Crystal structures of Ca_4+x Y_3–x Si₇O_15+x N_5–x (0 ≤ x ≤ 1) comprising of an isolated [Si₇(O,N)₁₉] unit

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 (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 [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.