Although silicon nitride is at present a leading contender for gas turbines and other hightemperature engineering applications, it is only the first of a wide field of nitrogen ceramics, other members of which offer better prospects for technological exploitation. "Sialons" are phases in the Si-AI-O-N and related systems and are comparable in variety and diversity with the mineral silicates. They are built up of one-, two-, and threedimensional arrangements of (Si, AI)(O, N)4 tetrahedra in the same way that the fundamental structural unit in the silicates is the SiO4 tetrahedron. These new oxynitrides include structure types based upon e and ~ silicon nitrides, silicon oxynitride, aluminium nitride and silicon carbide, eucryptite, spinel, melilite and apatite. They are being explored for their thermal, mechanical, chemical and electrical properties.

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At compositions near FesN and Fern the structures proposed for e iron nitrides by previous workers from incomplete X-ray data are confirmed. At intermediate nitrogen concentrations and at concentrations less than that of FesN, additional superlattice reflexions, unaccounted for by these previously proposed structures, are observed. Over the complete e range, the true unit of structure (space group D]-C312) has dimensions a' --~ (2V3)a, c' = c; where a and c are the dimensions of the approximately close-packed hexagonal unit of the iron-atom arrangement. The nitrogen atoms tend towards maximum separation and achieve considerable ordering at all concentrations within the phase limits. The proposed structures are correlated with the observed unit cell dimensional changes.In the ~-Fe~N structure the iron atoms retain the same relative positions as in e-Fe2N, but because of a rearrangement of the nitrogen atoms, which in ~ are packed more closely in one direction, the iron-atom lattice is anisotropically distorted. A mechanism for the e-F%N --> ~-F%N transition is suggested.A discussion of the structural features of the iron-nitrogen system indicates that the interstitialatom ordering shown by all phases except one, and particularly by the a" and e phases, and the sharp change from one ordered arrangement (8) to a different but equally ordered interstitial-atom arrangement (~) cannot be ascribed to ordinary close-packing or size factors.

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