Thermodynamic and mechanical properties of actinium and lanthanum dihydride

The mechanical and thermodynamic properties of actinium and lanthanum dihydride have been investigated by first-principles calculations based on the density functional theory with generalized gradient approximation. The equilibrium geometries, elastic constants, Poisson's ratio, various moduli, elastic wave velocities, and Debye temperature were systematically studied. All elastic constants of actinium dihydride were found to be smaller than those of lanthanum dihydride. Phonon band structures and partial density of states were also investigated. The two compounds have similar phonon band structures. The volume expansions, the heat capacities, and the isothermal bulk moduli at finite temperature were also obtained under the quasiharmonic approximation.