Evidence for an unusual phase in the perovskite-related system BaCoxMn1 –xO3 –y from EXAFS spectroscopy

Abstract

Three new phases in the system BaCo_xMn_{1 –x}O_{3 –y} have been characterized by X-ray powder diffraction, Mössbauer spectroscopy, and manganese and cobalt K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The 5H BaCo_0.5Mn_0.5O_2.87 and 12H BaCo_0.8Mn_0.2O_2.80 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 BaCo_0.5Mn_0.5O_2.70, 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 Å, 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.