Mean bond-length variation in crystal structures: a bond-valence approach

The distortion theorem of the bond-valence theory predicts that the mean bond length < D > increases with increasing deviation of the individual bond lengths from their mean value according to the equation < D > = (D' + Delta D), where D' is the length found in a polyhedron having equivalent bonds and Delta D is the bond distortion. For a given atom, D' is expected to be similar from one structure to another, whereas < D > should vary as a function of Delta D. However, in several crystal structures < D > significantly varies without any relevant contribution from Delta D. In accordance with bond-valence theory, < D > variation is described here by a new equation: < D > = (D-RU + Delta D-top + Delta D-iso + Delta D-aniso + Delta D-elec), where D-RU is a constant related to the type of cation and coordination environment, Delta D-top is the topological distortion related to the way the atoms are linked, Delta D-iso is an isotropic effect of compression (or stretching) in the bonds produced by steric strain and represents the same increase (or decrease) in all the bond lengths in the coordination sphere, Delta D-aniso is the distortion produced by compression and stretching of bonds in the same coordination sphere, Delta D-elec is the distortion produced by electronic effects. If present, Delta D-elec can be combined with Delta D-aniso because they lead to the same kind of distortions in line with the distortion theorem. Each D-index, in the new equation, corresponds to an algebraic expression containing experimental and theoretical bond valences. On the basis of this study, the Delta D index defined in bond valence theory is a result of both the bond topology and the distortion theorem (Delta D = Delta D-top + Delta D-aniso + Delta D-elec), and D' is a result of the compression, or stretching, of bonds (D' = D-RU + Delta D-iso). The deficiencies present in the bond-valence theory in explaining mean bond-length variations can therefore be overcome, and the observed variations of < D > in crystal structures can be described by a self-consistent model.