First-principles study of lattice dynamics and thermodynamic properties ofLaCl3andLaBr3

Abstract

We perform first-principles calculations of the lattice dynamics and the thermodynamic properties of $\mathrm{La}{\mathrm{Cl}}_{3}$ and $\mathrm{La}{\mathrm{Br}}_{3}$. Using density-functional perturbation theory, we obtain the Born effective charge tensors, the dielectric permittivity tensors, the phonon frequencies at the Brillouin zone center, and the phonon dispersion curves, as well as corresponding density of states. The Born effective charge and the dielectric permittivity tensors exhibit anisotropy, which are analyzed in detail. The calculated phonon frequencies at the $\ensuremath{\Gamma}$ point of the Brillouin zone show good agreement with the experimental values for most vibrational modes. The light yields of $\mathrm{La}{\mathrm{Cl}}_{3}:\mathrm{Ce}$ and $\mathrm{La}{\mathrm{Br}}_{3}:\mathrm{Ce}$ are theoretically estimated to be 62 400 and $71\phantom{\rule{0.2em}{0ex}}400\phantom{\rule{0.3em}{0ex}}\text{photons}∕\mathrm{MeV}$, respectively, on the basis of the calculated values of the dielectric constants and the highest longitudinal optical infrared phonon frequencies. The thermodynamics properties including the phonon contribution to the Helmholtz free energy $\ensuremath{\Delta}F$, the phonon contribution to the internal energy $\ensuremath{\Delta}E$, the entropy $S$, and the constant-volume specific heat ${C}_{v}$ are determined within the harmonic approximation based on the calculated phonon dispersion relations.