On the concept of “bond valence sums” and its applicability in solid state chemistry and physics

Abstract

A critical analysis is made of the current frequent use of “bond valence sums” (BVS) in the analysis of solid state phenomena, in particular those involving high-temperature super-conducting cuprates. It is shown that explicit expressions for the empirical bond valence concept can be derived on the basis of the Born-Mayer model of ionic solids. Whereas the “identity” of a cation-anion bond in terms of constant parameters ( R (0) and B) is preserved to good approximation upon changing the environment, bond lengths are not invariant under such changes. This invalidates conclusions drawn regarding ionic charges from measured bond lengths and applies in particular to the high- T c cuprates. The concept of “pressure-induced charge transfer” is shown to be both qualitatively in conflict with experimental observations and quantitatively unrealistic by several orders of magnitude. We also analyse time-dependent phenomena in the cuprates and the observed changes in core-level energies with varying chemical composition.