X-ray diffraction study of K3NdSi7O17: a new framework silicate with a linear Si-O-Si bond.
Hydrothermal investigations in the high-silica region of the K(2)O-Nd(2)O(3)-SiO(2) system, carried out in a search for novel fast-ion conductors (FICs), yielded the new compound tripotassium neodymium heptasilicate, K(3)NdSi(7)O(17). Single-crystal X-ray methods revealed that K(3)NdSi(7)O(17) crystallizes in space group P3, has lattice constants a = 16.131 (2) and c = 7.7146 (19) A, Z = 4, and 22 atoms in the asymmetric unit. Refinement was carried out to a residual, R(F), of 0.0253 and a weighted residual, wR(F(2)), of 0.0702 using anisotropic displacement parameters for all atoms. The silicate anion forms an interrupted framework, within which both Nd octahedra and K polyhedra are situated. The structure is unusual in that it contains a symmetry-constrained Si-O-Si bond angle of 180 degrees. No isomorphs to K(3)NdSi(7)O(17) are known.
Structural, spectroscopic, and computational studies on Tl4Si5O12: a microporous thallium silicate.
Single crystals of the previously unknown thallium silicate Tl4Si5O12 have been prepared from hydrothermal crystallization of a glassy starting material at 500 °C and 1 kbar. Structure analysis resulted in the following basic crystallographic data: monoclinic symmetry, space group C2/c, a = 9.2059(5) Å, b = 11.5796(6) Å, c = 13.0963(7) Å, β = 94.534(5)°. From a structural point of view the compound can be classified as an interrupted framework silicate with Q(3)- and Q(4)-units in the ratio 2:1. Within the framework 4-, 6-, and 12-membered rings can be distinguished. The framework density of 14.4 T-atoms/1000 Å(3) is comparable with the values observed in zeolitic materials like Linde type A, for example. The thallium cations show a pronounced one-sided coordination each occupying the apex of a distorted trigonal TlO3 pyramid. Obviously, this reflects the presence of a stereochemically active 6s(2) lone pair electron. The porous structure contains channels running along [110] and [-1 1 0], respectively, where the Tl(+) cations are located for charge compensation. Structural investigations have been completed by Raman spectroscopy. The interpretation of the spectroscopic data and the allocation of the bands to certain vibrational species have been aided by DFT calculations, which were also employed to study the electronic structure of the compound.
Preparation et affinement de la structure d'un silicate a double chaines d'yttrium et de trisodium
A new phase Na3YSi6O15 has been found in the ternary system SiO2Na2OY2O3. It is orthorhombic, space group Cmca, with cell dimensions: A = 14,744(8) A; b = 17,844(8) A; c = 10,610(3) A; Z = 8. The structure has been refined by a least square method to a final R-value of 0,048 (Rw = 0,031) for 1536 independent reflections. The crystal structure consists of corner sharing double silicate chains with a six [SiO4]motive sequences along the c-axis.