EVIDENCE FOR AN UNUSUAL PHASE IN THE PEROVSKITE-RELATED SYSTEM BACOXMN1-XO3-Y FROM EXAFS SPECTROSCOPY

Three new phases in the system BaCoxMn1-xO3-y have been characterized by X-ray powder diffraction, Mossbauer spectroscopy, and manganese and cobalt K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The 5H BaCo0.5Mn0.5O2.87 and 12H BaCo0.8Mn0.2O2.80 phases appear to be hexagonal perovskites with a mixture of face- and corner-sharing octahedra, although the manganese shows a strong preference for face-sharing sites, and there is evidence to suggest that some of the cobalt occupies tetrahedral sites by virtue of the introduction of BaO2 instead of BaO3 layers. The X-phase, as typified by BaCo0.5Mn0.5O2.70, occurs over a wide range of Co:Mn ratios close to the melting point. The Mn sites are similar to those in hexagonal perovskites, but the Co sites feature large numbers of Co-Co distances of ca. 3.0 angstrom, suggestive of widespread edge-sharing polyhedra, a feature not found in perovskites. A structural relationship with some incommensurate Sr phases is established, and it is suggested that the intergrowth of two different structural units may be involved.