BOND CHARACTER EFFECTS ON VECTORIAL BOND-VALENCE SUMS IN SULFIDES AND OXIDES

Previous work on oxide crystals has shown that vectorial bond-valence sums, which provide information about both bond lengths and angles, are roughly predictable based on the valence (length) of the bonds. We have found, however, that this predictability is partially an artifact of the restricted range of bond lengths found in crystals for bonds between a given atom pair. The expected bond valence values are highly correlated with the electronegativity of the cations, but a wider range of both bond valences and vectorial bond-valence sums are possible in molecules. The wider range in vectorial bond-valence sums is predictable on the basis of both the bond valence and the bond character (ionic vs. covalent), as described by electronegativity differences. Sulfides show similar patterns to oxides, and taking bond character effects into account allows us to make more accurate predictions of crystal structures.