Analysis of the structural parameters of phases that adopt brownmillerite-type structures suggests the distribution of the different complex ordering schemes adopted within this structure type can be rationalized by considering both the size of the separation between the tetrahedral layers and the tetrahedral chain distortion angle. A systematic study using structural data obtained from La1−x A x MnO2,5 (A = Ba, Sr, Ca,) phases, prepared by the topotactic reduction of the analogous La1−x A x MnO3 perovskite phases, was performed to investigate this relationship. By manipulating the A-cation composition, both the tetrahedral layer separation and tetrahedral chain distortion angle in the La1−x A x MnO2,5 phases were controlled and from the data obtained a “structure map” of the different brownmillerite variants was plotted as a function of these structural parameters. This map has been extended to include a wide range of reported brownmillerite phases showing the structural ideas presented are widely applicable. The complete structural characterization of La1−x A x MnO2,5 0.1 ≤ x ≤ 0.33, A = Ba; 0.15 ≤ x ≤ 0.5 A = Sr, and 0.22 ≤ x ≤ 0.5 A = Ca is described and includes compositions which exhibit complex intralayer ordered structures and Mn2+/Mn3+ charge ordering.

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.

A new A(n)B(n)O(3n-2) homologous series of anion-deficient perovskites has been evidenced by preparation of the members with n = 5 (Pb(2.9)Ba(2.1)Fe(4)TiO(13)) and n = 6 (Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)) in a single phase form. The crystal structures of these compounds were determined using a combination of transmission electron microscopy and X-ray and neutron powder diffraction (S.G. Ammm, a = 5.74313(7), b = 3.98402(4), c = 26.8378(4) Å, R(I) = 0.035, R(P) = 0.042 for Pb(2.9)Ba(2.1)Fe(4)TiO(13) and S.G. Imma, a = 5.7199(1), b = 3.97066(7), c = 32.5245(8) Å, R(I) = 0.032, R(P) = 0.037 for Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16)). The crystal structures of the A(n)B(n)O(3n-2) homologues are formed by slicing the perovskite structure with (101)(p) crystallographic shear (CS) planes. The shear planes remove a layer of oxygen atoms and displace the perovskite blocks with respect to each other by the 1/2[110](p) vector. The CS planes introduce edge-sharing connections of the transition metal-oxygen polyhedra at the interface between the perovskite blocks. This results in intrinsically frustrated magnetic couplings between the perovskite blocks due to a competition of the exchange interactions between the edge- and the corner-sharing metal-oxygen polyhedra. Despite the magnetic frustration, neutron powder diffraction and Mössbauer spectroscopy reveal that Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) are antiferromagnetically ordered below T(N) = 407 and 343 K, respectively. The Pb(2.9)Ba(2.1)Fe(4)TiO(13) and Pb(3.8)Bi(0.2)Ba(2)Fe(4.2)Ti(1.8)O(16) compounds are in a paraelectric state in the 5-300 K temperature range.

The crystal and magnetic structures of the x Ϸ 1 member of the Pb 2−x Ba x Fe 2 O 5 solid solution series have been studied using x-ray and neutron powder diffraction, electron diffraction, high-resolution electron microscopy, and Mössbauer spectroscopy. Pb 1.08 Ba 0.92 Fe 2 O 5 has two polymorphic forms with the orthorhombic unit cell with a Ϸ ͱ2a p , b Ϸ a p , and c Ϸ 4 ͱ 2a p ͑a p-the parameter of the perovskite subcell͒ with the Pnma space group of the low-temperature ͑LT͒ phase and the Imma space group of the high-temperature ͑HT͒ phase, which are related by a phase transition at T c Ϸ 540 K. The crystal structures of both polymorphs were refined from neutron powder-diffraction data at T = 14 K and T = 700 K. The structure consists of parallel perovskite blocks with the thickness of two FeO 6 octahedra linked together by infinite chains of edge-sharing distorted FeO 5 trigonal bipyramids with two columns of the Pb cations in between characterized by the asymmetric coordination environment due to localized 6s 2 lone electron pair. Two mirror-related configurations of the trigonal bipyramidal chains are ordered in the LT structure; their arrangement becomes disordered in the HT structure. Below T N = 625 K, Pb 1.08 Ba 0.92 Fe 2 O 5 transforms into an antiferromagnetically ordered state. The antiferromagnetic ͑AFM͒ structure with a propagation vector k = ͓0, 1 2 , 1 2 ͔ is characterized by an antiparallel spin alignment for all nearest-neighbor Fe atoms in the perovskite blocks, which stack on to each other at the trigonal bipyramidal chains, resulting in alternating antiparallel and parallel arrangement of spins on going along the common edge of the FeO 5 trigonal bipyramids. An unusual spin flipping dynamic behavior was revealed by Mössbauer spectroscopy and related to a specific character of superexchange interactions inside the chains of the FeO 5 trigonal bipyramids.

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Phases: Mapping the Variants of the Brownmillerite Structure. -The title phases are prepared by topotactic reduction with NaH of the corresponding La1-xMxMnO3 (M: Ba, Sr, Ca) perovskites obtained by a citrate gel method from stoichiometric mixtures of La2O3, MCO3, and MnO2 dissolved in a minimum quantity of 6 M HNO3 (1325-1500°C). The samples are characterized by powder XRD and neutron diffraction. La0.6Sr0.4MnO2.5 crystallizes in the space group Pcmb, La0.7Sr0.3MnO2.5 in the space group C2/c, La0.75Ca0.25MnO2.5 in the space group Pnma, and La0.8Ba0.2MnO2.5 in the space group I2mb. Careful analysis of the structural parameters of these phases allows a structure map to be plotted which correlates the appearance of different brownmillerite structural variants with simple crystallographic parameters.