pubs.acs.org/cm r XXXX American Chemical Society Chem. Mater. XXXX, XXX, 000–000 A DOI:10.1021/cm902535m Synthesis and Structural Characterization of La 1-x A x MnO 2.5 (A=Ba, Sr, Ca ) Phases: Mapping the Variants of the Brownmillerite Structure Thomas G. Parsons, † Hans D’Hondt, ‡ Joke Hadermann, ‡ and Michael A. Hayward* ,† † Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom, and ‡ EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium Received August 18, 2009. Revised Manuscript Received October 8, 2009 Analysis of the structural parameters of phases that adopt brownmillerite-type structures suggests the distribution of the different complex ordering schemes adopted within this structure type can be rationalized by considering both the size of the separation between the tetrahedral layers and the tetrahedral chain distortion angle. A systematic study using structural data obtained from La 1-x A x MnO 2,5 (A = Ba, Sr, Ca,) phases, prepared by the topotactic reduction of the analogous La 1-x A x MnO 3 perovskite phases, was performed to investigate this relationship. By manipulating the A-cation composition, both the tetrahedral layer separation and tetrahedral chain distortion angle in the La 1-x A x MnO 2,5 phases were controlled and from the data obtained a “structure map” of the different brownmillerite variants was plotted as a function of these structural parameters. This map has been extended to include a wide range of reported brownmillerite phases showing the structural ideas presented are widely applicable. The complete structural characterization of La 1-x A x MnO 2,5 0.1 e x e 0.33, A = Ba; 0.15 e x e 0.5 A = Sr, and 0.22 e x e 0.5 A =Ca is described and includes compositions which exhibit complex intralayer ordered structures and Mn 2þ / Mn 3þ charge ordering. Introduction Transition metal oxides have been of enduring interest to scientists in a wide range of fields because of the diverse physical properties they exhibit. These include super- conductivity, magnetoresistance, and wealth of coopera- tive magnetic and transport behaviors. The introduction of large numbers of anion vacancies into transition metal oxides allows the tuning of these physical properties by altering the oxidation state and local coordination of transition metal centers. In addition, the presence of a large number of vacant anion sites can induce good ionic conductivity, giving these phases applications as fuel cell electrodes and membranes. The efficiency of ionic trans- port in anion deficient systems has been shown to be strongly influenced by the degree of order or disorder in the anion vacancy lattice. The potential future impor- tance of these applications strongly motivates the study of anion deficient materials to investigate factors that direct this ordering behavior so that better property optimiza- tions can be achieved. 1,2 The brownmillerite structure is one of the most com- mon anion vacancy ordered structures. 3 It can be viewed as an anion-deficient variant of the ABO 3 cubic perovskite structure in which half the anions have been removed from alternate BO 2 layers. This gives the stacking sequence AO-BO 2 -AO-BO-AO- with alternating layers of apex- linked BO 6 octahedra and BO 4 tetrahedra (Figure 1). The anion vacancies are arranged within the BO layers in an ordered manner to yield chains of vacancies parallel to the [110] direction of the simple cubic perovskite lattice. The resulting layers consist of chains of apex-linked BO 4 tetrahedra which also run along the [110] direction of the simple cubic perovskite sublattice. The structure is complicated by the possibility that the chains of tetrahe- dra can undergo a cooperative twist, which can occur in either a clockwise or anti-clockwise sense, to yield “left”- or “right”-handed chains (Figure 2)-the two being re- lated by symmetry. The three-dimensional arrangement of the different twist directions within the structure leads to a number of distinct structural configurations (Figure 1). The sim- plest ordered arrangement in which all the tetrahedral chains are twisted in the same manner is described in space group I2mb. In contrast the configuration with Pnma symmetry has all the tetrahedral chains in a parti- cular layer twisted in the same manner, but the twist direction inverts between adjacent layers (interlayer order). There are also configurations that adopt an alter- nating arrangement of left and right handed chains within the tetrahedral layers (intralayer order). These ordered layers can be stacked in one of two ways: either with a *Corresponding author. E-mail: michael.hayward@chem.ox.ac.uk. Tel.: þ44 1865 272623. Fax: þ44 1865 272690. (1) Mohn, C. E.; S., S.; Norberg, S. T.; Hull, S. Phys. Rev. Lett. 2009, 102, 155502. (2) Stolen, S.; Mohn, C. E.; Ravindran, P.; Allan, N. L. J. Phys. Chem. B 2005, 109, 12362–12365. (3) Anderson, M. T.; Vaughey, J. T.; Poeppelmeier, K. R. Chem. Mater. 1993, 5, 151–165. Downloaded by OXFORD UNIV LIBR SVCS on October 30, 2009 | http://pubs.acs.org Publication Date (Web): October 29, 2009 | doi: 10.1021/cm902535m