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

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 C_v 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. (condensed matter: structural, mechanical, and thermal properties)