Xiang-Rong Chen

First-principles calculations of elastic and electronic properties of NbB 2 under pressure

The structural parameters, elastic constants and electronic structure of NbB(2) under pressure are investigated by using first-principles plane-wave pseudopotential density functional theory within the generalized gradient approximation (GGA). The obtained results are in agreement with the available theoretical data. It is found that the elastic constants and the Debye temperature of NbB(2) increase monotonically and the anisotropies weaken with pressure. The band structure and density of states (DOS) of NbB(2) under pressure are also presented. It is the σ hole that determines the superconductivity in NbB(2), and the features of the σ bands are unchanged after applying pressure except for a shift of position. The density of states (DOS) atxa0the Fermi level decreases with increasing pressure, in conjunction with Bardeen-Cooper-Schrieffer (BCS) theory, which can predict T(c) decreasing with pressure, in agreement with the trend of the theoretical T(c) versus pressure.

Phase transition and elastic constants of zirconium from first-principles calculations

Using the projector augmented wave (PAW) within the Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA), we investigate the effect of hydrostatic pressure on the structures of zirconium metal at zero temperature. We obtain the [Formula: see text] transition at around 26.8xa0GPa, which is in excellent agreement with the experimental values. Wexa0also find that the ω phase is most stable at 0xa0K and 0xa0GPa. This conclusion is supported by first-principles calculations of Schell et al and Jona et al. The elastic constants of ω-Zr under high pressures are calculated for the first time. We find that the compressional and shear wave velocities increase monotonically with increasing pressure and the results are in good agreement with the available experimental data. The pressure dependences of three anisotropies of elastic waves are also presented.

Ab initio refinement of the thermal equation of state for bcc tantalum: the effect of bonding on anharmonicity

We report a detailed ab initio study for body-centered-cubic (bcc) Ta within the framework of the quasiharmonic approximation (QHA) to refine its thermal equation of state and thermodynamic properties. Based on the excellent agreement of our calculated phonon dispersion curve with experiment, the accurate thermal equations of state and thermodynamic properties are well reproduced. The thermal equation of state (EOS) and EOS parameters are considerably improved in our work compared with previous results by others. Furthermore, at high temperatures, the excellent agreement of our obtained thermal expansion and Hugoniot curves with experiments greatly verifies the validity of the quasiharmonic approximation at higher temperatures. It is known that pressure suppresses the vibrations of atoms from their equilibrium positions, i.e.xa0the bondings among atoms are strengthened by pressure; for the same temperature, anharmonicity becomes less important at high pressure. Thus the highest valid temperature of the QHA can be reasonably extended to the larger range.

The pressure-induced phase transition of the solid -HMX

The structural, vibrational and thermodynamic properties of β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β–HMX) crystal have been studied using the isothermal-isobaric molecular dynamics (NPT-MD) simulations. The variations of cell volume, lattice constants and molecular geometry of solid β–HMX are presented and discussed at different pressure and temperature. It was found that the N–N bond is significantly lengthened with increasing temperature, which suggests that it is relevant to the initial decomposition. An abrupt change at 27 Gpa for the volume and internal geometrical parameters was observed. This is in good accord with the experimental observation that there is a phase transition at 27 GPa, which is clearly due to conformational change, not chemical reaction. The vibrational frequencies at ambient conditions agree well with experimental results, and the pressure/temperature-induced frequency shifts of these modes are discussed. Frequency discontinuity was also observed at pressure when the phase transition occurred. The Grüneisen parameter was obtained using the vibrational frequency.

Magnetism and phase transitions of iron under pressure

The spin-polarized generalized gradient approximation within the plane-wave pseudopotential density functional theory is employed to investigate the magnetism and phase transition of iron under pressure. It is found that iron has a ferromagnetic body-centered-cubic (bcc) ground state, while at high pressure (such as at the Earths lower mantle and core pressure), the most stable phase is the nonmagnetic hexagonal-close-packed (hcp) phase. For the face-centered-cubic (fcc) iron, we find that there is an intermediate-spin state (IS) during the transformation from the high-spin state (HS) to the low-spin (LS) state under pressure. The transition pressures of the HS -> IS and the IS -> LS are about 15 GPa and 50 GPa, respectively. The magnetism can affect the properties of iron up to 72.9 GPa. From the enthalpy difference between every two phases, we find the phase transition pressures of FM-bcc -> FM-hcp, FM-bcc -> NM-hcp and NM-bcc -> NM-hcp are 14.4 GPa, 29.5 GPa and 42.7 GPa, respectively.

Molecular dynamics study of the melting curve of NiTi alloy under pressure

The melting curve of NiTi alloy was predicted by using molecular dynamics simulations combining with the embedded atom model potential. The calculated thermal equation of state consists well with our previous results obtained from quasiharmonic Debye approximation. Fitting the well-known Simon form to our Tm data yields the melting curves for NiTi: 1850(1u2009+u2009P/21.938)0.328 (for one-phase method) and 1575(1u2009+u2009P/7.476)0.305 (for two-phase method). The two-phase simulations can effectively eliminate the superheating in one-phase simulations. At 1 bar, the melting temperature of NiTi is 1575u2009±u200925 K and the corresponding melting slope is 64 K/GPa.

Surface Corrugation in Rotational and Diffractive Scattering of O2 from LiF (001)

A quantum dynamic calculation on a five-dimensional O2/LiF (001) model system is performed using the multi-configuration time-dependent Hartree method. The obtained results show that the mechanism of rotational and diffractive excitation in details: Comparison with the rotational excited state, the initially non-rotational state is seen to favor the inelastic scattering in the rotational excitation process. The surface corrugation can damp the quantum interferences and produce a greater amount of rotational inelastic scattering at the expense of the elastic process in the rotational excitation process. The diffraction process and the average energy transferred into the rotational and diffractive mode are also discussed.