New perovskite-based manganite Pb 2Mn 2O 5

Abstract

A new perovskite based compound Pb 2Mn 2O 5 has been synthesized using a high pressure high temperature technique. The structure model of Pb 2Mn 2O 5 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 a=5.736(1) Å≈√2 a p , b=3.800(1) Å≈ a p , c=21.562(6) Å≈4√2 a p ( a p —the parameter of the perovskite subcell) and space group Pnma. The Pb 2Mn 2O 5 structure consists of quasi two-dimensional perovskite blocks separated by 1/2[110] p (1̄01) p crystallographic shear planes. The blocks are connected to each other by chains of edge-sharing MnO 5 distorted tetragonal pyramids. The chains of MnO 5 pyramids and the MnO 6 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 MnO 6 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.