Crystal structures of disordered A2Mn3+M5+O6 (A=Sr, Ca; M=Sb, Nb, Ru) perovskites

Polycrystalline samples of A2MnMO6 (A=Sr, Ca; M=Nb, Sb, Ru) were prepared by conventional solid state synthesis and their crystal structures were determined using neutron powder diffraction data. All six compounds can be classified as distorted, disordered perovskites. The Mn3+/M5+ distribution is disordered in all six compounds. The strontium containing compounds, Sr2MnMO6 (M=Nb, Sb, Ru), undergo out of phase rotations of the octahedra about the c-axis (tilt system a0a0c-) leading to tetragonal I4/mcm space group symmetry. The calcium containing compounds, Ca2MnMO6 (M=Nb, Ru, Sb), have orthorhombic Pnma space group symmetry, as a result of a GdFeO3-type octahedral tilting distortion (tilt system a-b+a-). A cooperative Jahn-Teller distortion is observed in Sr2MnSbO6 and Sr2MnRuO6, but it is much smaller than the distortion observed in LnMnO3 (Ln=lanthanide ion) perovskites. It is possible that Jahn-Teller distortions of the MnO6 octahedra take place on a short-range length scale in the other four compounds, but there is little or no evidence for cooperative ordering of the local distortions. These findings demonstrate a link between orbital ordering, cation ordering and octahedral tilting