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

A series of new nitrides and carbonitrides has been identified with crystal structures similar to those of the hexagonal quaternary nitrides of the type (Ba,Sr,Eu)YbSi4N7. The large divalent cations in these structures can be replaced by trivalent cations such as Ln and/or Y, if valency balance is preserved by the simultaneous substitution of carbon for nitrogen in the unique [4]-coordinated anion site. This has been demonstrated by carbon-13 magic-angle spinning NMR spectra which for the yttrium member of this series shows a peak at 36.7 ppm corresponding to carbon atoms occupying the central non-metal atom site in the characteristic [C(NSi3)4] structural unit. The resulting compounds have compositions of the types La2Si4N6C, Y2Si4N6C or (La,Y)2Si4N6C; the crystal structure of a related mixed (Ca,Y) derivative of composition (Ca,Y)2Si4(N,C)7 is reported in Part II of this series. When the two large cations are different, the hexagonal symmetry characteristic of the (Ba,Sr,Eu)YbSi4N7 compounds is maintained; when both cations are the same, lower symmetries are observed. The powder diffraction pattern of La2Si4N6C indexes on an orthorhombic unit cell with a=6.0360(7), b=10.1246(9), c=10.5664(11) Å and the crystal structure has been determined. An alternative way of achieving valency balance without incorporation of carbon is to replace some of the silicon by aluminium; related derivatives of the type M2Si3AlN7, where M=La, Y or mixed La,Y have been prepared and their unit cell dimensions are reported.