Infrared Dielectric Dispersion and Apparent Ionic Charges in Sodium Nitrite

Measurements at room temperature of the infrared reflectivity of oriented samples of orthorhombic NaNO₂ crystals have been carried out and analyzed by Kramers-Kronig relations to obtain the principal dielectric response functions. The eight infrared dispersion frequencies found are well accounted for by the expected polar vibrational normal modes of the NaNO₂ lattice, as are the observed mode polarization directions. The range in magnitude of the dielectric polarization associated with the various normal modes is exceptionally large, and this feature has been further investigated by the use of a formalism in which apparent charges are assigned to the individual ions. The influence of dipolar local-field corrections on the apparent charges is considered within the context of a simplified "shell model," leading to a set of apparent ionic charges which explain the unusual intensity distribution within the modes in a natural way. The failure to observe resonant contributions to the dielectric response from phonons or other elementary excitations associated with the reorientation of the NO₂^- group and the attendant ferroelectric-paraelectric transition is discussed.