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) angstrom, b = 15.6230(4) angstrom, c = 5.6075(1) angstrom, 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 Sr2Al1.07Mn0.93O5 and its A(2)B'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.