Lattice-dynamical and ground-state properties of CaF2 studied by inelastic neutron scattering and density-functional methods -: art. no. 144301

We report results on the lattice dynamics of the superionic conductor CaF2 from experiment and theory. Coherent inelastic neutron scattering between room temperature and 1000degreesC gives an unusually large temperature dependence of the phonon frequencies (dispersion) and widths. Electronic and lattice-dynamical properties are obtained from ab initio density-functional theory employing pseudopotentials and plane-wave basis sets, the electronic properties also from full-potential linearized augmented plane waves-linearized muffin-tin orbital methods. From density-functional perturbation theory we have calculated the dispersion of the phonon frequencies and of the mode-Gruneisen parameters. The contribution from thermal expansion to the experimental line shift is separated from the other anharmonic (phonon-phonon) contributions by comparison with the theoretical line shift from the volume expansion. The temperature dependence of the line broadening can in most cases be rationalized in terms of the two-phonon combined density of states, but the width of the zone-boundary X-2(') mode is unexpectedly large, even though the two-phonon combined density of states shows that there are only few two-phonon decay channels, and the frequency becomes critically soft at higher volumes. The effective charges and the dielectric constant depend nonlinearly on volume.