Crystal structures of Ca(4+x)Y(3–x)Si(7)O(15+x)N(5–x) (0 ≤ x ≤ 1) comprising of an isolated [Si(7)(O,N)(19)] unit

Single crystals of the solid solution series Ca(4+x)Y(3–x)Si(7)O(15+x)N(5–x) were obtained by a solid-state reaction method using a flux for x = 0, 0.5 and 1, resulting in Ca(4)Y(3)Si(7)O(15)N(5) (tetra­calcium triyttrium hepta­silicon oxynitride), Ca(4.5)Y(2.5)Si(7)O(15.5)N(4.5) and Ca(5)Y(2)Si(7)O(16)N(4) (penta­calcium diyttrium hepta­silicon oxynitride). Single-crystal X-ray analysis revealed that the three compounds are isotypic and belong to space-group type P6(3)/m. Ca(2+) and Y(3+) cations are distributed over two crystallographic sites (site symmetry [Image: see text].. 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 [Si(7)(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 [Si(6)O(15)N(3)]. The present compounds are the first to have such an [Si(7)(O,N)(19)] unit in their structures.