The elastic constants of cubic crystals with covalent and partially covalent bonds

General formulae for the elastic constants of purely and partially covalent cubic crystals are derived. For the mixed covalent‐ionic bond a new model of electron locale‐transition is proposed, according to which the ionicity of a bond depends on the bonding angle. This model can account, qualitatively, for the elastic properties of ZnS‐compounds. Also discussed are some earlier theories, which appear to be in error, and some broader implications of the proposed model.