The crystal and magnetic structures of brownmillerite-like Sr(2)Co(1.2)Ga(0.8)O(5) with a stable Co(3+) oxidation state at both octahedral and tetrahedral sites are refined using neutron powder diffraction data collected at 2 K (S.G. Icmm, a = 5.6148(6) Å, b = 15.702(2) Å, c = 5.4543(6) Å; R(wp) = 0.0339, R(p) = 0.0443, χ(2) = 0.775). The very large tetragonal distortion of CoO(6) octahedra (1.9591(4) Å for Co-O(eq) and 2.257(6) Å for Co-O(ax)) could be beneficial for the stabilization of the long-sought intermediate-spin state of Co(3+) in perovskite-type oxides. However, the large magnetic moment of octahedral Co(3+) (3.82(7)μ(B)) indicates the conventional high-spin state of Co(3+) ions, which is further supported by the results of a combined theoretical and experimental soft X-ray absorption spectroscopy study at the Co-L(2,3) edges on Sr(2)Co(1.2)Ga(0.8)O(5). A high-spin ground state of Co(3+) in Sr(2)Co(1.2)Ga(0.8)O(5) resulted in much lower in comparison with a LaCoO(3) linear thermal expansion coefficient of 13.1 ppm K(-1) (298-1073 K) determined from high-temperature X-ray powder diffraction data collected in air.

About 20 new compounds with the Sillén-Aurivillius intergrowth structure, Me(1)Me(2)Bi3Nb2O11X (Me(1) = Pb, Sr, Ba; Me(2) = Ca, Sr, Ba; X = Cl, Br, I), have been prepared. They are composed of stacking of [ANb2O7] perovskite blocks, fluorite-type [M2O2] blocks and halogen sheets. The cation distribution between the fluorite and perovskite layers has been studied for Ba2Bi3Nb2O11I, Ca1.25Sr0.75Bi3Nb2O11Cl, BaCaBi3Nb2O11Br and Sr2Bi3Nb2O11Cl. The smaller Me cations tend to reside in the perovskite block while the larger ones are situated in the fluorite-type block. The distribution of the elements was confirmed for BaCaBi3Nb2O11Br using energy dispersive X-ray analysis combined with scanning transmission electron microscopy (STEM-EDX). An electron diffraction study of this compound reveals a local symmetry lowering caused by weakly correlated rotation of NbO6 octahedra. Based on our findings, we suggest a new stability criterion for mixed-layer structures, which is that net charges of any two consecutive layers do not compensate for each other and only the whole layer sequence is electroneutral.

The phase relations along the Nd 2 MoO 6 -PbO join of the ternary Nd 2 O 3 -MoO 3 -PbO system have been studied by means of solid-state synthesis in air. The samples with high Pb content underwent a reversible first-order phase transition near 820°C. XRD analysis revealed two tetragonal phases, the hightemperature centric phase (I4 1 /acd) and the low-temperature acentric phase (I42m). In the region of the phase transition, the permittivity of the Pb-containing samples show a strong lambda-type anomaly and electrical conductivity increasesIn the 1960s and 1970s, rare-earth molybdates of various compositions (1:3, 1:2, 5:6, and 1:1) were synthesized as Ln 2 O 3 -MoO 3 binary systems containing 25-50 mol-% Ln 2 O 3 . Many of these molybdates possess functional properties of practical interest. Borchardt and Bierstedt found that the 1:3 compounds, Ln 2 (MoO 4 ) 3 , show ferroelectric properties. [1] In 2000, Lacorre et al. reported La 2 Mo 2 O 9 (1:2 compound) to have high oxygen-ion conductivity (0.06 S/cm at 800°C). [2] In air, 1:2 compounds can only be synthesized for Ln = La and Pr. [2][3][4][5] A number of Ln 2 O 3 -MoO 3 systems have been found to contain Ln 5 Mo 3 O 16+δ compounds of variable composition with a fluorite-like cubic structure. [6][7][8][9][10][11] Compounds with such a structure can be synthesized for Ln = La, Pr, Nd, Sm, and Gd under reducing conditions, [9] and only for Pr and Nd in air. [6,7,10,11] The Ln 5 Mo 3 O 16+δ compounds were found to have mixed oxygen-ion/electronic conductivity, which opens up the possibility of using them in electrochemical devices. [9,10] At a slightly higher rare-earth content, 1:1 Ln 2 MoO 6 compounds, so-called oxymolybdates, [12][13][14] exist for all of the rare earths and have different structures depending on the ionic radius of the rare-earth element and the synthesis temperature.The structures and polymorphism of the oxymolybdates have been the subject of several studies. [12][13][14][15][16][17] The 1:1 molybdates have a tetragonal (La 2 MoO 6 -type) or monoclinic (Nd 2 WO 6type [18] ) structure depending on the ionic radius of the rareearth element and the synthesis temperature. The tetragonal layered structure of Ln 2 MoO 6 (Ln = La, Pr, Nd) compounds is [a]

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