First-principles investigations on structure and thermodynamic properties of CaS under high pressures

Abstract

First-principles calculations of the electronic structure and thermodynamic properties of calcium sulfide (CaS) have been carried out by the plane-wave pseudopotential density functional theory method. The calculated values of lattice constant, elastic modulus and its derivative for CaS under zero pressure and zero temperature, agree well with the experimental data and some of the existing model calculations. The band structure and density of states are discussed in detail. Moreover, the dependences of the volume variation, bulk elastic modulus, thermal expansion coefficient and heat capacity on pressure have been investigated for the first time, so far as we know. It is concluded that under the condition of zero temperature (0 K) and zero pressure (0 GPa), the volume is 44.6 3 when the energy of the crystal unit cell reaches a minimum in the structural model of CaS, which is the most stable system. The energy band of CaS is mainly composed of low band gap, valence band and conduction band, the GV-XC band gap of CaS is 2.435 eV. The DOS results show that the valence band is mainly of Ca 3s and S 3p, while the conduction band is mainly of Ca 4d and a small amount of S 3p. At a certain temperature, the volume change rate, heat capacity and thermal expansion coefficient decrease with rising pressure, and the body elastic modulus B increases simultaneously. In contrast, when the pressure is constant, the volume change rate and body elastic modulus B decrease with the increase of temperature, while the thermal expansion coefficient and heat capacity increase as the temperature rises. When the temperature is higher than a certain value, the heat capacity CV is close to the Dulong-Petit limit, and the effect of temperature on the heat capacity is minimal. Furthermore, under the condition of low pressures, the influence of temperature on thermal expansion coefficient is greater than that of the pressure on it.