Crystal structures and chemistry of double perovskites Ba2M(II)M'(VI)O6 (M=Ca, Sr, M'=Te, W, U)

Structures of the double perovskites Ba2M(II)M '(VI)O6 (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 R3-bar (M'=Te), respectively. In the case of M=Ca, the space groups are either monoclinic P21/n (M'=U) or cubic Fm3-bar m (M'=W and Te). The tetragonal and orthorhombic symmetry reported earlier for Ba2SrTeO6 and Ba2CaUO6, respectively, were not observed. In addition, non-ambient X-ray diffraction data were collected and analyzed for Ba2SrWO6 and Ba2CaWO6 in the temperature range between 80 and 723 K. It was found that the rhombohedral R3-bar structure exists in Ba2SrWO6 above room temperature between the monoclinic and the cubic structure, whereas the cubic Ba2CaWO6 undergoes a structural phase transition at low temperature to the tetragonal I4/m structure. - Graphical abstract: Evolution of structures as function of temperature in Ba2SrWO6 (left) and Ba2CaWO6 (right). The existence of two phase region in Ba2SrWO6 can be clearly seen by the progressive increase of the rhombohedral R3-bar phase marked by asterisk (*)