M2(Si,Al)4(N,C)7 (M = La,Y,Ca) carbonitrides II.: The crystal structure of Ca0.8Y1.2Si4N6.8C0.2

A new (Ca,Y)Si-4(N,C)(7) phase has been characterised lying between the two end-members Y2Si4N6C and CaYSi4N7. This phase is similar to BaYbSi4N7, which is made up of network of [N(SiN3)(4)] structural units linked together in a three-dimensional network, with the large cations located in the interstices, but (Ca,Y)Si-4(N,C)(7) is a disordered variant, with nitrogen atoms partially occupying two sets of equivalent sites related by the combined operations of rotation and tilt. The crystal of (Ca,Y)Si-4(N,C)(7) used for structure determination contained Ca and Y in the atomic ratio 2:3, the excess positive charge in the cation sites being balanced by the partial replacement of nitrogen by carbon in the central non-metal site of the [N(SiN3)(4)] unit. Powder diffraction data are listed for Ca0.8Y1.2Si4N6.8C0.2, which is hexagonal with a = 5.9874(4), c = 9.7849(8) Angstrom at ambient temperature. The crystal structure has been determined from single crystal data; Z = 2; S.G. P6(3)mc (no, 186); R-int = 0.0274, R1 = 0.0384, wR2 = 0.0993 for all data. (C) 2004 Elsevier Ltd. All rights reserved.