POLARIZABILITIES OF ANIONS IN ANISOTROPIC ENVIRONMENTS - THE FLUORIDE-ION IN THE PEROVSKITE LATTICES NAMGF3, KMGF3 AND KCAF3

Coupled Hartree-Fock calculations are used to find the polarizability of the in-crystal fluoride ion in the perovskite-like compounds NaMgF3, KMgF3 and KCaF3. In all three cases calculations on ion clusters embedded in idealized lattices give a mean polarizability <(alpha)over bar>(F-) in the range 58 +/- 0.1 au and a small negative anisotropy Delta alpha(F-) (0.3% of the mean for NaMgF3 and 4-5% for the potassium compounds). The sign of the anisotropy denotes a smaller polarizability along the direction of contact with the divalent cation. A correlated MP2 calculation for F- in NaMgF3 predicts an increase of 1.4 au in the mean and brings the fractional anisotropy up to 5%. An extended Clausius-Mossotti treatment is used to derive the analytical relationship between the dielectric constant/refractive index of a general undistorted perovskite and the effective polarizabilities of ions in the unit cell. Derived fluoride polarizabilities <(alpha)over bar>(F-) for the three compounds fall in the range 7.1 +/- 0.7 au.