Lattice dynamics and elastic properties of PbF\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\mathsf{_2}$\end{document} and BaF\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\mathsf{_2}$\end{document} from quantum mechanical calculations

The lattice dynamics of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\beta$\end{document}-PbF2, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\alpha$\end{document}-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 ~100 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.