Crystal structures of Ca_4+xY_3−xSi₇O_15+xN_5−x (0 ≤ x ≤ 1) comprising of an isolated [Si₇(O,N)₁₉] unit

Abstract

　Single crystals of the solid solution series Ca_4+xY_3−xSi₇O_15+xN_5−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), Ca_4.5Y_2.5Si₇O_15.5N_4.5 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 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.