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A new classification of the crystal structures of normal valence compounds with simple atomic ratios is discussed. This classification is based on two atomic parameters which gauge the directional properties of the bonds in a compound. The parameters are (1) the average principal quantum number, ~, of the valence shell of the component atoms and (2) the difference,/lx, between the electronegativities of anion and cation. It is shown that the most common structures of compounds of composition AiX 1 (i, j = 1, 2, 3) occur only in certain well defined regions of a ~ versus ~Ix diagram, and the proposed classification, therefore, is of considerable help in understanding and predicting the structures of normal valence compounds.

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Optical absorption measurements on thick monocrystalline samples of GaSel-& show that in these mixed crystals the lowest energy gap is indirect in the complete range 0 5 x 5 1. These results are contrary to earlier reports which indicate that GaSe is a direct gap semiconductor. I n the case of Gas the absorption spectra allow a tentative identification of the phonons involved in the indirect transition.Optische Absorptionsmessungen an dicken, einkristallinen Proben von GaSel -%SX zeigen, On the basis of their optical absorption the semiconducting compounds Gas and GaSe have been reported to have an indirect and a direct energy gap, respectively [I, 21. Furthermore, the work of Basov et al.[3] appears t o indicate that stimulated emission is possible from GaSe crystals under electron bombardment, and there has been speculation on the occurrence of such emission in electroluminescence [4]. Stimulated emission would of course most readily be reconciled with the presence in GaSe of a direct gap.We now present evidence that as in Gas the smallest energy gap in GaSe is indirect and lies about 53 meV below the direct gap. The earlier failure to recognize the indirect gap is due to the crystal habit of the then available transport-reacted single crystals of GaSe. With very few exceptions these crystals had the form of thin flakes (thickness to 10-1 mm) whose faces were perpendicular to the c-axis, and most experiments were carried out with light incident normal to the flake faces so that c-axis. In this geometry the indirect tran-2 sition is very weak because in the absence

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A new electron-phonon interaction characteristic of layer structures is described. Depending on the strength of the coupling between electron and lattice, this interaction, which involves short-range forces, leads to free or self-trapped charge carriers. The theoretical findings are compared with the experimentallv determined charge-carrier mobilities in GaSe and MoS 2 .

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7 8 WAVELENGTH IN MICRONS FIG. 1. Spectral distribution of recombination radiation from InSb.