14286-95-4Relevant articles and documents
β-Y2Si2O7, a new thortveitite-type compound, determined at 100 and 280 K
Redhammer, Guenther J.,Roth, Georg
, p. i103-i106 (2003)
A new form of Y2Si2O7 (diyttrium heptaoxodisilicate) has been synthesized which is isotypic with thortveitite, Sc2Si2O7, and crystallizes in the centrosymmetric space group C2/m, both at 100 and 280 K. The Y3+ cation occupies a distorted octahedral site, with Y-O bond lengths in the range 2.239 (2)-2.309 (2) A. The SiO4 tetrahedron is remarkably regular, with Si-O bond lengths in the range 1.619 (2)-1.630 (2) A. The bridging O atom of the Si2O7 pyrosilicate group shows a large anisotropic displacement perpendicular to the Si-O bond. Changes in lattice and structural parameters upon cooling are small with, however, a distinct decrease of the anisotropic displacement of the briding O atom. Structure solution and refinement in the non-centrosymmetric space group C2 are possible but do not yield a significantly different structure model. The Si-O-Si bond angle of the isolated Si2O7 groups is 179.2 (1)° at 280 K in C2 and 180° per symmetry in C2/m. The C2/m structure model is favoured.
Stability of rare-earth disilicates: Ionic radius effect
Galunin, Evgeny,Alba, Maria D.,Vidal, Miquel
, p. 1568 - 1574 (2011)
Rare-earth (RE) disilicates, of general formula RE2Si 2O7, are one of the products of the chemical reaction between RE (elements), which are actinide simulators, and the silicates used in the engineered barrier systems of deep geological repositories (DGPs). The aim of this paper is to establish the stability range of the disilicate phase as function of the nature of the RE (RE=Sc, Lu, or Y) and examine whether this phase would permit RE leaching under experimental conditions simulating those of the DGP. The β-polymorphs of the RE disilicates were synthesized by the sol-gel method and subsequently submitted to a pHstat leaching test. The rates of RE and Si leaching were measured and the transformation of the crystalline and amorphous phases was examined by X-ray powder diffraction and nuclear magnetic resonance techniques. The results indicate that the disilicate phases were stable within a wide range of pH, their stability being related to the hydrated ratio of the RE. Disilicate stability increased with the ionic radius of the RE. As a result, the Sc disilicate was stable throughout the pH range tested, whereas Y and Lu disilicate leaching was only observed at pH4. Thus, it was confirmed that the formation of the disilicate phases could contribute to the confinement of radioactive wastes in engineered barriers.
ζ-Y2[Si2O7]: A new structure type within the yttrialite series
Hartenbach, Ingo,Meier, Steffen F.,Schleid, Thomas
, p. 1054 - 1060 (2006)
During attempts of preparing yttrium oxotellurates(IV) using Y 2O3 and TeO2 in YCl3 fluxes, the occasional reaction of these educts with the walls of the evacuated silica ampoules led to colourless, lath-shaped single crystals of Y2[Si 2O7] in the new ζ-type structure as a minor by-product which was investigated by X-ray diffraction. The title compound crystallizes monoclinically in the space group P21/m (a = 503.59(5), b = 806.47(8), c = 732.65(7) pm, β = 108.633(6)°) with two formula units per unit cell. The crystallographically unique Y3+ cation is coordinated by seven oxygen atoms (d(Y-O = 221-248 pm) arranged in the shape of a slightly distorted monocapped octahedron. The isolated oxodisilicate units [Si2O7]6- consist of two Si4+ cations and seven O2- anions of which five are crystallographically independent. These pyroanions (d(Si-O) = 161-168 pm, ≮(O-Si-O) = 91-117°, ≮(Si-O-Si) = 156°) exhibit an almost perfectly eclipsed conformation built of a horseshoe-shaped backbone with the two silicon and three of the oxygen atoms situated on the mirror planes of the unit cell. The remaining four oxide anions complete this [Si2O7] 6- entity of two vertex-sharing [SiO4]4- tetrahedra as terminal ligands for silicon. Assembled in planar layers parallel to (-1 0 1), the [Si2O7]6- anions are packed with their wide basal faces of the tetrahedra pointing towards the small waist of the adjacent units and vice versa. The yttrium cations reside between these layers in order to interconnect them three-dimensionally.
