Lattice dynamics and elastic properties of PbF2 and BaF2 from quantum mechanical calculations

The lattice dynamics of beta-PbF2, alpha-PbF2 and cubic BaF2 were studied using density-functional perturbation theory. These calculations show that the bonding in the two PbF2 polymorphs is very similar. Phonon densities of states and heat capacities have been calculated and compared to the available experimental data. The results imply that anharmonicity begins to become significant already at temperatures as low as similar to100 K. The computed elastic stiffness coefficients of the PbF2 polymorphs are used to discuss the unusual observation that the denser orthorhombic polymorph, thought to be the high pressure modification, is significantly more compressible than the cubic form.