Using Bond Valence sums as restraints in XAFS analysis

Abstract

The Bond Valence model gives a simple, accurate relationship between the bond distance, co-ordination number, and formal valence of ions in many systems. This empirical model thus gives complementary information to EXAFS and XANES, and has been used as an independent check of XAFS results by many workers. More generally, the Bond Valence represents prior knowledge of the system that can be used to constrain the XAFS analysis itself. Since Bond Valence sums are not perfectly satisfied, their use as exact constraints in XAFS analysis may not be warranted and they are probably best suited as restraints – inexact conditions placed on the analysis that act as additional data or prior knowledge of the system. The practical use of Bond Valence sums as constraints and restraints in EXAFS analysis is demonstrated for iron oxides, and the use of Bond Valence sums as prior knowledge that can be applied to a wide-range of XAFS analyses is discussed.