Synthesis, structures and phase transitions in the double perovskites Sr2−xCaxCrNbO6

The synthesis and crystal structures of nine members of the rock-salt ordered double perovskites Sr2−xCaxCrNbO6 is presented. The crystal structures of the end members of the series Sr2CrNbO6 and Ca2CrNbO6 were refined using powder neutron diffraction data and are cubic in image and monoclinic in P21/n, respectively, in both cases there being considerable anti-site Cr–Nb mixing. Variable temperature and/or composition studies suggest a direct first-order P21/n to image transition, a suggestion supported by selected area electron diffraction studies.