Volume dependent vibrational properties of cerium hydrides from first principles

We have performed an ab initio study of structural, volume-dependent elastic and lattice dynamical properties of rare-earth metal-hydrides CeH2 and CeH3. The calculations have been carried out within the density functional theory and linear response formalism using norm-conserving pseudopotentials and a plane-wave basis. The hydrogen incorporation into the octahedral sites of the cubic CeH2 to obtain CeH3 leads to stiffening of the lattice as indicated by a volume reduction and an up to 60% increase of the elastic constants of the trihydride compared to that of CeH2. The phonon density of states has been found to show well separated and narrow two (centered at 15 and 110 meV) and three (centered at 15, 60 and 120 meV) groups of vibrations for CeH2 and CeH3, respectively. The transverse acoustic mode Grüneisen parameters of the dihydride phase are found to be negative in the most part of the Brillouin zone which leads to lattice dynamical unstability above a pressure of 14 GPa as indicated by imaginary phonon frequencies.