ABSTRACTThe drive for improved refractoriness in nitrogen ceramics, which has motivated the removal of residual glass from grain-boundaries by heat-treatment at sub-solidus temperatures, has resulted in the discovery of a wide range of crystalline oxynitride phases. Generally, these phases are oxygen-rich, and can be classified in the same way as the mineral silicates, in which oxygen atoms are coordinated to a maximum of two silicon atoms, with SiO4 tetrahedra joined at either 0,1,2,3 or 4 corners to other tetrahedra. However, the field of oxynitride crystal chemistry is wider than this, because nitrogen in SiN4 tetrahedra commonly occurs coordinated to three silicon atoms, and an additional range of structures exist with nitrogen in this coordination.Oxynitride analogues of mineral silicates are well known in neso-, soro- and cyclostructure types; new structures have recently been reported which belong to the pyroxene family (ino-silicates), and information on their preparation and crystal chemistry is reported here. The sialon U-phase (typical composition Ln3Si3Al3O12N2, Ln = La,Ce,Nd,Sm), is an example of an oxynitride with a structure intermediate between layer (phyllo-) and framework (tecto-) types. Sialon W-phase (approximate composition Ln4Si9Al5O30N, Ln = La,Ce,Nd) has not been completely characterized, but appears to have a structure related to the amphibole group of double-chain silicates.Comments are made on the many oxynitride structures which are still uncharacterized. The suitability of all these oxynitrides as grain-boundary phases in silicon nitride and sialon ceramics is discussed.