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 La1−x A x MnO2,5 (A = Ba, Sr, Ca,) phases, prepared by the topotactic reduction of the analogous La1−x A x MnO3 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 La1−x A x MnO2,5 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 La1−x A x MnO2,5 0.1 ≤ x ≤ 0.33, A = Ba; 0.15 ≤ x ≤ 0.5 A = Sr, and 0.22 ≤ x ≤ 0.5 A = Ca is described and includes compositions which exhibit complex intralayer ordered structures and Mn2+/Mn3+ charge ordering.

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Phases: Mapping the Variants of the Brownmillerite Structure. -The title phases are prepared by topotactic reduction with NaH of the corresponding La1-xMxMnO3 (M: Ba, Sr, Ca) perovskites obtained by a citrate gel method from stoichiometric mixtures of La2O3, MCO3, and MnO2 dissolved in a minimum quantity of 6 M HNO3 (1325-1500°C). The samples are characterized by powder XRD and neutron diffraction. La0.6Sr0.4MnO2.5 crystallizes in the space group Pcmb, La0.7Sr0.3MnO2.5 in the space group C2/c, La0.75Ca0.25MnO2.5 in the space group Pnma, and La0.8Ba0.2MnO2.5 in the space group I2mb. Careful analysis of the structural parameters of these phases allows a structure map to be plotted which correlates the appearance of different brownmillerite structural variants with simple crystallographic parameters.

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