Crystal structures, charge and oxygen-vacancy ordering in oxygen deficient perovskites SrMnOx (x<2.7)

Bulk SrMnOx samples with oxygen contents 2.5 <= x < 2.7 have been studied using a combination of neutron time-of-flight and high-energy high-resolution synchrotron X-ray diffraction measurements along with thermogravimetric analysis. We report the identification and characterization of two new oxygen-vacancy ordered phases, Sr5Mn5O13 (SrMnO2.6-tetragonal P4/m a = 8.6127(3) angstrom, c = 3.8102(2) angstrom) and Sr7Mn7O19 (SrMnO2.714-monoclinic P2/m a = 8.6076(4) angstrom, b = 12.1284(4) angstrom, c = 3.8076(2) angstrom, gamma = 98.203(2)degrees). The nuclear and magnetic structures of Sr2MnO2.5 are also reported (SrMnO2.5 nuclear: orthorhombic Pbam, magnetic: Orthorhombic Ay type P(c)bam with c(M) = 2c). In the three phases, oxygen-vacancies are ordered in lines running along one of the (100) directions of the parent cubic perovskite system. Oxygen-vacancy ordering allows the charge and orbital ordering of the Mn3+ and Mn4+ cations in the new phases. (c) 2007 Elsevier Inc. All rights reserved.