Raman and infrared spectra of crystals with α-NaFeO 2 structure

Abstract

We have studied the Raman spectra of twelve NaBX 2 compounds, which crystallize in the common α-NaFeO 2 structure, namely B=Cr, In, Er, Ho, Y, Yb, and X=0, S. Adopting a simple force model for the Raman active normal modes, the repulsive forces between the vibrating anion layers are calculated and correlated with the structural parameters of the crystals. For NaCrS 2 we present a detailed analysis of the Raman and i.r. spectra which is based on the rigid-ion approximation. The potential energy is given by V=V s+V c , where V s is the contribution of the short range overlap forces and V c represents the long range Coulomb interactions. V s is expressed in terms of a simple force model which contains three important short range force constants. In order to account for the observed TO-LO splitting of the i.r. active modes, a rigid-ion model is used for V c which contains two independent effective charges. The comparatively small effective charges obtained from this analysis suggest that the valence electrons in NaCrS 2 are delocalized to a large extent.