Crystal structures of Ca4+x Y3–x Si7O15+x N5–x (0 ≤ x ≤ 1) comprising of an isolated [Si7(O,N)19] unit

Single crystals of the solid solution series Ca4+xY3–xSi7O15+xN5–x were obtained by a solid-state reaction method using a flux for x = 0, 0.5 and 1, resulting in Ca4Y3Si7O15N5 (tetra­calcium triyttrium hepta­silicon oxynitride), Ca4.5Y2.5Si7O15.5N4.5 and Ca5Y2Si7O16N4 (penta­calcium diyttrium hepta­silicon oxynitride). Single-crystal X-ray analysis revealed that the three compounds are isotypic and belong to space-group type P63/m. Ca2+ and Y3+ cations are distributed over two crystallographic sites (site symmetry \overline{3}.. and 1) in a disordered manner. The corresponding (Ca,Y)-centred polyhedra are connected by edge-sharing, resulting in the formation of a layer structure extending parallel to (001). Three [Si(O,N)]4 tetra­hedra (two with point group symmetry m.., one with 3.. and half-occupancy) are condensed into an isolated [Si7(O,N)19] unit, in which an [Si(O,N)]4 tetra­hedron is located at the center of a 12-membered oxynitride ring with composition [Si6O15N3]. The present compounds are the first to have such an [Si7(O,N)19] unit in their structures.