Thortveitite-type Tm(2)Si(2)O(7)

Single crystals of dithulium disilicate, Tm(2)Si(2)O(7), were obtained in flux synthesis experiments in the system SiO(2)–Tm(2)O(3)–LiF at ambient pressure. The compound belongs to the group of sorosilicates, i.e. it is based on [Si(2)O(7)]-units and crystallizes in the thortveitite (Sc(2)Si(2)O(7)) structure type. The Tm(3+) cation (site symmetry .2.) occupies a distorted octa­hedral site, with Tm—O bond lengths in the range 2.217 (4)–2.289 (4) Å. Each of the octa­hedra shares three of its edges with adjacent [TmO(6)] groups, resulting in the formation of layers parallel to (001). The individual [SiO(4)] tetra­hedra are more regular, i.e. the differences between the bond lengths between Si and the bridging and non-bridging O atoms are not very pronounced. The layers containing the octa­hedra and the sheets containing the [Si(2)O(7)] groups (point group symmetry 2/m) form an alternating sequence. Linkage is provided by sharing common oxygen vertices.