Interpretation of elastic properties of ionic crystals. Validity of a quasi-additivity rule?

Abstract

Abstract In groups of isotypic ionic crystals the product of the mean value of the principal elastic constants and of the molecular volume, the S-value, is an almost constant quantity. Even chemically related ionic compounds possess similar S-values. The different “hardness” of the ions and the number of contacts of the lattice particles are responsible for certain modifications of the S-values. In ionic crystals a quasi-additivity rule is valid which allows the prediction of S-values and therefore also of mean elastic stiffnesses from the S-values of the constituent components in a surprisingly good approximation. Deviations from this behaviour provide a tool to detect and interpret anomalous bonding contributions. Crystals with dominant non-ionic bonding exhibit characteristic differences from the behaviour of ionic crystals.