Compositionally induced phase transition in the Ca2MnGa1-xAlxO5 solid solutions:: Ordering of tetrahedral chains in brownmillerite structure

The Ca2MnGa1-xAlxO5 solid solutions (0.2 <= x <= 1.0) with brownmillerite-type structure were synthesized by solid state reaction at 1250 degrees C for 40 h in Ar flow. The structures of the solid solutions were studied using X-ray powder diffraction, transmission electron microscopy and high resolution electron microscopy. Replacing Ga by A1 introduces a phase transformation from the brownmillerite structure with the Pnma space symmetry (x <= 0.5) to a structure with 12mb space symmetry (x > 0.5). The structures differ by the ordering pattern of the mirror-related tetrahedral chains (L and R): in the primitive structure the L and R chains form alternating layers whereas in the body-centered phase all chains are of the same type. The crystal structure of Ca2MnGa0.5Al0.5O5 was refined from X-ray powder diffraction data (space group Pnma, a = 5.25175(5) angstrom, b = 15.1426(2) angstrom, c = 5.46029(6) angstrom, R-I = 0.042, R-P = 0.017). A specific feature of this structure is disorder in the Ga layer with intermixing of the L and R chains in a approximate to 2:1 ratio. The disorder is related to the formation of numerous antiphase boundaries (APBs) with R = 1/2[111] as a displacement vector, which produces two adjacent tetrahedral layers with the same type of chains in the initial -L-R-L-R-L- layer sequence of the Pnma phase. The density of APBs increases with increasing x resulting in the formation of slabs of the 12mb phase up to a complete phase transformation. Dipole-dipole interactions between the tetrahedral chains are discussed as a possible driving force causing various patterns of tetrahedral chain ordering. (c) 2005 Elsevier SAS. All rights reserved.