Anomalous octahedral distortion and multiple phase transitions in the metal-ordered manganite YBaMn2O6

摘要

By means of powder X-ray diffraction, powder neutron diffraction and transmission electron microscopy (TEM), we determined the crystal structures of a metal-ordered manganite YBaMn2O6 which undergoes successive phase transitions. A high-temperature metallic phase (T-cl = 520 K < T) crystallizes in a triclinic P1 with the following unit cell: Z = 2. a = 5.4948(15) Angstrom, b = 5.4920(14) Angstrom, c = 7.7174(4) Angstrom, alpha = 89.804(20)degrees, beta = 90.173(20)degrees, gamma = 91.160(4)degrees. The MnO6 octahedral tilting is approximately written as a(0)b(-)c(-) leading to a significant structural anisotropy within the ab plane. The structure for T-c2 < T < T-c1 is a monoclinic P2 (Z = 2, 5.5181(4) Angstrom, b = 5.5142(4) Angstrom, c = 7.6443(3) Angstrom, beta = 90.267(4)degrees) with an a(-)b(-)c(-) tilting. The structural features suggest a d(x2)-(y2) orbital ordering (OO). Below T-c2 = 480 K, crystallographically inequivalent two octahedra show distinct volume difference, due to the Mn3+/Mn4+ charge ordering. The TEM study furthermore revealed a unique d(3x2-r2)/d(3y2-r2) OO with a modified CE structure. It was found that the obtained crystal structures are strongly correlated to the unusual physical properties. In particular, the extremely high temperature at which charge degree of freedom freezes, T-c2, should be caused by the absence of the structural disorder and by heavily distorted MnO6 octahedra. (C) 2003 Elsevier Inc. All rights reserved. [References: 24]