HIGH-PRESSURE CRYSTAL CHEMISTRY AND INSIGHTS IN TO THE NATURE OF THE CHEMICAL BOND

Over the past 30 years high-pressure single-crystal diffraction methods have progressed from the very first experiments to almost routine high-precision determinations of small changes in bond lengths and angles between the most strongly-bonded atoms. The use of high pressure now allows us to probe the fundamentals of inter-atomic interactions over ranges of bond lengths inaccessible to experiments at ambient pressures. For example, it is commonly assumed that the relationship between bond strength and bond length for a particular pair of atoms is a simple and single-valued one for a given coordination environment; longer bonds are weaker. This is the basis of the concept of bond valence, for example. Indeed, in strongly-bonded oxide minerals, the range of bond lengths found for a given cation-anion polyhedron is so small that it was long thought that the polyhedral bulk moduli were essentially independent of structure type and thus the environment of the polyhedron.

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.