Pressure dependent electronic properties of MgO polymorphs: A first-principles study of Compton profiles and autocorrelation functions

The first-principles periodic linear combination of atomic orbitals method within the framework of density functional theory implemented in the CRYSTAL06 code has been applied to explore effect of pressure on the Compton profiles and autocorrelation functions of MgO. Calculations are performed for the B1, B2, B3, B4, B8_1 and h-MgO polymorphs of MgO to compute lattice constants and bulk moduli. The isothermal enthalpy calculations predict that B4 to B8_1, h-MgO to B8_1, B3 to B2, B4 to B2 and h-MgO to B2 transitions take place at 2, 9, 37, 42 and 64 GPa respectively. The high pressure transitions B8_1 to B2 and B1 to B2 are found to occur at 340 and 410 GPa respectively. The pressure dependent changes are observed largely in the valence electrons Compton profiles whereas core profiles are almost independent of the pressure in all MgO polymorphs. Increase in pressure results in broadening of the valence Compton profiles. The principal maxima in the second derivative of Compton profiles shifts towards high momentum side in all structures. Reorganization of momentum density in the B1 to B2 structural phase transition is seen in the first and second derivatives before and after the transition pressure. Features of the autocorrelation functions shift towards lower r side with increment in pressure.Comment: 19 pages, 8 figures, accepted for publication in Journal of Materials Scienc

Simulation of thermal conductivity and heat transport in solids

Using molecular dynamics (MD) with classical interaction potentials we present calculations of thermal conductivity and heat transport in crystals and glasses. Inducing shock waves and heat pulses into the systems we study the spreading of energy and temperature over the configurations. Phonon decay is investigated by exciting single modes in the structures and monitoring the time evolution of the amplitude using MD in a microcanonical ensemble. As examples, crystalline and amorphous modifications of Selenium and $\rm{SiO_2}$ are considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in PR

Direct Observation of Martensitic Phase-Transformation Dynamics in Iron by 4D Single-Pulse Electron Microscopy

The in situ martensitic phase transformation of iron, a complex solid-state transition involving collective atomic displacement and interface movement, is studied in real time by means of four-dimensional (4D) electron microscopy. The iron nanofilm specimen is heated at a maximum rate of ∼10^(11) K/s by a single heating pulse, and the evolution of the phase transformation from body-centered cubic to face-centered cubic crystal structure is followed by means of single-pulse, selected-area diffraction and real-space imaging. Two distinct components are revealed in the evolution of the crystal structure. The first, on the nanosecond time scale, is a direct martensitic transformation, which proceeds in regions heated into the temperature range of stability of the fcc phase, 1185−1667 K. The second, on the microsecond time scale, represents an indirect process for the hottest central zone of laser heating, where the temperature is initially above 1667 K and cooling is the rate-determining step. The mechanism of the direct transformation involves two steps, that of (barrier-crossing) nucleation on the reported nanosecond time scale, followed by a rapid grain growth typically in ∼100 ps for 10 nm crystallites

Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press

Sound velocity in shock compressed molybdenum obtained by ab initio molecular dynamics

The sound velocity of Mo along the Hugoniot adiabat is calculated from first principles using density-functional theory based molecular dynamics. These data are compared to the sound velocity as measured in recent experiments. The theoretical and experimental Hugoniot and sound velocities are in very good agreement up to pressures of 210 GPa and temperatures of 3700 K on the Hugoniot. However, above that point the experiment and theory diverge. This implies that Mo undergoes a phase transition at about the same point. Considering that the melting point of Mo is likely much higher at that pressure, the related change in the sound velocity in experiment can be ascribed to a solid-solid transition.Funding Agencies|Swedish Research Council (VR) [2013-5767, 2014-4750]</p

Molecular dynamics study of phase transitions in Xe

International audienc

New Data on the Stability of bcc Iron in the Earth Inner Core

International audienceA comparison between recent experimental [1] and theoretical [2] data on melting of Xe at high pressure has demonstrated that the close-packed structure to bcc structure transition might be misinterpreted as a melting transition. This suggested that the so-called "low" iron melting curve [3], measured using the same experimental technique as in the case of Xe, might also represent a solid-solid temperature induced transition. We studied this possibility using molecular dynamics employing first principles, pairwise and EAM models of interatomic interactions, as well as the first principles full potential linear muffin tin orbitals method. The results demonstrate that the bcc high-TP phase is at least dynamically stable. The stabilization occurs due to effective mixing of first and second neighbours in the bcc crystal. The iron atoms become effectively 14-coordinated, which removes the dynamic instability of bcc iron at low temperature and high pressure. This identification of a hcp-bcc transition brings all the data on iron, both theoretical [4-6] and experimental [3,7,8], into total agreement. It is thus possible that the Earth inner core is not hcp, but rather bcc iron. [1] R. Boehler et al., Phys. Rev. Lett. 86, 5731(2001). [2] A. B. Belonoshko, R. Ahuja and B. Johansson, Phys. Rev. Lett. 87,(2001). [3] R. Boehler, Nature 363, 534(1993). [4] D. Alfe, M. J. Gillan and G. D. Price, Nature 401, 462(1999). [5] A. Laio et al., Science 287, 1027(2000). [6] A. B. Belonoshko, R. Ahuja and B. Johansson, Phys. Rev. Lett. 84, 3638(2000). [7] J. M. Brown and R. G. McQueen, J. Geophys. Res. 91, 7485(1986]. [8] Q. R. Williams et al., Science 236, 181(1987)