Synthesis and crystal structure of the Sr₂Al_1.07Mn_0.93O₅ brownmillerite

A new brownmillerite-type compound Sr₂Al_1.07Mn_0.93O₅ 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) Å, b = 15.6230(4) Å, c = 5.6075(1) Å, R_I = 0.036, R_P = 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 Sr₂Al_1.07Mn_0.93O₅ 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 T_N = 105 K.