β-Y2Si2O7, a new thortveitite-type compound, determined at 100 and 280 K

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) Angstrom. The SiO4 tetrahedron is remarkably regular, with Si-O bond lengths in the range 1.619 (2) - 1.630 (2) Angstrom. 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)degrees at 280 K in C2 and 180degrees per symmetry in C2/m. The C2/m structure model is favoured.