Prediction of the influence of additives on the density of valence-alternation centres in lone-pair semiconductors

Although the addition of small concentrations of additives to chalcogenide semiconductors can change the density of valence-alternation centres, the effect may be much smaller than expected. A general approach is presented for predicting the fraction x of additive that is actually effective in changing the density of centres. This fraction is less than unity because some of the additive has its valence requirements satisfied (in part by coordinate bonds). Only additive atoms in unusual (charged) bonding configurations can influence the density of valence-alternation centres, x is enhanced by entropy effects but reduced by the presence of the intrinsic density of valence-alternation centres and by the higher energy of the charged bonding configuration.