Charge ordering and anisotropic thermal expansion of the manganese perovskite CaMn7O12

Abstract The crystal structure of CaMn 7 O 12 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 CaMn 7 O 12 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 MnO 6 octahedra around Mn 3+ ions are apically Jahn-Teller distorted with two pairs of long ∼2.04,∼2.03 A and one short ∼1.89 A 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.