M2(Si,Al)4(N,C)7 (M = La,Y,Ca) carbonitrides

A new (Ca,Y)Si4(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 a 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)Si4(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)Si4(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) Å at ambient temperature. The crystal structure has been determined from single crystal data; Z=2; S.G. P63mc (no. 186); Rint=0.0274, R1=0.0384, wR2=0.0993 for all data.