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 °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 Al introduces a phase transformation from the brownmillerite structure with the Pnma space symmetry (x⩽0.5) to a structure with I2mb 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) Å,b=15.1426(2) Å,c=5.46029(6) Å, RI=0.042,RP=0.017). A specific feature of this structure is disorder in the Ga layer with intermixing of the L and R chains in a ≈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 LRLRL layer sequence of the Pnma phase. The density of APBs increases with increasing x resulting in the formation of slabs of the I2mb 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.