M. Kobayashi, T. Yasunaga, H. Kato, K. Fujii, M. Yashima, M. Kakihana, *Acta Crystallographica Section E*, **E75**, 260–263 (2019). DOI: 10.1107/S2056989019001257

#### Abstract

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 *P*6_{3}/*m*. Ca^{2+} and Y^{3+} cations are distributed over two crystallographic sites 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.