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  • 1. Asker, C.
    et al.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Mikhaylushkin, A. S.
    Abrikosov, I. A.
    First-principles solution to the problem of Mo lattice stability2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 22Article in journal (Refereed)
    Abstract [en]

    The energy differences between the ground state body-centered structure and closed-packed face-centered structure for transition metals in the middle of the series show unusually large disagreements when they are obtained by the thermochemical approach based on the analysis of experimental data or by first-principles electronic structure calculations. Considering a typical example, the lattice stability of Mo, we present a solution to this long-standing problem. We carry out ab initio molecular dynamics simulations for the two phases at high temperature and show that the configurational energy difference approaches the value derived by means of the thermochemical approach. The main contribution to the effect comes from the modification of the canonical band structure due to anharmonic thermal motion at high temperature.

  • 2.
    Belonoshko, A. B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory. KTH, School of Biotechnology (BIO), Centres, Albanova VinnExcellence Center for Protein Technology, ProNova.
    Ab Initio Study of Water Interaction with a Cu Surface2010In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 26, no 21, 16267-16270 p.Article in journal (Refereed)
    Abstract [en]

    We have performed a first principles investigation of water interaction with a Cu surface. The calculated surface energy of a Cu(100) slab is in reasonable agreement with experimental data. The energy of water dissociation is in agreement with experiment. The results of the ab initio calculations are compared to experimental data on hydrogen partial pressure. It is concluded that Cu(OH)(ads) is formed due to a reaction between Cu and anoxic water. The energy of the Cu(100) slab with OH and H adsorbed is lower than the energy of the same slab with an adsorbed water molecule.

  • 3.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Arapan, S.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    An ab initio molecular dynamics study of iron phases at high pressure and temperature2011In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 48Article in journal (Refereed)
    Abstract [en]

    The crystal structure of iron, the major component of the Earth's inner core (IC), is unknown for the IC high pressure (P; 3.3-3.6 Mbar) and temperature (T; 5000-7000 K). There is mounting evidence that the hexagonal close-packed (hcp) phase of iron, stable at the high P of the IC and a low T, might be unstable under the IC conditions due to the impact of high T and impurities. Experiments at the IC P and T are difficult and do not provide a conclusive answer as regards the iron stability at the pressure of the IC and temperatures close to the iron melting curve. Recent theory provides contradictory results regarding the nature of the stable Fe phase. We investigated the possibility of body-centered cubic (bcc) phase stabilization at the P and T in the vicinity of the Fe melting curve by using ab initio molecular dynamics. Thermodynamic calculations, relying on the model of uncorrelated harmonic oscillators, provide nearly identical free energies within the error bars of our calculations. However, direct simulation of iron crystallization demonstrates that liquid iron freezes in the bcc structure at the P of the IC and T = 6000 K. All attempts to grow the hcp phase from the liquid failed. The mechanism of bcc stabilization is explained. This resolves most of the earlier confusion.

  • 4.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Equation of state for epsilon-iron at high pressures and temperatures2010In: Condensed Matter Physics, ISSN 1607-324X, Vol. 13, no 2, 23605-23615 p.Article in journal (Refereed)
    Abstract [en]

    The equation of state for hexagonal close packed (hcp or ∈) phase of Fe at high pressure is created by employing molecular dynamics (MD) simulations in conjunction with the embedded atom method based on the full potential linear muffin tin orbital (FPLMTO) method. Comparison between the existing experimental data and our calculations suggests that the obtained equation of state can be reliably used for calculating iron volumetric properties under conditions appropriate for the Earth's core. We demonstrate that some experimental data on iron might be subjected to a systematic error. I suggest a model which describes the temperature dependence of the volume better than the Mie-Grüneisen equation.

  • 5.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Triple fcc-bcc-liquid point on the Xe phase diagram determined by the N-phase method2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 17Article in journal (Refereed)
    Abstract [en]

    There is a discrepancy between the fcc-bcc phase boundaries in Xe determined by the two-phase and the lambda-integration methods. To resolve this issue, I performed large scale (4x10(6) atoms) molecular-dynamics simulations of fcc and bcc Xe phases embedded in liquid Xe. Such simulations, which I call N-phase method, allows for the hydrostatic freezing or melting and direct competition of the phases under consideration. As a result of these long (over several nanoseconds) simulations, I can place the triple fcc-bcc-liquid point on the melting curve of Xe between temperatures of 3470 and 4000 K. This suggests that certain effects are not taken into account in the previous work. Possible reasons are discussed.

  • 6. Belonoshko, Anatoly B.
    et al.
    Ahuja, R.
    Eriksson, O.
    Johansson, Börje
    Quasi ab initio molecular dynamic study of Cu melting2000In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 61, no 6, 3838-3844 p.Article in journal (Refereed)
    Abstract [en]

    We have investigated the melting of Cu theoretically by means of a molecular dynamic method employing the Sutton-Chen model for the interatomic interaction. This interaction has been fitted to reproduce results from first-principles self-consistent total-energy calculations within the local-density approximation using the full-potential linear-muffin-tin-orbital method for the bcc, fee, hcp, and liquid configurations. No experimental data were used to tune the potential. A large number of properties including equation of state, melting temperature, high-pressure melting curve, change of volume and entropy at melting, liquid structure, diffusion coefficient in liquid, and vacancy formation energy are all in good agreement with experimental data. Inclusion of the full potential energy of a liquid configuration in the fitting procedure is critical for obtaining good agreement with experiment. Different ways to calculate the melting transition are shown to produce very different results. The use of a large number of particles in combination with the solid-liquid interface as an initial configuration in the simulation is essential in order to obtain the correct melting temperatures.

  • 7. Belonoshko, Anatoly B.
    et al.
    Ahuja, R.
    Johansson, Börje
    Mechanism for the kappa-Al2O3 to the alpha-Al2O3 transition and the stability of kappa-Al2O3 under volume expansion2000In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 61, no 5, 3131-3134 p.Article in journal (Refereed)
    Abstract [en]

    The kappa-Al2O3 metastable phase is an important material for producing cutting tools. However, at temperatures above 1000 K the kappa phase transforms into the stable modification alpha-Al2O3. We have investigated mechanisms for this transformation by means of molecular dynamic simulations using pair potentials. We have found that for the temperature range above 1000 K the mean square displacement of the atoms at the free surface changes its behavior drastically. Since, as was calculated, all other possible driving mechanisms of the phase transition such as pressure and/or temperature without a free surface are not sufficient to cause the transition, the free surface is the major factor initiating the unwanted transition. To hinder the transition one has to slow down the diffusion at the free surface. As an alternative to chemical vapor deposition of thin films of kappa-Al2O3 phase at surfaces of cutting tools, it is found that it is thermodynamically possible to obtain kappa-Al2O3 in a stable phase at a volume expansion of the alpha-Al2O3 phase at a negative pressure of about - 40 kbar.

  • 8. Belonoshko, Anatoly B.
    et al.
    Ahuja, R.
    Johansson, Börje
    Molecular dynamics of LiF melting2000In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 61, no 18, 11928-11935 p.Article in journal (Refereed)
    Abstract [en]

    We performed molecular-dynamics simulations of the melting and/or freezing of LiF. The simulations were done using the Tosi-Fumi model and our own model of interatomic interactions. The latter was verified by ab initio calculations of the equation of state for LiF. We show that the recent molecular-dynamics calculations by Boehler and co-workers are not adequate and their model for the interactions is not capable of providing melting temperatures in agreement with experiment. Our calculated pressure dependence of the melting temperatures gives valuable information. We found that the B1-B2 transition in LiF at around 1 Mbar removes the discrepancy between the diamond-anvil cell and shockwave melting temperatures. An explanation of the controversy between low and high melting temperatures obtained from diamond-anvil cell experiments is suggested.

  • 9. Belonoshko, Anatoly B.
    et al.
    Ahuja, R.
    Johansson, Börje
    Quasi - Ab initio molecular dynamic study of Fe melting2000In: Physical Review Letters, ISSN 0031-9007, Vol. 84, no 16, 3638-3641 p.Article in journal (Refereed)
    Abstract [en]

    We have investigated the melting of hcp Fe at high pressure by employing molecular dynamics simulations in conjunction with the full potential linear muffin tin orbital method. Apart from being of fundamental value. the melting of iron at high pressure is also important for our understanding of the Earth. The subject of iron melting at high pressures is controversial. The experimental data for the iron melting temperature can be separated into two regions. low and high. Here we present an ab initio simulated iron melting curve which is in agreement with the low temperatures at lower pressures, but is in excellent agreement with the high-mostly shockwave-temperatures at high pressure. A comparison with available data lends support to the presented iron melting curve.

  • 10.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Molecular dynamics study of melting and fcc-bcc transitions in Xe2001In: Physical Review Letters, ISSN 0031-9007, Vol. 8716, no 16, art. no.-165505 p.Article in journal (Refereed)
    Abstract [en]

    We have investigated the phase diagram of Xe over a wide pressure-temperature range by molecular dynamics. The calculated melting curve is in good agreement with earlier experimental data. At a pressure of around 25 GPa and a temperature of about 2700 K we find a triple fcc-bcc liquid point. The calculated fcc-bcc boundary is in nice agreement with the experimental points, which, however, were interpreted as melting. This finding suggests that the transition from close-packed to bcc structure might be more common at high pressure and high temperature than was previously anticipated.

  • 11.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Ahuja, Rajeev
    KTH, Superseded Departments, Materials Science and Engineering.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Stability of the body-centred-cubic phase of iron in the Earth's inner core2003In: Nature, ISSN 0028-0836, Vol. 424, no 6952, 1032-1034 p.Article in journal (Refereed)
    Abstract [en]

    Iron is thought to be the main constituent of the Earth's core(1), and considerable efforts(2-14) have therefore been made to understand its properties at high pressure and temperature. While these efforts have expanded our knowledge of the iron phase diagram, there remain some significant inconsistencies, the most notable being the difference between the 'low' and 'high' melting curves(15). Here we report the results of molecular dynamics simulations of iron based on embedded atom models fitted to the results of two implementations of density functional theory. We tested two model approximations and found that both point to the stability of the body-centred-cubic (b.c.c.) iron phase at high temperature and pressure. Our calculated melting curve is in agreement with the 'high' melting curve, but our calculated phase boundary between the hexagonal close packed (h. c. p.) and b.c.c. iron phases is in good agreement with the 'low' melting curve. We suggest that the h.c.p.-b.c.c. transition was previously misinterpreted as a melting transition, similar to the case of xenon(16-18), and that the b.c.c. phase of iron is the stable phase in the Earth's inner core.

  • 12.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Arapan, Sergiu
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Martonak, R.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    MgO phase diagram from first principles in a wide pressure-temperature range2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 5, 054110-1-054110-9 p.Article in journal (Refereed)
    Abstract [en]

    Recent laser-initiated strong shockwave measurements at Livermore provide the opportunity for verification of the MgO phase diagram at extreme pressures and temperatures. This calls for a comprehensive study of the MgO phase diagram. The phase diagram is obtained by ab initio molecular dynamics (two phase and Z method) and phonon-based thermodynamic calculations. Energies and forces are computed from first principles projector augmented wave method. The B1 transforms to B2 phase at about 490 GPa. Melting temperatures of B1 are consistent with the two-phase melting curve (Alfe, 2005). The triple point B1-B2-liquid is located at about 2.4 Mbar and 9000 K. The melting curve of the B2 phase rises rather steeply from the triple point. The Hugoniot is likely to cross the B1-B2 boundary first and then to cross the melting curve of B2, therefore, the melting curve of periclase is not relevant for the Hugoniot. MgO melts between 11.3 and 12.5 thousand K and 4.3 and 5 Mbar along the Hugoniot from the B2 phase. The two-phase melting curves of B1 computed with various semiempirical potentials are remarkably close to each other and are consistent with the B1 first principles melting curve at low pressure. This suggests the MgO melting temperatures to be in the close proximity of this determination. The search for new phases of MgO by first principles metadynamics has not produced unknown phases. Therefore, the suggested discontinuity of the Hugoniot at 170 GPa and 3000 K remains enigmatic.

  • 13.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Bryk, T.
    Rosengren, Anders
    Shear Relaxation in Iron under the Conditions of Earth's Inner Core2010In: Physical Review Letters, ISSN 0031-9007, Vol. 104, no 24, 245703- p.Article in journal (Refereed)
    Abstract [en]

    Large scale molecular dynamics simulations of iron at high pressure and temperature are performed to investigate the physics of shear softening. A solid 16 x 10(6) atoms sample of iron is grown out of the liquid with a small solid immersed in it at the start of simulation. We observe that diffusion in the sheared solid is similar to that in liquid, even though at different time scales. This allows us to describe the time dependence of shear stress in terms of elastic and hydrodynamic relaxation. The elastic response of the sample is close to the elastic response of Earth's inner core. This explains the abnormally low shear modulus in the core. The reason for the low shear modulus is the presence of defects of the crystal structure.

  • 14.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Burakovsky, L.
    Chen, S. P.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Mikhaylushkin, A. S.
    Preston, D. L.
    Simak, S. I.
    Swift, D. C.
    Molybdenum at high pressure and temperature: Melting from another solid phase2008In: Physical Review Letters, ISSN 0031-9007, Vol. 100, no 13Article in journal (Refereed)
    Abstract [en]

    The Gibbs free energies of bcc and fcc Mo are calculated from first principles in the quasiharmonic approximation in the pressure range from 350 to 850 GPa at room temperatures up to 7500 K. It is found that Mo, stable in the bcc phase at low temperatures, has lower free energy in the fcc structure than in the bcc phase at elevated temperatures. Our density-functional-theory-based molecular dynamics simulations demonstrate that fcc melts at higher than bcc temperatures above 1.5 Mbar. Our calculated melting temperatures and bcc-fcc boundary are consistent with the Mo Hugoniot sound speed measurements. We find that melting occurs at temperatures significantly above the bcc-fcc boundary. This suggests an explanation of the recent diamond anvil cell experiments, which find a phase boundary in the vicinity of our extrapolated bcc-fcc boundary.

  • 15.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Derlet, P. M.
    Mikhaylushkin, A. S.
    Simak, S. I.
    Hellman, O.
    Burakovsky, L.
    Swift, D. C.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Quenching of bcc-Fe from high to room temperature at high-pressure conditions: a molecular dynamics simulation2009In: New Journal of Physics, ISSN 1367-2630, Vol. 11Article in journal (Refereed)
    Abstract [en]

    The new high-temperature (T), high-pressure (P), body-centered cubic (bcc) phase of iron has probably already been synthesized in recent diamond anvil cell (DAC) experiments (Mikhaylushkin et al 2007 Phys. Rev. Lett. 99 165505). These DAC experiments on iron revealed that the high-PT phase on quenching transforms into a mixture of close-packed phases. Our molecular dynamics simulation and structural analysis allow us to provide a probable interpretation of the experiments. We show that quenching of the high-PT bcc phase simulated with the embedded-atom model also leads to the formation of the mixture of close-packed phases. Therefore, the assumption of the stability of the high-PT bcc iron phase is consistent with experimental observation.

  • 16.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Dorogokupets, P. I.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Saxena, S. K.
    Koci, L.
    Ab initio equation of state for the body-centered-cubic phase of iron at high pressure and temperature2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 10Article in journal (Refereed)
    Abstract [en]

    The solid inner core of the Earth consists mostly of iron. There is accumulating evidence that, at the extreme pressures and temperatures of the deep Earth interior, iron stabilizes in the body-centered-cubic phase. However, experimental study of iron at those conditions is very difficult at best. We demonstrate that our ab initio approach is capable of providing volumetric data on iron in very good agreement with experiment at low temperature and high pressure. Since our approach treats high-temperature effects explicitly, this allows us to count on similar precision also at high temperature and high pressure. We perform ab initio molecular-dynamics simulations at a number of volume-temperature conditions and compute the corresponding pressures. These points are then fitted with an equation of state. A number of parameters are computed and compared with existing data. The obtained equation of state for high pressure and temperature nonmagnetic body-centered-cubic phase allows the computation of properties of iron under extreme conditions of the Earth's inner core.

  • 17.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Gutierrez, G.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Molecular dynamics simulation of the structure of yttria Y2O3 phases using pairwise interactions2001In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 6418, no 18Article in journal (Refereed)
    Abstract [en]

    We have studied the structure of yttria (Y2O3) by means of ab initio and molecular dynamics methods. The suggested simple model for the interatomic interaction is shown to produce reasonable results at moderate pressures for a wide range of temperatures. The calculated x-ray structure factor is in good agreement with experimental data obtained by the x-ray levitation technique at the temperature of 2526 K. The quality of the agreement decreases with increasing temperature. We demonstrate that it is not necessary to assume nonstoichiometry of liquid yttria, as was done in a recent publication, to obtain agreement with experiment. The structure of liquid yttria can be considered as a mixture of 4- and 6-coordinated Y atoms. We also show the possibility of a light amorphous yttria phase, which possibly can be obtained by quenching from a vapor instead of conventional amorphous yttria quenched from a liquid.

  • 18.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Isaev, E. I.
    Skorodumova, N. V.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Stability of the body-centered-tetragonal phase of Fe at high pressure: Ground-state energies, phonon spectra, and molecular dynamics simulations2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 21Article in journal (Refereed)
    Abstract [en]

    It is well established that at a pressure of several megabars and low temperature Fe is stable in the hexagonal-close-packed (hcp) phase. However, there are indications that on heating a high-pressure hcp phase of Fe transforms to a less dense (open structure) phase. Two phases have been suggested as candidates for these high-temperature stable phases: namely, body-centered-cubic and body-centered-tetragonal (bct) phases. We performed first-principles molecular dynamics and phonon analysis of the bct Fe phase and demonstrated its dynamical instability. This allows us to dismiss the existence of the bct Fe phase under the high-pressure high-temperature conditions of the Earth's inner core.

  • 19.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    LeBacq, O.
    Ahuja, Rajeev
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Molecular dynamics study of phase transitions in Xe2002In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 117, no 15, 7233-7244 p.Article in journal (Refereed)
    Abstract [en]

    A full account is given of our recent theoretical discovery [A. B. Belonoshko, R. Ahuja, and B. Johansson, Phys. Rev. Lett. 87, 165505 (2001)] of the fcc-bcc transition in Xe at high pressure and temperature. The interaction model and method for calculating phase boundaries are exhaustively tested by independent methods. The model was carefully checked against experimental data and results of ab initio molecular dynamics and it was found to perform very well. The two-phase method employed for finding the melting transition was compared with the robust thermodynamic approach and was found to provide data in exact agreement with the latter. The deviation of the calculated melting curve from the experimental one is quite tolerable at low pressures. After a reinterpretation of the experimental data, our results are also in good agreement with recent diamond anvil cell experiments. At a pressure of around 25 GPa and a temperature of about 2700 K, we find a triple fcc-bcc-liquid point. The fcc-bcc boundary is calculated without reference to the experimental data, in contrast to our previous work, and found to be in nice agreement with previous calculations as well as with the experimental data points, which, however, were interpreted as melting. Our finding concerning the fcc-bcc transition is confirmed by the direct molecular dynamics simulation of the fcc, bcc, and liquid phases in the same computational cell. In this simulation, it was observed that while the fcc phase melts, the bcc structure solidifies. Since Xe is a typical rare-gas solid, the fcc-bcc transition can now be expected for a number of other van der Waals systems, first of all in Ar and Kr. Our finding suggests, that the transition from close packed to bcc structure might be more common at high pressure and high temperature than was previously anticipated. The performed thorough test of methods and models in this study leads us to suggest that the original interpretation of experimental results is erroneous.

  • 20.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Li, S.
    Ahuja, R.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    High-pressure crystal structure studies of Fe, Ru and Os2004In: Journal of Physics and Chemistry of Solids, ISSN 0022-3697, E-ISSN 1879-2553, Vol. 65, no 09-aug, 1565-1571 p.Article in journal (Refereed)
    Abstract [en]

    In order to reveal structural trends with increasing pressure in d transition metals, we performed full potential linear muffin-tin orbital calculations for Fe, Ru, and Os in the hexagonal close packed structure. The calculations cover a wide volume range and demonstrate that all these hexagonal close-packed metals have non-ideal c/a at low pressures which, however, increases with pressure and asymptotically approaches the ideal value at very high compressions. These results are in accordance with most recent experiment for Ru and Os. The experimental data for iron is not conclusive, but it is believed that the c/a ratio decreases weakly with increasing pressure at moderate compression. Since, the experimental and calculated equations of state for iron are in increasingly good agreement with increasing pressure, it is possible that either the negative c/a trend is valid only for a restricted pressure range, or related to the experimental difficulties (e.g. non-hydrostaticity).

  • 21.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Lukinov, Timofei
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Burakovsky, Leonid
    Preston, Dean L.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Melting of a polycrystalline material2013In: The European Physical Journal Special Topics, ISSN 1951-6355, Vol. 216, no 1, 199-204 p.Article in journal (Refereed)
    Abstract [en]

    Calculating the melting temperature of a solid with a known model of interaction between atoms is nowadays a comparatively simple task. However, when one simulates a single crystal by molecular dynamics method, it does not normally melt at the melting temperature. Instead, one has to significantly overheat it. Yet, a real material melts at the melting point. Here we investigate the impact of the defects and the grain boundaries on melting. We demonstrate that defects and grain boundaries have similar impact and make it possible to simulate melting in close vicinity of thermodynamic melting temperature. We also show that the Z method might be non-applicable in discriminating a stable submelting phase.

  • 22.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Lukinov, Timofei
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Fu, Jie
    KTH, School of Engineering Sciences (SCI), Physics, Condensed Matter Theory.
    Zhao, Jijun
    Davis, Sergio
    Simak, Sergei I.
    Stabilization of body-centred cubic iron under inner-core conditions2017In: Nature Geoscience, ISSN 1752-0894, E-ISSN 1752-0908, Vol. 10, no 4, 312-+ p.Article in journal (Refereed)
    Abstract [en]

    The Earth's solid core is mostly composed of iron. However, despite being central to our understanding of core properties, the stable phase of iron under inner-core conditions remains uncertain. The two leading candidates are hexagonal close-packed and body-centred cubic (bcc) crystal structures, but the dynamic and thermodynamic stability of bcc iron under inner-core conditions has been challenged. Here we demonstrate the stability of the bcc phase of iron under conditions consistent with the centre of the core using ab initio molecular dynamics simulations. We find that the bcc phase is stabilized at high temperatures by a diffusion mechanism that arises due to the dynamical instability of the phase at lower temperatures. On the basis of our simulations, we reinterpret experimental data as support for the stability of bcc iron under inner-core conditions. We suggest that the diffusion of iron atoms in solid state may explain both the anisotropy and the low shear modulus of the inner core.

  • 23.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Lukinov, Timofiy
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory. KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. AlbaNova University Center, Sweden.
    Bryk, Taras
    Litasov, Konstantin D.
    Synthesis of heavy hydrocarbons at the core-mantle boundary2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, 18382Article in journal (Refereed)
    Abstract [en]

    The synthesis of complex organic molecules with C-C bonds is possible under conditions of reduced activity of oxygen. We have found performing ab initio molecular dynamics simulations of the C-O-H- Fe system that such conditions exist at the core-mantle boundary (CMB). H2O and CO2 delivered to the CMB by subducting slabs provide a source for hydrogen and carbon. The mixture of H2O and CO2 subjected to high pressure (130 GPa) and temperature (4000 to 4500 K) does not lead to synthesis of complex hydrocarbons. However, when Fe is added to the system, C-C bonds emerge. It means that oil might be a more abundant mineral than previously thought.

  • 24.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Ramzan, Muhammad
    Mao, Ho-kwang
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Atomic Diffusion in Solid Molecular Hydrogen2013In: Scientific Reports, ISSN 2045-2322, Vol. 3, 2340- p.Article in journal (Refereed)
    Abstract [en]

    We performed ab initio molecular dynamics simulations of the C2c and Cmca-12 phases of hydrogen at pressures from 210 to 350 GPa. These phases were predicted to be stable at 0 K and pressures above 200 GPa. However, systematic studies of temperature impact on properties of these phases have not been performed so far. Filling this gap, we observed that on temperature increase diffusion sets in the Cmca-12 phase, being absent in C2c. We explored the mechanism of diffusion and computed melting curve of hydrogen at extreme pressures. The results suggest that the recent experiments claiming conductive hydrogen at the pressure around 260 GPa and ambient temperature might be explained by the diffusion. The diffusion might also be the reason for the difference in Raman spectra obtained in recent experiments.

  • 25.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    A possible mechanism of copper corrosion in anoxic water2012In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 92, no 36, 4618-4627 p.Article in journal (Refereed)
    Abstract [en]

    Recent experiments show that solid Cu reacts with anoxic water. The reaction is observed by measuring the hydrogen release. This release is continuous and stable over a period of months. We have since theoretically found that water adsorbs dissociatively at a copper surface. But this adsorption is not enough to explain the amount of hydrogen released in the experiment. This observation calls for the explanation of the removal of the reaction product from the surface to provide a clean Cu surface where the water dissociation takes place. In this paper we investigate, by first-principles calculations, two possible mechanisms for this removal: first the possibility of Cu-O-H nanoparticulate formation, and second the diffusion of the dissociation products into Cu. We show that while the formation of nanoparticulates is energetically unfavorable, the diffusion of OH along grain boundaries can be substantial. The OH being placed in a grain boundary of the Cu sample quickly dissociates and O and H atoms diffuse independently of each other. Such a diffusion is markedly larger than the diffusion in bulk Cu. Thus, grain boundary diffusion is a viable mechanism for providing a clean Cu surface for the dissociation of water at the Cu surface. An order-of-magnitude estimate of the amount of hydrogen released in this case agrees with experiment. But this mechanism is not enough to explain the result of the experiment. We propose the formation of nanocrystals of copper oxide as a second step. A decisive experiment is proposed. 

  • 26.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    High-pressure melting curve of platinum from ab initio Z method2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 17, 174104- p.Article in journal (Refereed)
    Abstract [en]

    Pt is widely used as a standard in high-pressure high-temperature experiments. The available experimental and theoretical data on Pt thermal stability is not consistent. We address the issue of high-pressure Pt melting by ab initio molecular dynamics. We demonstrate a remarkable consistency of our computed melting curve with the experimental data by N. R. Mitra, D. L. Decker, and H. B. Vanfleet [Phys. Rev. 161, 613 (1967)]. The extrapolation of their data, based on the Simon equation, nearly coincides with our ab initio computed melting curve. We propose the Pt melting curve in the form P-m(kbar) = 443.0[(T/T-m)(1.14) - 1].

  • 27.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory. KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Burakovsky, Leonid
    Preston, Dean L.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Melting of Fe and Fe0.9375Si0.0625 at Earth's core pressures studied using ab initio molecular dynamics2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 22Article in journal (Refereed)
    Abstract [en]

    The issue of melting of pure iron and iron alloyed with lighter elements at high pressure is critical to the physics of the Earth. The iron melting curve in the relevant pressure range between 3 and 4 Mbar is reasonably well established from the theoretical point of view. However, so far no one attempted a direct atomistic simulation of iron alloyed with light elements. We investigate here the impact of alloying the body-centered cubic (bcc) Fe with Si. We simulate melting of the bcc Fe and Fe0.9375Si0.0625 alloy by ab initio molecular dynamics. The addition of light elements to the hexagonal-close-packed (hcp) iron is known to depress its melting temperature (T-m). We obtain, in marked contrast, that alloying of bcc Fe with Si does not lead to T-m depression; on the contrary, the T-m slightly increases. This suggests that if Si is a typical impurity in the Earth's inner core, then the stable phase in the core is bcc rather than hcp.

  • 28.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Dong, Qian
    KTH, Superseded Departments, Materials Science and Engineering.
    Hultquist, Gunnar
    KTH, Superseded Departments, Materials Science and Engineering.
    Leygraf, Christofer
    KTH, Superseded Departments, Materials Science and Engineering.
    First-principles study of hydrogen diffusion in alpha-Al2O3 and liquid alumina2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 69, no 2Article in journal (Refereed)
    Abstract [en]

    We have studied the energetics and mobility of neutral hydrogen in alumina Al2O3 using ab initio density-functional calculations. The mobility of hydrogen was studied in corundum (alpha-Al2O3) as well as in liquid alumina. Using both static as well as molecular-dynamics calculations, and applying classical transition state theory, we derive the temperature-dependent diffusivity of hydrogen in alpha-Al2O3 as D(T)=(21.7x10(-8) m(2)/s)exp(-1.24 eV/kT). The corresponding diffusivity of hydrogen in liquid/amorphous alumina, derived directly from ab initio molecular dynamics calculations, is D(T)=(8.71x10(-7) m(2)/s)exp(-0.91 eV/kT). The computed diffusivity compares very well to experimental data. We conclude that diffusion of neutral hydrogen through the bulk of alumina is a good approximation of the mechanism for hydrogen mobility in corrosion scales. The representation of grain-boundary structures by amorphous alumina is, probably, realistic at higher temperatures.

  • 29.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Rosengren, Anders
    KTH, Superseded Departments, Physics.
    Dong, Qian
    KTH, Superseded Departments, Materials Science and Engineering.
    Hultquist, Gunnar
    KTH, Superseded Departments, Materials Science and Engineering.
    Leygraf, Christofer
    KTH, Superseded Departments, Materials Science and Engineering.
    First-principles study of hydrogen diffusion in α-Al 2O3 and liquid alumina2004In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 69, no 2, 243021-243026 p.Article in journal (Refereed)
    Abstract [en]

    We have studied the energetics and mobility of neutral hydrogen in alumina Al2O3 using ab initio density-functional calculations. The mobility of hydrogen was studied in corundum (α-Al2O 3) as well as in liquid alumina. Using both static as well as molecular-dynamics calculations, and applying classical transition state theory, we derive the temperature-dependent diffusivity of hydrogen in α-Al 2O3 as D(T)=(21.7 × 10-8 m 2/s)exp(-1.24 eV/kT). The corresponding diffusivity of hydrogen in liquid/amorphous alumina, derived directly from ab initio molecular dynamics calculations, is D(T)=(8.71 × 10-7 m2/s)exp(-0.91 eV/kT). The computed diffusivity compares very well to experimental data. We conclude that diffusion of neutral hydrogen through the bulk of alumina is a good approximation of the mechanism for hydrogen mobility in corrosion scales. The representation of grain-boundary structures by amorphous alumina is, probably, realistic at higher temperatures.

  • 30.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Hultquist, Gunnar
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Corrosion Science.
    Thermal regimes of passivative oxide film formation on Al surface: Theoretical and experimental study2006In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 600, no 20, 4796-4800 p.Article in journal (Refereed)
    Abstract [en]

    We report results of ab initio molecular dynamics simulations of an Al surface exposed to an oxygen atmosphere. The results, supported by experiments performed in this study, demonstrate that the Al surface, by reacting with the oxygen molecules, can be heated above melting temperature and transformed into a liquid. This process is potentially capable of creating an amorphous corrosion scale which might possess an enhanced resistance to deterioration.

  • 31.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Skorodumova, N. V.
    Bastea, S.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Shock wave propagation in dissociating low-Z liquids: D-22005In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 122, no 12Article in journal (Refereed)
    Abstract [en]

    We present direct molecular dynamics simulations of shock wave propagation in liquid deuterium for a wide range of impact velocities. The calculated Hugoniot is in perfect agreement with the gas-gun data as well as with the most recent experimental data. At high impact velocities we observe a smearing of the shock wave front and propagation of fast dissociated molecules well ahead of the compressed region. This smearing occurs due to the fast deuterium dissociation at the shock wave front. The experimental results are discussed in view of this effect.

  • 32.
    Belonoshko, Anatoly B.
    et al.
    KTH, Superseded Departments, Physics.
    Simak, S. I.
    Kochetov, A. E.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Burakovsky, L.
    Preston, D. L.
    High-pressure melting of molybdenum2004In: Physical Review Letters, ISSN 0031-9007, Vol. 92, no 19Article in journal (Refereed)
    Abstract [en]

    The melting curve of the body-centered cubic (bcc) phase of Mo has been determined for a wide pressure range using both direct ab initio molecular dynamics simulations of melting as well as a phenomenological theory of melting. These two methods show very good agreement. The simulations are based on density functional theory within the generalized gradient approximation. Our calculated equation of state of bcc Mo is in excellent agreement with experimental data. However, our melting curve is substantially higher than the one determined in diamond anvil cell experiments up to a pressure of 100 GPa. An explanation is suggested for this discrepancy.

  • 33.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Skorodumova, N. V.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    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.
    Burakovsky, L.
    Preston, D. L.
    High-pressure melting of MgSiO32005In: Physical Review Letters, ISSN 0031-9007, Vol. 94, no 19Article in journal (Refereed)
    Abstract [en]

    The melting curve of MgSiO3 perovskite has been determined by means of ab initio molecular dynamics complemented by effective pair potentials, and a new phenomenological model of melting. Using first principles ground state calculations, we find that the MgSiO3 perovskite phase transforms into post perovskite at pressures above 100 GPa, in agreement with recent theoretical and experimental studies. We find that the melting curve of MgSiO3, being very steep at pressures below 60 GPa, rapidly flattens on increasing pressure. The experimental controversy on the melting of the MgSiO3 perovskite at high pressures is resolved, confirming the data by Zerr and Boehler.

  • 34.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Skorodumova, N. V.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Melting and critical superheating2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 1Article in journal (Refereed)
    Abstract [en]

    Two mechanisms of melting are known, heterogeneous, where melting starts at surfaces, and homogeneous, where the liquid nucleates in the bulk crystal. If melting occurs homogeneously, a crystal can be superheated significantly above its melting temperature (T-m). At present, the physical meaning of the limit of superheating (T-LS) is unknown. We demonstrate, by molecular dynamics simulations, that the total energy of a solid at T-LS is equal to the total energy of its liquid at T-m at the same volume. In the high pressure limit T-LS and T-m are connected by the constant k(AB)=ln 2/3 via the relation k(AB)=T-LS/T-m-1.

  • 35.
    Belonoshko, Anatoly B.
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Skorodumova, Natalia V.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic anisotropy of Earth's inner core2008In: Science, ISSN 0036-8075, Vol. 319, no 5864, 797-800 p.Article in journal (Refereed)
    Abstract [en]

    Earth's solid- iron inner core is elastically anisotropic. Sound waves propagate faster along Earth's spin axis than in the equatorial plane. This anisotropy has previously been explained by a preferred orientation of the iron alloy hexagonal crystals. However, hexagonal iron becomes increasingly isotropic on increasing temperature at pressures of the inner core and is therefore unlikely to cause the anisotropy. An alternative explanation, supported by diamond anvil cell experiments, is that iron adopts a body- centered cubic form in the inner core. We show, by molecular dynamics simulations, that the body- centered cubic iron phase is extremely anisotropic to sound waves despite its high symmetry. Direct simulations of seismic wave propagation reveal an anisotropy of 12%, a value adequate to explain the anisotropy of the inner core.

  • 36.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Davis, Sergio
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Ahuja, Rajeev
    Department of Physics, Uppsala University.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Simak, Sergei
    Burakovsky, Leonid
    Preston, D. L.
    Xenon melting: Density functional theory versus diamond anvil cell experiments2006In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 74, 054114- p.Article in journal (Refereed)
    Abstract [en]

    We performed two-phase ab initio density functional theory based molecular dynamics simulations of Xe melting and demonstrated that, contrary to claims in the recent literature, the pressure dependence of the Xe melting curve is consistent with the corresponding-states theory as well as with the melting curve obtained earlier from classical molecular dynamics with a Xe pair potential. While at low pressure the calculated melting curve is in perfect agreement with reliable experiments, our calculated melting temperatures at higher pressures are inconsistent with those from the most recent diamond anvil cell experiment. We discuss a possible explanation for this inconsistency.

  • 37.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Davis, Sergio
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Skorodumova, Natalia
    Department of Physics, Uppsala University.
    Lundow, Per-Håkan
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Properties of the fcc Lennard-Jones crystal model at the limit of superheating2007In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 76, 064121- p.Article in journal (Refereed)
    Abstract [en]

    The face-centered-cubic (fcc) Lennard-Jones (LJ) model can be considered as a representative model of a simple solid. We investigate the mechanism of melting at the limit of superheating in the fcc LJ solid by means of the procedure recently developed by us [Phys. Rev. B 73, 012201 (2006)]. Insight into the mechanism of melting was gained by studying diffusion and defects in the fcc LJ solid by means of molecular dynamics simulations. We found that the limit of superheating achieved by us is likely to be the highest so far. We also found that the size of the cluster which ignites the melting is very small (down to five to six atoms, depending on the size of the supercell) and closely correlates with the linear size of a supercell when the number of atoms varies between 500 and 13 500.

  • 38.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Koči, L.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Stability of the bcc phase of 4He close to the melting curve: A molecular dynamics study2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 1, 012503- p.Article in journal (Refereed)
    Abstract [en]

    We have investigated whether the Aziz et al. [J. Chem. Phys. 70, 4330 (1979)] model for (4)He renders the body-centered cubic phase more stable than the face-centered cubic phase in the proximity of the melting curve. Using molecular dynamics, we have simulated these solid phases in equilibrium with the liquid at a number of densities. In contrast to previous free energy molecular dynamics calculations, the model stabilizes the body-centered cubic phase. The stability field is just 5 degrees. wide below the melting curve at pressures around 140 Kbar and about 70 degrees wide at pressures around 750 Kbar. Considering that the body-centered cubic phase is dynamically unstable at low temperature, this result bears striking similarities to transition metal phase diagrams.

  • 39.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Lukinov, Tymofiy
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Zhao, Jijun
    Dalian University of Technology, China.
    Fu, Jie
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Davis, Sergio
    Simak, Sergei
    Mechanism of the body-centered cubic iron stabilization under the Earth core conditionsManuscript (preprint) (Other academic)
  • 40.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Iron shear modulus in the Earth's inner core2010In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 0016-1258, Vol. 74, no 12, A75-A75 p.Article in journal (Other academic)
  • 41.
    Belonoshko, Anatoly
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Skorodumova, Natalia
    Department of Physics, Uppsala University.
    Davis, Sergio
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Osiptsov, Alexander
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Origin of the Low Rigidity of the Earth's Inner Core2007In: Science, ISSN 0036-8075, Vol. 316, 1603- p.Article in journal (Refereed)
    Abstract [en]

    Earth's solid-iron inner core has a low rigidity that manifests itself in the anomalously low velocities of shear waves as compared to shear wave velocities measured in iron alloys. Normally, when estimating the elastic properties of a polycrystal, one calculates an average over different orientations of a single crystal. This approach does not take into account the grain boundaries and defects that are likely to be abundant at high temperatures relevant for the inner core conditions. By using molecular dynamics simulations, we show that, if defects are considered, the calculated shear modulus and shear wave velocity decrease dramatically as compared to those estimates obtained from the averaged single-crystal values. Thus, the low shear wave velocity in the inner core is explained.

  • 42. Benazzouz, Brahim K.
    et al.
    Zaoui, Ali
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Determination of the melting temperature of kaolinite by means of the Z-method2013In: American Mineralogist, ISSN 0003-004X, Vol. 98, no 10, 1881-1885 p.Article in journal (Refereed)
    Abstract [en]

    The melting temperature of materials is an important thermodynamic property. Despite the importance of kaolinite, one of the most common clay minerals on the Earth's surface, its thermal and melting behavior is poorly understood. We apply here the Z-method to determine the melting temperature (T-m) and the limit of superheating (T-LS) of kaolinite. The T-m is found at 1818 K (8.85 GPa), and T-LS at 1971 K (6.8 GPa). The diffusion coefficient for all atoms has been calculated in a broad temperature range. The calculated characteristics and, in particular, their dependence on temperature have confirmed the solid-liquid transition and strongly support the calculated melting point. In addition, some computed quantities, such as the radial distribution function, coordination numbers and mean-square displacement, were used to confirm the liquid state of kaolinite from the melting temperature as well as at other temperatures in the liquid branch. The diffusion coefficient for different atoms has been calculated throughout the isochore. These quantities and in particular their evolution under temperature have confirmed the solid-liquid states of kaolinite and the presence of the melting point. The latter quantity constitutes the first ever melting simulation of a clay mineral with close agreement to the experimental one.

  • 43. Bryk, T.
    et al.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Collective excitations in molten iron above the melting point: A generalized collective-mode analysis of simulations with embedded-atom potentials2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 2, 024202- p.Article in journal (Refereed)
    Abstract [en]

    It is shown, that the embedded-atom potential nicely describing structural properties of high pressure Fe can be successfully used for description of collective dynamics of liquid iron. A combination of molecular dynamics simulations and a fit-free analysis based on the approach of generalized collective modes (GCM) is used for calculations of spectra of collective excitations and relaxing modes at 1843 K. The obtained spectrum of acoustic excitations in the long-wavelength region perfectly agrees with the experimental speed of sound and reproduces the dispersion estimated from inelastic X-ray scattering (IXS) experiments. Heat fluctuations in liquid Fe were studied and resulted in calculated ratio of specific heats γ-1.40 being in agreement with the IXS-experiment estimate. We report analysis of the wave-number dependence of relaxation processes and their contributions to dynamic structure factors. This permits estimation of most important relaxation processes contributing to the shape of dynamic structure factors of liquid Fe in different regions of wave numbers.

  • 44. Burakovsky, L.
    et al.
    Chen, S. P.
    Preston, D. L.
    Belonoshko, Anatoly
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Mikhaylushkin, A. S.
    Simak, S. I.
    Moriarty, J. A.
    High-Pressure-High-Temperature Polymorphism in Ta: Resolving an Ongoing Experimental Controversy2010In: Physical Review Letters, ISSN 0031-9007, Vol. 104, no 25, 255702- p.Article in journal (Refereed)
    Abstract [en]

    Phase diagrams of refractory metals remain essentially unknown. Moreover, there is an ongoing controversy over the high-pressure melting temperatures of these metals: results of diamond anvil cell (DAC) and shock wave experiments differ by at least a factor of 2. From an extensive ab initio study on tantalum we discovered that the body-centered cubic phase, its physical phase at ambient conditions, transforms to another solid phase, possibly hexagonal omega phase, at high temperature. Hence the sample motion observed in DAC experiments is very likely not due to melting but internal stresses accompanying a solid-solid transformation, and thermal stresses associated with laser heating.

  • 45. Cricchio, F.
    et al.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Burakovsky, L.
    Preston, D. L.
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    High-pressure melting of lead2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 14Article in journal (Refereed)
    Abstract [en]

    The melting curve of the hexagonal close-packed (hcp) phase of lead (Pb) has been determined over a wide pressure range using both ab initio molecular dynamics (AIMD) simulations and classical molecular dynamics (CMD) employing an effective pair potential. The AIMD simulations are based on a density functional theory (DFT) in the generalized gradient approximation (GGA). The Pb melting curve, constructed using a well-established theoretical scheme, is in excellent agreement with the AIMD results. Our calculated equation of state (EOS) of hcp Pb is in excellent agreement with experimental data up to 40 GPa. Our melting curve agrees very well with melting temperatures obtained in both shock-wave and diamond-anvil cell (DAC) experiments, but at higher pressures our curve lies between the two data sets.

  • 46.
    Davis, Sergio
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Belonoshko, Anatoly B.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Model for diffusion at the microcanonical superheating limit from atomistic computer simulations2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 6, 064102- p.Article in journal (Refereed)
    Abstract [en]

    The diffusion statistics of atoms in a crystal close to the critical superheating temperature was studied in detail using molecular dynamics and Monte Carlo simulations. We present a continuous random-walk model for diffusion of atoms hopping through thermal vacancies. The results obtained from our model suggest that the limit of superheating is precisely the temperature for which dynamic percolation happens at the time scale of a single individual jump. A possible connection between the critical superheating limit and the maximization of the Shannon entropy associated with the distribution of jumps is suggested. As a practical application of our results, we show that an extrapolation of the critical superheating temperature (and therefore an estimation of the melting point) can be performed using only the dynamical properties of the solid state.

  • 47.
    Davis, Sergio
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Belonoshko, Anatoly
    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.
    SearchFill: A stochastic optimization code for detecting atomic vacancies in crystalline and non-crystalline systems2011In: Computer Physics Communications, ISSN 0010-4655, E-ISSN 1879-2944, Vol. 182, no 5, 1105-1110 p.Article in journal (Refereed)
    Abstract [en]

    We present an implementation of a stochastic optimization algorithm applied to location of atomic vacancies. Our method labels an empty point in space as a vacancy site, if the total spatial overlap of a "virtual sphere", centered around the point, with the surrounding atoms (and other vacancies) falls below a tolerance parameter. A Metropolis-like algorithm displaces the vacancies randomly, using an "overlap temperature" parameter to allow for acceptance of moves into regions with higher overlap, thus avoiding local minima. Once the algorithm has targeted a point with low overlap, the overlap temperature is decreased, and the method works as a steepest descent optimization.

    Our method, with only two free parameters, is able to detect the correct number and coordinates of vacancies in a wide spectrum of condensed-matter systems, from crystals to amorphous solids, in fact in any given set of atomic coordinates, without any need of comparison with a reference initial structure.

  • 48.
    Davis, Sergio
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Belonoshko, Anatoly
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    An atomistic model for homogeneous meltingManuscript (preprint) (Other academic)
    Abstract [en]

    The diffusion statistics of atoms in a crystal close to the critical superheating temperature wasstudied in detail using molecular dynamics (MD) and Monte Carlo (MC) simulations. We present ageneral dynamic percolation model for diffusion of atoms hopping through thermal vacancies. Theresults obtained from our model suggest that the limit of superheating is precisely the temperaturefor which dynamic percolation happens at the time scale of a single individual jump. We show thatthis prediction of the critical superheating temperature can give an estimate of the melting pointusing only the dynamical properties of the solid state.

  • 49.
    Davis, Sergio
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Belonoshko, Anatoly
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Rosengren, Anders
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    van Duin, Adri
    Johansson, Börje
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Molecular dynamics simulation of zirconia melting2010In: Central European Journal of Physics, ISSN 1644-3608, Vol. 8, no 5, 789-797 p.Article in journal (Refereed)
    Abstract [en]

    The melting point for the tetragonal and cubic phases of zirconia (ZrO2) was computed using Z-method microcanonical molecular dynamics simulations for two different interaction models: the empirical Lewis-Catlow potential versus the relatively new reactive force field (ReaxFF) model. While both models reproduce the stability of the cubic phase over the tetragonal phase at high temperatures, ReaxFF also gives approximately the correct melting point, around 2900 K, whereas the Lewis-Catlow estimate is above 6000 K.

  • 50.
    Davis, Sergio
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Belonoshko, Anatoly
    KTH, Superseded Departments, Physics.
    Skorodumova, Natalia
    Department of Physics, Uppsala University.
    Johansson, Börje
    KTH, Superseded Departments, Materials Science and Engineering.
    Relative stability of phases of iron in the Earth's inner coreManuscript (preprint) (Other academic)
    Abstract [en]

    We have investigated the phase diagram of iron in a range close to the conditions inside the Earth’sinner core, using molecular dynamics with a semi–empirical, embedded atom interatomic potential.Our calculated melting curves for the body centered cubic (bcc) and hexagonal close packed (hcp)phases of iron are in good agreement with experimental and first–principles data. Interestingly ourmodel suggests evidence of a crossover in relative stability of these crystalline phases: the moststable phase being bcc below 315 GPa, and hcp above that point. The dependence of structuraland dynamical properties of crystallized iron on the temperature, at the pressure of the inner core,were determined.

12 1 - 50 of 75
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