Fundamental Transition in The Electronic Nature of Solids

Abstract

The fundamental electronic properties of non- metallic, inorganic, crystalline solids depend in a natural manner on the character of the chemical bond which is developed between constituent atoms. For example, the familiar Group-IV semiconductors are totally covalent and exhibit characteristics which are qualitatively different from highly ionic materials, such as the alkali halides. However, many materials of fundamental and practical importance are intermediate between these two extremes; it is not immediately evident how their properties may be treated. We will demonstrate here that if crystalline solids are ordered by some measure of their bond ionicity, then one finds striking evidence for a universal and surprisingly abrupt transition in many electronic properties associated with the quantum mechanical valence state. This transition divides crystalline solids into two well-defined classes: “covalent” and “ionic.” Within each class there is a unifying character to the observed properties.