Volume and pressure dependence of ground-state and lattice-dynamical properties of BaF₂ from density-functional methods

We have performed an ab initio study of BaF₂ by employing different program packages. Ground-state and lattice-dynamical properties are obtained from ab initio density-functional theory within local-density approximation (LDA) and generalized-gradient approximation (GGA) employing pseudopotentials and plane-wave basis sets, the electronic properties also from full-potential LAPW+LMTO methods. The results for the lattice constant and the electronic gap energies agree well within the LDA and GGA. For the band structure we found a valence-band maximum in the Σ direction in contrast to previous works. From density-functional perturbation theory we have calculated the phonon properties. Phonon dispersion curves, elastic constants, high-frequency dielectric constant, and effective charges and their volume and pressure dependence are presented. From the calculation of the anharmonic process of thermal expansion a softening of the X₂^′ mode was found similar to that in CaF₂.