beta-Y(2)Si(2)O(7), a new thortveitite-type compound, determined at 100 and 280 K.

A new form of Y(2)Si(2)O(7) (diyttrium heptaoxodisilicate) has been synthesized which is isotypic with thortveitite, Sc(2)Si(2)O(7), and crystallizes in the centrosymmetric space group C2/m, both at 100 and 280 K. The Y(3+) cation occupies a distorted octahedral site, with Y-O bond lengths in the range 2.239 (2)-2.309 (2) A. The SiO(4) 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 Si(2)O(7) 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 Si(2)O(7) groups is 179.2 (1) degrees at 280 K in C2 and 180 degrees per symmetry in C2/m. The C2/m structure model is favoured.