INTERPRETATION OF ELASTIC PROPERTIES OF IONIC-CRYSTALS - VALIDITY OF A QUASI-ADDITIVITY RULE

In groups of isotypic ionic crystals the product of the mean value of the principal elastic constants and of the molecular volume, the S-val ue, is an almost constant quantity. Even chemically related ionic comp ounds possess similar S-values. The different ''hardness'' of the ions and the number of contacts of the lattice particles are responsible f or certain modifications of the S-values. In ionic crystals a quasi-ad ditivity rule is valid which allows the prediction of S-values and the refore also of mean elastic stiffnesses from the S-values of the const ituent components in a surprisingly good approximation. Deviations fro m this behaviour provide a tool to detect and interpret anomalous bond ing contributions. Crystals with dominant non-ionic bonding exhibit ch aracteristic differences from the behaviour of ionic crystals.