Independent structural and valence state transitions in the cation-ordered double perovskites Ba2−xSrxTbIrO6

Synchrotron X-ray and neutron powder diffraction were used to investigate the formation, structure and bonding in the double perovskite Ba2−xSrxTbIrO6 solid solutions. The results showed that these oxides all exhibit ordering of the Tb and Ir cations in a double perovskite-type structure. Three distinct structural types differing in symmetry and/or valence states were formed depending on the precise Ba:Sr ratio on the perovskite A   site; x⩽0.3x⩽0.3 cubic (Fm3¯m) with Tb4+ and Ir4+; 0.4⩽x⩽1.00.4⩽x⩽1.0 cubic (Fm3¯m) with Tb3+ and Ir5+ and x⩾1.2x⩾1.2 monoclinic (P21/n) with Tb3+ and Ir5+. The transitions between these appear to be first order in nature.

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