Effective charges and their pressure dependence in α-LiIO3

A simple static charge model is formulated for the zone-center optical modes of α-LiIO3 which are simultaneously Raman and ir-active. The model predicts successfully the experimental values of the oscillator strengths for all but one mode (E1-librational) and serves to clarify previous discrepancies in mode assignments resulting from Raman and ir-reflectivity measurements. From previously reported experimental data the pressure dependence is calculated of the effective charges and the angle between the I-O bond and the C/sub 3v/ axis of the IO3- pyramids. The self-consistency of these results is discussed using simple arguments based on rigid spheres and ionic radii. (author)