Compositionally induced phase transition in the Ca 2MnGa 1− xAl xO 5 solid solutions: Ordering of tetrahedral chains in brownmillerite structure

Abstract

The Ca 2MnGa 1− x Al x O 5 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 P n m a space symmetry ( x ⩽ 0.5 ) to a structure with I 2 m b 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 Ca 2MnGa 0.5Al 0.5O 5 was refined from X-ray powder diffraction data (space group P n m a , a = 5.25175 ( 5 ) Å , b = 15.1426 ( 2 ) Å , c = 5.46029 ( 6 ) Å , 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 ≈ 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 P n m a phase. The density of APBs increases with increasing x resulting in the formation of slabs of the I 2 m b 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.