Evaluation of bond covalency and bond valence in Sr-doped perovskite-type compounds

Formulas for decomposing of complex crystals to a sum of binary crystals are described and applied to the study of bond covalency in La1-xSrxFeO3 (0.0 less than or equal to x less than or equal to 0.9) and Ca1-xSrxMnO3 (0.0 less than or equal to x less than or equal to 0.5). The bond valence is treated by bond-valence sums scheme. The results indicate that, for both compounds, with the increasing doping level, the bond covalency and bond valence show the same trend, namely, larger bond covalency corresponds to higher bond valence. For La1-xSrxFeO3, with the increase of doping level, the bond covalency of La-O, Ca-O decreases in the orthorhombic (0.0 less than or equal to x less than or equal to 0.2) and rhombohedral (0.4 less than or equal to x less than or equal to 0.7) systems, then increases slightly for the cubic (0.8 less than or equal to x less than or equal to 0.9) system, but that of Fe-O increases for all crystal systems. A sharp decrease in bond covalency was observed where the crystal changes from orthorhombic to rhombohedral, while a smooth trend was seen for the rhombohedral-to-cubic transition. On the other hand, for orthorhombic Ca1-xSrxMnO3, the bond covalency of Ca-O, Sr-O, and Mn-O (4-coordinate site) decreases with the increasing doping level, that of Mn-O (2-coordinate site) increases.