SHEAR-INDUCED METALLIZATION

It is well known that compression causes insulators and semiconductors to become metallic when the concentration of the matrix atoms (or of impurity atoms) reaches a critical value given by the theories of Herzfeld, Mott, Hubbard, Edwards and Sienko, and others. It is argued here, based on the observed transitions in diamond-framework semiconductors, that microscopic shear strains are sometimes more important than dilatations. The simple theory of hybridized covalent bonds indicates that the energy gaps in these materials should vanish when compression causes the tetrahedral bond angle (109.5-degrees) to increase to 148.2-degrees. This is close to the average observed transformation angle of 149.2-degrees (homopolar crystals). Implications for various phenomena are outlined.