HIGH-PRESSURE CRYSTAL CHEMISTRY AND INSIGHTS IN TO THE NATURE OF THE CHEMICAL BOND
This view is incompatible with the discovery that the response of the perovskite structure to high pressures is controlled by the equipartition of bond-valence strain between the A and B cation sites within the structure [Acta Cryst. B60:263]. The same appears to be true, within experimental uncertainties, for all framework minerals. In perovskites, this explicitly implies that the octahedral compressibility depends not only upon the octahedral cation, but also upon the compressibility of the cation-oxygen bonds of the extra-framework (nominally dodecahedral) site. Thus the octahedral compressibility of a B cation site must change as the A-site cation is changed, whether or not the B-O bond lengths change as a result of the substitution on the A site. The strength of bonds is thus dependent upon the crystal environment and not solely upon the bond length. The observation of a plateau effect in the variation of octahedral compressibilities in perovskite solid solutions suggests that the bond-valence matching principle is followed not just globally, but on a local scale as well. Such observations should allow the change with pressure of the excess thermodynamic properties of solid solutions to be directly related to the microscopic (atomic scale) evolution of the structure.