Crystal structures of disordered A2Mn 3+M5+O 6 ( A=Sr, Ca; M=Sb, Nb, Ru) perovskites

Abstract

Polycrystalline samples of A 2Mn MO 6 ( 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 Mn 3+/ M 5+ distribution is disordered in all six compounds. The strontium containing compounds, Sr 2Mn MO 6 ( M=Nb, Sb, Ru), undergo out of phase rotations of the octahedra about the c-axis (tilt system a 0 a 0 c −) leading to tetragonal I4/ mcm space group symmetry. The calcium containing compounds, Ca 2Mn MO 6 ( M=Nb, Ru, Sb), have orthorhombic Pnma space group symmetry, as a result of a GdFeO 3-type octahedral tilting distortion (tilt system a − b + a −). A cooperative Jahn–Teller distortion is observed in Sr 2MnSbO 6 and Sr 2MnRuO 6, but it is much smaller than the distortion observed in LnMnO 3 ( Ln=lanthanide ion) perovskites. It is possible that Jahn–Teller distortions of the MnO 6 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.