Crystal structures of disordered A(2)Mn(3+) M5+O6 (A= Sr, Ca; M = Sb, Nb, Ru) perovskites

摘要

Polycrystalline samples of A(2)MnMO(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 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 a(0)a(0)c(-)) 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 SrMnSbO6 and Sr2MnRuO6, 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 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. (C) 2004 Elsevier Inc. All rights reserved.