First-principles Investigation On Physical Properties Of Stable Hexagonal Ge₂Sb₂Te₅under Pressure

Abstract:The structural, electronic and optical properties of stable hexagonal Ge2Sb2Te5with the petrov sequence have been studied by the density functional theory (DFT) method within the generalized gradient approximation (GGA). The calculated equilibrium lattice parameters are in reasonable agreement with the available experimenal data and other theoretical results. Both the band structure and density of states (DOS) show that the stable hexagonal Ge2Sb2Te5with the petrov sequence possesses metallic properties. The pressure-induced variations of cell volume indicate that the structure occurs unstable at17GPa and34GPa, suggesting that the phase transitions may occur at the two pressure points, which are good agreement with experimental results (Krbal. et al.2009). In addition, the optical properties of the stable hexagonal Ge2Sb2Te5with the petrov sequence under pressure are also investigated. The dielectric function, absorption coefficient, optical reflectivity and refractive index, extinction coefficient, complex dielectric function, as well as electron energy loss are systematically obtained for radiation up to20eV. In the meantime, the thermal properties of the stable hexagonal Ge2Sb2Te5with the petrov sequence are also calculated and summarized The heat capacity, debye temperature and thermal expansion coefficient curves under the temperature and pressure are given. The results show that in a constant temperature, heat capacity decreases with the increase of pressure, and debye temperature increases with the pressure increases. In a constant pressure, heat capacity increases with the increases of temperature, and the debye temperature decreases with the increases of temperature, and the higher of temperature, the faster it decreases. In low pressure, the thermal expansion coefficient is very high. When the pressure increases, the thermal expansion coefficient presents linear decrease phenomenon. But for a given pressure, the thermal expansion coefficient increases rapidly with the temperature increases, the curve presents linear increasing trend.