Anna S. Kalyuzhnaya

Synthesis and crystal structure of the Sr2Al1.07Mn0.93O5 brownmillerite

A new brownmillerite-type compound Sr2Al1.07Mn0.93O5 was synthesized. The crystal structure was determined using electron diffraction and high resolution transmission electron microscopy and refined from X-ray powder diffraction data (space group Imma, a = 5.4358(1) A, b = 15.6230(4) A, c = 5.6075(1) A, RI = 0.036, RP = 0.023). The structure is characterized by a disordered distribution of the tetrahedral chains in L and R configuration and a partial occupation of the octahedral position by the Mn3+ and Al3+ cations. The relationships between the crystal structures of Sr2Al1.07Mn0.93O5 and its A2B′MnO5 analogues (A = Ca, Sr, B′ = Al, Ga) and the structural reasons for the different types of tetrahedral chain ordering in brownmillerites are discussed. The temperature dependences of the magnetic susceptibility and specific heat reveal that the compound is antiferromagnetically ordered below TN = 105 K.

Synthesis and crystal structure of the Sr_{2}Al_{1.07}Mn_{0.93}O_{5} brownmillerite

A new brownmillerite-type compound Sr2Al1.07Mn0.93O5 was synthesized. The crystal structure was determined using electron diffraction and high resolution transmission electron microscopy and refined from X-ray powder diffraction data (space group Imma, a = 5.4358(1) A, b = 15.6230(4) A, c = 5.6075(1) A, RI = 0.036, RP = 0.023). The structure is characterized by a disordered distribution of the tetrahedral chains in L and R configuration and a partial occupation of the octahedral position by the Mn3+ and Al3+ cations. The relationships between the crystal structures of Sr2Al1.07Mn0.93O5 and its A2B′MnO5 analogues (A = Ca, Sr, B′ = Al, Ga) and the structural reasons for the different types of tetrahedral chain ordering in brownmillerites are discussed. The temperature dependences of the magnetic susceptibility and specific heat reveal that the compound is antiferromagnetically ordered below TN = 105 K.

Compositionally induced phase transition in the Ca_{2}MnGa_{1-x}Al_{x}O_{5} solid solutions: ordering of tetrahedral chains in brownmillerite structure

The Ca 2 MnGa 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 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 Ca 2 MnGa 0.5 Al 0.5 O 5 was refined from X-ray powder diffraction data (space group Pnma, a = 5.25175(5) A, b = 15.1426(2) A, c = 5.46029(6) A, R I = 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 -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.