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

Bulk SrMnO x 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) A, c=3.8102(2) A) and Sr7Mn7O19 (SrMnO2.714-monoclinic P2/m a=8.6076(4) A, b=12.1284(4) A, c=3.8076(2) A, γ=98.203(2)o). The nuclear and magnetic structures of Sr2Mn2O5 are also reported (SrMnO2.5 nuclear: orthorhombic Pbam, magnetic: Orthorhombic Ay type Pcbam 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. - Graphical abstract: The structures of Sr5Mn5O13 and Sr7Mn7O19 (shown) were determined using synchrotron X-rays and neutron time-of-flight powder diffraction and the Rietveld method on multiphase bulk samples. Charge, orbital and oxygen vacancy-ordering has been observed in the novel compounds where Mn4+ octahedra and Mn3+ pyramids are linked through the corners leaving lines of vacant oxygen sites lying along the c-axis