The response of polyhedra in close packed structures to temperature and pressure

The mean quadratic elongation, < A >, is used to measure the distortions of close packed arrays of anions in crystals. It is also used to trace how such distortions vary with temperature (T) and pressure (P). A calculation of < ..\ > for arrays of anions in materials that adopt the rutile and olivine structure-types shows that the arrays exhibit increased distortions with increasing P whereas those in materials that adopt the dolomite structure-type exhibit a decrease in distortion. On the other hand, the close packed arrays in the dolomite and calcite structure-types become more distorted with increasing T. The array of oxide anions in rutile show an increased distortion while those in stishovite show a decreased distortion with increasing T. An analysis of apparent bulk moduli and linear thermal expansion coefficients calculated for the constituent polyhedra shows that the distortion of a material with a close packed structure is governed by its unoccupied as well as its occupied polyhedra. It also indicates that the rearrangement of the anions in such materials induced by an increase in P is governed by the apparent bulk moduli of its constituent polyhedra rather than by a tendency of the anions to adopt a more perfect close packed array.