Crystal structures and chemistry of double perovskites Ba 2M(II) M′(VI)O 6 ( M=Ca, Sr, M′=Te, W, U)

Abstract

Structures of the double perovskites Ba 2 M(II) M ′(VI)O 6 ( M=Ca, Sr, M′=Te, W, U) at room temperature have been investigated by the Rietveld method using X-ray and neutron powder diffraction data. For double perovskites with M=Sr, the observed space groups are I2/ m ( M′ =W) and R 3 ¯ ( M′=Te), respectively. In the case of M=Ca, the space groups are either monoclinic P2 1/ n ( M′=U) or cubic Fm 3 ¯ m ( M′=W and Te). The tetragonal and orthorhombic symmetry reported earlier for Ba 2SrTeO 6 and Ba 2CaUO 6, respectively, were not observed. In addition, non-ambient X-ray diffraction data were collected and analyzed for Ba 2SrWO 6 and Ba 2CaWO 6 in the temperature range between 80 and 723 K. It was found that the rhombohedral R 3 ¯ structure exists in Ba 2SrWO 6 above room temperature between the monoclinic and the cubic structure, whereas the cubic Ba 2CaWO 6 undergoes a structural phase transition at low temperature to the tetragonal I4/ m structure.