Showing papers in "Acta Crystallographica in 1997"

Synchrotron X-ray Studies of LiNbO3 and LiTaO3

Abstract: The structural geometry, vibrations and deformation density Ap for lithium niobate, LiNbO3, and lithium tantalate, LiTaO3, are derived from synchrotron radiation diffraction measurements. Electron density is transferred from the Nb (Ta) atom towards the large 03 triangle near the Li atom. Spontaneous polarizations of 0.43 (LiNbO3) and 0.33 cm -2 (LiTaO3), calculated with Hirshfeld charges, agree qualitatively with experimental values of 0.71 and 0.50 cm -2, respectively. Both strong Li...Nb (Ta) interactions in the structure are markedly shorter than all Li...Li, Nb...Nb or Ta..-Ta vectors, as expected if the Li and Nb (Ta) cations carry charges of opposite sign. The Li atom's negative charge plays a pivotal role in the polarization responsible for ferroelectricity. LiNbO3, space group R3c, rhombohedral, Mr = 147.8, a = 5.493 (2)/~, o~ = 55.89 (3) °, V= 106.02 (4)/~3, Z = 2, Dx = 4.629 Mg m -3, F(000) = 136, lz(0.7/~) = 5.01 mm -l, Ymin = 0.27, T = 293 K, R = 0.015, wR = 0.015, S = 3.276 (84) for 773 unique reflections. LiTaO3, R3c, rhombohedral, Mr = 235.9, a = 5.471 (2)/~, c~ = 56.16(3) °, V= 105.51 (4)/~,3, Z = 2, Dx = 7.412 Mg m -3, F(000) = 200, #(0.7/~) = 50.03 mm -~ , Ymin = 0.32, T = 293 K, R = 0.016, wR = 0.022, S = 1.372(35) for 773 unique reflections.

Structure d''un Diphosphate Synthetique de Cobalt : Co2P2O7

Abstract: The title compound, a new polymorphic phase of dicobalt diphosphate, Co2P2O7, was prepared by hydrothermal methods. The structure contains CoO6 coordination octahedra and P2O7 radicals. The CoO6 radicals, connected by edge sharing, form six-membered rings, which are further interconnected to form two-dimensional zigzag sheets parallel to (001). The P2O7 groups link these sheets to build three-dimensional networks.

Bis(piperazinium) Pentakis(trioxomolybdo-)bis(ethylphosphonate) Dihydrate

Abstract: The title compound, bis(piperazine-1,4-diylium) bis(ethylphosphonato)-1κO,2:3κ2O',4:5κ2O′′;1:2κ2O,3:4κ2O',5κO′′- penta-μ-oxo-1:2κ2O;1:5κ2O;2:3κ2O;3:4κ2O;4:5κ2O-decaoxo-1κ2O,2κ2O,3κ2O,4κ2O,5κ2O-pentamolybdenum(4−) dihydrate, (C4H12N2)2[Mo5O15(C2H5O3P)2]2H2O, crystallizes as isolated five-unit rings of vertex- and edge-sharing distorted octahedral MoO6 groups, doubly capped by PC2H5 entities {as [(C2H5)PO3]2− ethylphosphonate groups}, resulting in [(MoO3)5{(C2H5)PO3}2]4− {or [Mo5P2O21(C2H5)2]4−} anions Doubly protonated piperazinium cations and water molecules complete the structure This phase shows reversible dehydration/rehydration without loss of structure