Zeng Zhao-Yi

First-principles calculations for electronic, optical and thermodynamic properties of ZnS

The electronic, optical and thermodynamic properties of ZnS in the zinc-blende (ZB) and wurtzite (WZ) structures are investigated by using the plane-wave pseudopotential density functional theory (DFT). The results obtained are consistent with other theoretical results and the available experimental data. When the pressures are above 20.5 and 27 GPa, the ZB-ZnS and the WZ-ZnS are converted into indirect gap semiconductors, respectively. The critical point structure of the frequency-dependent complex dielectric function is investigated and analysed to identify the optical transitions. Moreover, the values of heat capacity CV and Debye temperature ? at different pressures and different temperatures are also obtained successfully.

First-Principles Calculations for Elastic Properties of ZnS under Pressure

The pressure dependence of elastic properties of ZnS in zinc-blende (ZB) and wurtzite (WZ) structures are investigated by the generalized gradient approximation (GGA) within the plane-wave pseudopotential density functional theory (DFT). Our results are in good agreement with the available experimental data and other theoretical results. From the high-pressure elastic constants obtained, we find that the ZB and WZ structures of ZnS are unstable when the applied pressures are larger than 15.8 GPa and 21.3 GPa, respectively. The sound velocities along different directions for the two structures are also obtained. It is shown that as pressure increases, the sound velocities of the shear wave decrease, and those of all the longitudinal waves increase. An analysis has been made to reveal the anisotropy and highly noncentral forces in ZnS.

Pressure and Temperature Induced Phase Transition of ZnS from First-Principles Calculations

The pressure induced phase transition of ZnS from the wurtzite (WZ) and the zincblende (ZB) structures to the rocksalt (RS) structure and the temperature induced phase transition from the ZB structure to the WZ structure axe investigated by ab initio plane-wave pseudopotential density-functional theory (I)FT), together with the quasi-harmonic Debye model. It is found that the zero-temperature transition pressures from the WZ-ZnS and the ZB-ZnS to the RS-ZnS are 17.20 and 17.37 GPa, respectively. The zero-pressure transition temperature from the ZB-ZnS to the WZ-ZnS is 1199 K. All these results are consistent with the available experimental data. Moreover, the dependences of the normalized primitive cell volume V/V-0 on pressure and thermal expansion coefficient alpha on temperature are also obtained successfully.

Phase Transition and Melting Curves of Calcium Fluoride via Molecular Dynamics Simulations

The phase transition and melting curves of CaF2 are investigated by using the general utility lattice programme (GULP) via the shell model with molecular dynamics method. By calculating the entropy H (at 0K) and Gibbs free energy G ⁄ (at 300K), we flnd that the phase transition pressure from the face-centred cubic (fcc) structure to the orthorhombic structure is 11.40GPa and 9.33GPa at 0K and 300K, respectively. The modifled melting point of the fcc CaF2 is in the range of 1650{1733K at 0GPa. All these results are well consistent with the available experimental data and other theoretical results. We also obtain that the melting temperature of high pressure phase is 990{1073K at 10GPa. Moreover, the temperature dependences of the elastic constants Cij, bulk module B and shear module G are also predicted.

Structural and thermodynamic properties of wurtzitic boron nitride from first-principle calculations

Using ab initio plane-wave pseudopotential density functional theory method, we have studied thestructural and thermodynamic properties of the wurtzite boron nitride (w-BN). Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependencies of the normalized lattice parameters a/a(0) and c/c(0), the axial ratio c/a, and the normalized primitive cell volume V/V-0 on pressure P and temperature T are investigated. Furthermore, the Debye temperature circle minus, the variation of thermal expansion alpha, as well as the heat capacity C-V as functions of P and T are studied systematically.

Shell Model for Elastic and Thermodynamic Properties of Gallium Nitride with Hexagonal Wurtzite Structure

Shell model molecular dynamic simulation with interatomic pair potential is utilized to investigate the elastic and thermodynamic properties of gallium nitride with hexagonal wurtzite structure (w-GaN) at high pressure. The calculated elastic constants Cij at zero pressure and 300 K agree well with the experimental data and other calculated values. Meanwhile, the dependences of the relative volume V/V0, elastic constants Cij, entropy S, enthalpy H, and heat capacities CV and CP on pressure are successfully obtained. From the elastic constants obtained, we also calculate the shear modulus G, bulk modulus B, Youngs modulus E, Poissons ratio v, Debye temperature D, and shear anisotropic factor Ashear on pressures.

Phonon Dispersion and Thermodynamics Properties of CaF2 via Shell Model Molecular Dynamics Simulations

The phonon and thermodynamics properties of face-centered cubic CaF2 at high pressure and high temperature are investigated by using the shell model interatomic pair potential within General Utility Lattice Program (GULP). The phonon dispersion curves and the corresponding density of state (PDOS) in this work are consistent with the experimental data and other theoretical results. The transverse optical (TO) and longitudinal optical (LO) mode splitting as well as heat capacity at constant volume CV and entropy S versus pressure and temperature are also obtained.

Structures and Equation of State of "-Fe under High Pressure ⁄

The equation of state (EOS) and the axial ratio c/a of -Fe at high pressures are investigated by using the generalized gradient approximation (GGA) within the plane-wave pseudopotential density functional theory (DFT). The results show that at the lower pressure, the EOS of ferromagnetic -Fe is consistent with the experimental result. While at higher pressure, the EOS of the nonmagnetic -Fe is in good agreement with the experimental result. Meanwhile, we find an obvious increase of the axial ratio c/a with pressure, and there is only a small increase with increasing temperature at high pressure.

First principle calculation of elastic and thermodynamic properties of stishovite

Using ab initio plane-wave pseudo-potential density functional theory method, the elastic constants and band structures of stishovite were calculated. The calculated elastic constants under ambient conditions agree well with previous experimental and theoretical data. C13, C33, C44, and C66 increase nearly linearly with pressure while C11 and C12 show irregularly changes with pressure over 20 GPa. The shear modulus (C11-C12)/2 was observed to decrease drastically between 40 GPa and 50 GPa, indicating acoustic mode softening in consistency with the phase transition to CaCl2-type structure around 50 GPa. The calculated band structures show no obvious difierence at 0 and 80 GPa, being consistent with the high incompressibility of stishovite. With a quasi-harmonic Debye model, thermodynamic properties of stishovite were also calculated and the results are in good agreement with available experimental data.