Trends in energy gaps of binary compounds: an approach based upon electron transfer parameters from optical spectroscopy

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Abstract
Energy-gap (Eg) data for I-VII, II-VI and III-V binary compounds (MX) are examined in an attempt to correlate Eg with the electron-transfer (charge-transfer) spectra of the related complex ions M'Xnm- and MX'nm-. Reasons are advanced for excluding oxides and fluorides in this study. Electron-transfer spectra can be parametrised in terms of optical electronegativity ( chi opt) values which are assigned to cations and anions. It is found that the chi opt values can be modified slightly to take account of the discrete energy levels changing to energy bands in transferring from the complexes, M'Xnm- and MX'nm-, to the compounds MX. These modified values then provide a simple means for predicting energy gaps both of N-(8-N) and non-N-(8-N) binary compounds. There are theoretical grounds for anticipating the correlation to be inapplicable to the 'inter-shell type' compounds such as the copper(I) and silver(I) halides, and the experimental data bear this out.