Mechanical and thermodynamic properties of cubic YH2 under high pressure: Prediction from first-principles study

Abstract

First-principles calculations are used to investigate the mechanical and thermodynamic properties of cubic YH2 at different pressures and temperatures. The generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) method is used to describe the exchange–correlation energy in the present work. The calculated equilibrium lattice constant a and bulk modulus B are in good accordance with the available experimental values. According to the Born–Huang criteria for mechanical stability, elastic constants are calculated from the strain-induced stress method in a pressure range from 0 to 67.1 GPa. Isotropic wave velocities and sound velocities are discussed in detail. It is found that the Debye temperature decreases monotonically with the increase of pressure and that YH2 has low anisotropy in both longitudinal and shear-wave velocities. The calculated elastic anisotropic factors indicate that YH2 has low anisotropy at zero pressure and that its elastic anisotropy increases as pressure increases. Through the quasi-harmonic Debye model, in which phononic effects are considered, the thermodynamic properties of YH2, such as the relations of (V–V0)/V0 to the temperature and the pressure, the dependences of heat capacity Cv and thermal expansion coefficient α on temperature and pressure ranging from 0 to 2400 K and from 0 to 65 GPa, respectively, are also discussed.