Synthesis and Structural Characterization of La_1−xA_xMnO_2.5 (A = Ba, Sr, Ca) Phases: Mapping the Variants of the Brownmillerite Structure

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 ≤ 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 Mn 2+ /Mn 3+ charge ordering.