Lattice dynamics of magnesium fluoride

Neglecting the relative core-shell displacements at equilibrium the usual shell model formalism is applied to the rutile structure. A number of models of increasing complexity ranging from the simplest rigid-ion model to the full shell model (with both ions polarizable) are presented. When applying these various models to magnesium fluoride (MgF 2 ) it is found that a simplified shell model is already able to account for the experimental phonon spectrum. Such a comparison is based on optical data for the zone-centre frequencies, and on extensive inelastic neutron scattering measurements of dispersion curves in three symmetry directions and over a frequency range from 0 to 10 THz. Using best-fit values for the model parameters a number of dielectric properties for MgF 2 are calculated and compared with existing data.