The Real Structure of Defect Solids

Abstract

Any theoretical analysis of thermodynamic equilibrium, transport and diffusion, electronic properties, and other properties of solids needs to be based on an understanding of the role played by defects from perfect crystalline order. When one turns from crystals that deviate from ideal composition to a degree that is barely detectable chemically to solidsolution systems, one finds a situation that differs in a vital respect from that discussed by point-defect theory, viz., in the concentration of lattice sites that appear to be “wrongly” occupied. The validity of basic point-defect theory is not open to question; it is firmly rooted in the statistical thermodynamics of crystals. The equilibria between defects, and between electrons and positive holes, are defined by the energetic cost of creating defects and by the band structure of the crystal. There is a wide range of experimental evidence, drawn from nuclear magnetic resonance, electron-spin resonance, and other techniques, that, at low concentrations, the defects have the site symmetry and properties associated with a point-defect description in terms of vacant sites or interstitials.