Charge ordering and anisotropic thermal expansion of the manganese perovskite CaMn7O12

The crystal structure of CaMn7O12 has been studied by high-resolution neutron and synchrotron radiation diffraction between 10 and 400 K. The unusual geometrical properties of the Mn-O bonds network in CaMn7O12 are discussed. The thermal lattice expansion is anisotropic-the hexagonal lattice parameter c has a local maximum around 50 K and a local minimum at 250 K. The MnO6 octahedra around Mn3+ ions are apically Jahn-Teller distorted with two pairs of long similar to 2.04, similar to 2.03 Angstrom and one short similar to 1.89 Angstrom bonds without significant changes with temperature. There are weak peaks in the SR diffraction patterns which are due to a charge modulation, and which disappear above T-CM = 250 K. The maximum and minimum of the c-lattice parameter correlates with the magnetic phase transition at T-C = 49 K and the charge ordering transition T-CM = 250 K, respectively. (C) 2003 Elsevier B.V. All rights reserved.