Elastic Properties of ZnS Structure Semiconductors

A simple phenomenological theory of the elastic constants of sphalerite structure crystals is presented and shown to apply within reasonable errors to the known experimental constants. The theory utilizes a form for bond-stretching () and bending () forces first used by Keating, to which are added effective point-ion Coulombic forces. Also it is pointed out that regularities in the experimental elastic constants of these crystals are readily explained in terms of the ionicity f₈ defined by Phillips and Van Vechten. Of particular note are the shear constants which decrease markedly with ionicity. It is found that this decrease is described quantitatively by (1-f₈), which confirms the interpretation of, since bond-bending forces should vanish in the ionic limit f₈1. Other equally simple formulas for the forces in terms of only the bond length and f₈ are shown to predict all the constants with a rms accuracy of 10%.