Bond-Valence Analysis of the Structure of (Ba0.875Sr0.125)RuO3

The structure ofTRuO3(T=Ba0.875Sr0.125) has been analyzed using the bond valence procedures developed by Brown [e.g.,Acta Crystallogr. Sect. B48, 553 (1992)] and OʹKeeffe [Structure and Bonding71, 161 (1989)]. The experimental results, which show "overbonded" and "underbonded"Tand Ru cations, respectively, are explained by the fact that the theoreticalT–O and Ru–O distances that completely satisfy the valence requirements of the atoms are incommensurate under the constraints imposed by the site symmetries of the special positions occupied by the atoms. Starting from a model derived from sphere packing geometry and the theoretical bond lengths, it is possible to evaluate the lattice parameters and the atomic shifts that minimize the strains introduced into the structure by the incommensurability of the theoretical Ru–O andT–O distances, and to arrive at a model quite close to the experimental results without making use of any prior knowledge of the structure. According to this analysis, therefore, the observed structure is interpreted as resulting from a compromise between the different bond distance requirements of the Ru andTcations.