scheffler, Phys. Rev. Lett. 94 (2005) 126101]. The corresponding Pendry reliability factor is 0.34. Furthermore, we investigate the influence of the preparation conditions (temperature, oxygen pressure) on the LEED intensities of natural and synthetic samples. The electron scattering phase shifts used in the analysis of the LEED spectra are derived from two methods, one based on the DFT electron densities and another employing an overlap of atomic potentials with optimized muffin-tin radii. Both approaches lead to similar results.

  • 15. Sirtl, T.
    et al.
    Jelic, J.
    Meyer, J.
    Das, K.
    Heckl, W. M.
    Moritz, W.
    Rundgren, John
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Schmittel, M.
    Reuter, K.
    Lackinger, M.
    Adsorption structure determination of a large polyaromatic trithiolate on Cu(111): Combination of LEED-I(V) and DFT-vdW2013In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 26, p. 11054-11060Article in journal (Refereed)
    Abstract [en]

    The adsorption geometry of 1,3,5-tris(4-mercaptophenyl)benzene (TMB) on Cu(111) is determined with high precision using two independent methods, experimentally by quantitative low energy electron diffraction (LEED-I(V)) and theoretically by dispersion corrected density functional theory (DFT-vdW). Structural refinement using both methods consistently results in similar adsorption sites and geometries. Thereby a level of confidence is reached that allows deduction of subtle structural details such as molecular deformations or relaxations of copper substrate atoms.

  • 16.
    Vitos, Levente
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Magyari-Kope, B.
    Ahuja, Rajeev
    Condensed Matter Theory Group, Physics Department, Uppsala University.
    Kollar, J.
    Grimvall, Göran
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theory of Materials.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Phase transformations between garnet and perovskite phases in the Earth's mantle: A theoretical study2006In: Physics of the Earth and Planetary Interiors, ISSN 0031-9201, E-ISSN 1872-7395, Vol. 156, no 1-2, p. 108-116Article in journal (Refereed)
    Abstract [en]

    Using first-principles theories, we show that the stability of garnet and perovskite phases in an Al-free system is strongly influenced by both pressure and temperature, giving rise to a sequence of phase changes. Around 17 +/- 3 GPa pressure, the (Mg1-yCay)(3)(MgSi)Si3O12 majorite garnet dissociates into Ca- and Mg-perovskites. This divariant transition is associated with structural, density and elastic changes, and for y approximate to 0. 13 it has a width of similar to 0.6 GPa. In CaSiO3 Plus MgSiO3 aggregate, a (Mg, Ca)SiO3 solid solution with an intermediate orthorhombic perovskite structure can be formed. The (Mg1-xCax)SiO3 solid solution with x approximate to 0.04-0.06 is calculated to be stable at the transition zone base and uppermost lower mantle conditions, and with increasing pressure it separates into perovskite end-members. The pressure-temperature stability limit for the perovskite solid solutions is close to the mantle geotherms, suggesting the appearance of structural and chemical inhomogeneities driven by temperature anomalies within the Earth's lower mantle.

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