Anisotropy of effective charges in NaNO₂, NaNO₃, KNO₃and CaCO₃

Abstract

Effective charges of the constituents of NaNO2, NaNO3, KNO3 and CaCO3 are calculated from the splitting of polar optical modes into transverse and longitudinal components, obtained from the simulation of the infrared reflectivity spectra polarised along the different crystallographic directions. Effective charges within planar molecular ions are found to be highly anisotropic, whereas the effective charge of external cations depends little on polarisation. The temperature dependence of the effective charges is reported in NaNO2, NaNO3 and KNO3. The dependences are discussed in terms of correlation of the cation-anion distance with the iono-covalent character of the bond, in particular near structural phase transitions where the cation-anion distances may change.