New perovskite-based manganite Pb2Mn2O5

A new perovskite based compound Pb2Mn2O5 has been synthesized using a high pressure high temperature technique. The structure model of Pb2Mn2O5 is proposed based on electron diffraction, high angle annular dark field scanning transmission electron microscopy and high resolution transmission electron microscopy. The compound crystallizes in an orthorhombic unit cell with parameters 0=5.736(1) angstrom approximate to root 2a(p), b 3.800(1) angstrom approximate to a(p), c= 21.562(6) angstrom approximate to 4 root 2a(p) (a(p) - the parameter of the perovskite subcell) and space group Puma. The Pb2Mn205 structure consists of quasi two-dimensional perovskite blocks separated by 1/2[110](p)((1) over bar 01)(p) crystallographic shear planes. The blocks are connected to each other by chains of edge-sharing MnO5 distorted tetragonal pyramids. The chains of Mn05 pyramids and the Mn06 octahedra of the perovskite blocks delimit six-sided tunnels accommodating double chains of Pb atoms. The tunnels and pyramidal chains adopt two mirror-related configurations (left L and right R) and layers consisting of chains and tunnels of the same configuration alternate in the structure according to an -L-R-L-R-sequence. The sequence is sometimes locally violated by the appearance of -L-L- or -R-R-fragments. A scheme is proposed with a Jahn-Teller distortion of the MnO6 octahedra with two long and two short bonds lying in the a-c plane, along two perpendicular orientations within this plane, forming a d-type pattern. (C) 2010 Elsevier Inc. All rights reserved.