First-Principles Calculations For Properties Of GaAs,GaN And MgO Under High Pressure

As semiconductors, GaAs and GaN are the most suitable candidates for fabrication of photovoltaic devices, semiconductor lasers and light emitting diodes. Despite the technological developments of GaAs, the high pressure behavior of GaAs has been the subject of considerable attention. Many fundamental problems for GaAs under high pressure condition, such as the structural, electronic and bonding mechanisms, still remain unsolved. As one of the major earth-forming minerals, periclase (MgO) is stable in rocksalt (RS) structure with the pressure up to 227 GPa, The knowledge of the elastic properties of MgO is most important for interpreting the seismically observed velocity variations in the earth's mantle and for constructing reliable mineralogical models of the earth's interior. Recently, properties of material at high pressures and high temperatures have been the objects of intensive experimental and theoretical investigations.Firstly, the transition phase of GaAs from the ZB to the RS structure is investigated by ab initio plane-wave pseudopotential density functional theory, and the lattice constant a, the bulk modulus B₀ and the first order pressure derivative of bulk modulus B′₀ are obtained. According to the usual condition of equal enthalpies, we find that the transition from the ZB structure to the RS structure occurs at the pressure of 16.3 GPa, as is well consistent with the experimental data and other theoretical results. The thermodynamic properties of the ZB and RS structures are obtained through the quasi-harmonic Debye model. Meanwhile, we calculate the heat capacities and the Debye temperatures at different temperatures and different pressures, and find that as pressure increases, the heat capacity C_V decreases and the...