Relationship between bond valence and bond softness of alkali halides and chalcogenides

Established bond-valence parameter tables rely on the assumption that the bond-valence sum of a central atom is fully determined by interactions to atoms in its first coordination shell. In this work the influence of higher coordination shells is tested in detail for bonds between lithium and oxygen. It is demonstrated that the sum of the weak interactions with atoms of the second coordination shells significantly contributes to the valence sum and should therefore not be neglected. Since the independent refinement of the two parameters R(0) and b is hardly possible from the limited range of bond lengths occurring in the first coordination shell, the restriction of bond-valence sums to contributions from nearest neighbours implicated another far-reaching simplification: the postulation of a universally fixed value of the bond-valence parameter b which characterizes the shape of the bond-valence pseudopotential for the respective atom pair. However, recent more sophisticated applications of the bond-valence concept, e.g. to model ion-transport pathways in solid electrolytes, demand sensible estimates of the bond-valence sums for mobile ions not only at their equilibrium sites but also at interstitial sites and bottle-necks of transport pathways. Calculations of bond valences at these non-equilibrium sites require the knowledge of the actual shape of the bond-valence pseudopotential. A systematic route to a more realistic estimate of b for alkali halides and chalcogenides is developed in this work from an empirical correlation between b and the absolute softnesses of the interacting particles.