ON THE CONCEPT OF BOND VALENCE SUMS AND ITS APPLICABILITY IN THE ANALYSIS OF HIGH-TC SUPERCONDUCTORS

A critical analysis is made of the at present frequency use of "bond valence sums" (BVS) in the analysis of high-temperature superconducting 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 the (cation) valence from measured bond lengths and applies, in particular, to the high-T(c) cuprates (Cu valence). The concept of "pressure induced charge transfer" is shown to be quantitatively unrealistic by several orders of magnitude.