First-principles calculations on phase transition and elastic and thermodynamic properties of NaH

Abstract:

The structural phase transition, elastic and thermodynamic properties of NaH were investigated by the plane-wave pseudopotential method within the density functional theory. The equilibrium lattice parameters a0 , elastic constants Cij, bulk modulus B0 and it’s derivative B0′ were calculated. The results are consistent with the experimental and theoretical data. From the usual condition of equal enthalpy, the phase transition of NaH from B1 to B2 phase occurs at 30.5 GPa with a volume collapse of about 4.5%, consistent with the experimental (29.3±0.9GPa) and calculated (37.0 GPa) results. The static finite strain technique was used to calculate elastic constants Cij at the different pressures. Furthermore, the elastic wave velocities, Debye temperature Θ and elastic anisotropic factors of NaH were also investigated in detail. Through the quasi-harmonic Debye model, in which the phononic effects were considered, the thermodynamic properties such as the specific heat capacity and thermal expansion coefficient of NaH were obtained in the whole pressure range from 0 to 50 GPa and temperature range from 0 to 1500 K.