The crystal structure of the Sr 2 Fe 2 O 5 brownmillerite has been investigated using electron diffraction and high resolution electron microscopy. The Sr 2 Fe 2 O 5 structure demonstrates two-dimensional order: the tetrahedral chains with two mirror-related configurations (L and R) are arranged within the tetrahedral layers according to the -L-R-L-Rsequence, and the layers themselves are displaced with respect to each other over 1/2[111] or 1/2[111 j ] vectors of the brownmillerite unit cell, resulting in different ordered stacking variants. A unified superspace model is constructed for ordered stacking sequences in brownmillerites based on the average brownmillerite structure with a ) 5.5298(4)Å, b ) 15.5875(12)Å, c ) 5.6687(4)Å, and (3 + 1)-dimensional superspace group I2/m(0βγ)0s, q ) βb* + γc*, 0 e β e 1/2, 0 e γ e 1.

A new brownmillerite-type compound Sr 2 Al 1.07 Mn 0.93 O 5 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 ˚, 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 Mn 3+ and Al 3+ cations. The relationships between the crystal structures of Sr 2 Al 1.07 Mn 0.93 O 5 and its A 2 B9MnO 5 analogues (A = Ca, Sr, B9 = 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.

The crystal structure of alpha-K(3)AlF(6) was solved and refined from a combination of powder X-ray and neutron diffraction data (a = 18.8385(3)A, c = 33.9644(6)A, S.G. I4(1)/a, Z = 80, R(P)(X-ray) = 0.037, R(P)(neutron) = 0.053). The crystal structure is of the A(2)BB'X(6) elpasolite type with the a = b approximately a(e) square root(5), c = 4a(e) superstructure (a(e), parameter of the elpasolite subcell) and rock-salt-type ordering of the K and Al cations over the B and B' positions, respectively. The remarkable feature of alpha-K(3)AlF(6) is a rotation of 2/5 of the AlF(6) octahedra by approximately pi/4 around one of the crystal axes of the elpasolite subcell, coinciding with the 4-fold symmetry axes of the AlF(6) octahedra. The rotation of the AlF(6) octahedra replaces the corner-sharing between the K and Al polyhedra by edge-sharing, resulting in an increase of coordination numbers of the K cations at the B positions up to 7 and 8. Due to significant deformations of the K polyhedra, the corner-sharing connectivity of the octahedral elpasolite framework is broken and the rotations of the AlF(6) octahedra do not have a cooperative character. Elpasolites and double perovskites with similar structural organization are discussed. The difference in ionic radii of the B and B' cations as well as the tolerance factor are proposed to be the parameters governing the formation of elpasolites and double perovskites with broken corner-sharing connectivity of the octahedral framework.

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