Effective charges and ionicity in alkali halides: An analysis

Based on the well-known Phillips - Van Vechten and Penn-type models, we present here an analysis of the effective charges and ionicity in alkali halides. This analysis is made in the light of the recently reported experimental results of Lowndes and Martin. It is essentially seen that Zeff=8 gives results which are in good accord with those of Van Vechten and Lawaetz. The power law governing the variation of the homopolar gap with the nearest-neighbor distance, which has earlier been proposed for elemental semiconductors, is also shown to hold for the alkali halides. We then extrapolate this result to obtain an estimate of the hitherto-unknown homopolar gap of cesium fluoride. Other related properties of CsF are then derived.