Syntheses, crystal structures, and characterization of two new Tl +-Cu 2+-Te 6+ oxides: Tl 4CuTeO 6 and Tl 6CuTe 2O 10

Crystals and polycrystalline powders of two new oxide materials, Tl 4CuTeO6 and Tl 6CuTe 2O 10, have been synthesized by hydrothermal and solid-state methods. The materials were structurally characterized by single-crystal X-ray diffraction. Tl 4CuTeO6 and Tl 6CuTe 2O 10 exhibit one dimensional anionic slabs of [CuTeO 6] 4� and [CuTe 2O 10] 6�, respectively. Common to both slabs is the occurrence of Cu 2+O 4 distorted squares and Te 6+O 6 octahedra. The slabs are separated by Tl + cations. For Tl 4CuTeO 6, magnetic measurements indicate a maximum at �8 K in the temperature dependence of the susceptibility. Low temperature neutron diffraction data confirm no long-range magnetic ordering occurs and the susceptibility was adequately accounted for by fits to a Heisenberg alternating chain model. For Tl 6CuTe 2O 10 on the other hand, magnetic measurements revealed paramagnetism with no evidence of long-range magnetic ordering. Infrared, UV-vis spectra, thermogravimetric, and differential thermal analyses are also reported. Crystal data: Tl 4CuTeO 6, Triclinic, space group P-1 (No. 2), a=5.8629(8)A, b=8.7848(11) A, c=9.2572(12)A, α = 66.0460(10), β = 74.2010(10), γ = 79.254(2), V=417.70(9) A 3, andZ=2; Tl 6CuTe 2O 10, orthorhombic, space group Pnma (No. 62), a=10.8628(6) �, b=11.4962(7)�, c=10.7238(6) �, V= 1339.20(13) � 3, and Z=4. © 2012 Elsevier Inc. All rights reserved

Bi2W2O9: A potentially antiferroelectric Aurivillius phase

Ferroelectric tungsten-based Aurivillius oxides are naturally stable superlattice structures, in which A-site deficient perovskite blocks [WnO3n+1]-2 (n = 1, 2, 3, ...) interleave with fluorite-like bismuth oxide layers [Bi2O2]+2 along the c -axis. In the n = 2 Bi2W2O9 phase, an in-plane antipolar distortion dominates but there has been controversy as to the ground state symmetry. Here we show, using a combination of first-principles density functional theory calculations and experiments, that the ground state is a non-polar phase of Pnab symmetry. We explore the energetics of metastable phases and the potential for antiferroelectricity in this n=2 Aurivillius phase

Labile Degree of Disorder in Bismuth-Oxophosphate Compounds: Illustration through Three New Structural Types

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Labile Degree of Disorder in Bismuth-Oxophosphate Compounds: Illustration through Three New Structural Types

Here, we analyze the crystal structures of three new Bi/M oxophosphates, focusing on the ambiguity between order and disorder in different structural subunits. The three structures are original but systematically built on the assembly of O­(Bi,M)₄ tetrahedra into various 1D-oxocenterd units, separated by PO₄ groups that create cationic channels. Two main subunits show versatile degrees of disorder, i.e., the cationic channels and some of the terminal O­(Bi,M)₄ entities. (a) In the compound [Bi₂(Bi_1.56K_0.44)^disO₃]­K_0.88^dis(PO₄)₂, the K/K and K/Bi disorder is total on both <i>nano-</i> and <i>micro</i>-sized domains. (b) In the incommensurately modulated [Bi₁₀(Bi_∼0.5Cd_∼0.5)₈^disO₁₆]­(Bi_0.6Cd_0.8)₂^ord(PO₄)₈, only the cationic channels show an ordered Bi/Cd arrangement which can be modified by minor stoichiometric changes between domains. (c) In [Bi₁₈Zn₁₀O₂₁]^ordZn₅^ord(PO₄)₁₄, both subunits are almost perfectly ordered (complex Bi/Zn sequence) into a 7-fold supercell, but this order strongly depends on the observation scale and is mainly lost in micronic-grains also due to slight compositional changes. However, the refined noncentrosymmetric organization is maintained (SHG tests) in the bulk. The relative stability of ordered versus disordered sites is discussed on the basis of the existence of two possible mixed sites and probably depends on the M chemical nature. Disorder was characterized by use of solid-state ³¹P NMR probing for the first two cases. Finally, the observed disordered or long periodicities along the infinite dimension suggest the sketch of a periodic/rigid skeleton of O­(Bi,M)₄ units with counterions filling the interspace in more or less disordered arrangements