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; image cubic (image) with Tb4+ and Ir4+; image cubic (image) with Tb3+ and Ir5+ and image monoclinic (P21/n) with Tb3+ and Ir5+. The transitions between these appear to be first order in nature.