The Structural Characterisation of Sialon Polytypoids

Abstract

The six polytypoid phases in the Si-Al-O-N system (8H, 15R, 12H, 21R, 27R, 2H^s) form a series of structurally similar compounds, analogues of which occur in most metal sialon systems. The structures are determined by their metal: non-metal atom-ratio M:X which for the above series is of the type mM:(m + 1)X where m has the values 4, 5, 6, 7, 9 and 11 respectively. Beryllium, magnesium, scandium and possibly other metal cations can be incorporated into these structures provided that charge balance is preserved and that the overall M:X ratio is retained.

Compounds with an M:X ratio greater than unity have identical structures but with metal and non-metal atoms reversed; such antitypes are observed in the Be-Si-O-N, Al-N-C and Al-Si-C systems. In the Mg-Si-Al-O-N system, two other series of compounds occur; Mg₄N₂O and Mg₄SiN₄ have layer-structures related to Mg₃N₂, and three phases 6H, 14H and 8H form a polytypoid series at MgO-rich compositions in the 6M:7X, 7M:8X and 8M:9X planes respectively.

The structures of the sialon polytypoids have been determined by X-ray powder methods and consist of a wurtzite sequence of tetrahedra broken up by single layers of octahedra inserted every mth layer. Half way between these layers, metal atoms are shared between two adjacent layers of tetrahedra to give an MX₂ unit which reduces the M:X ratio below unity. In the 6H, 14H, 8H series, octahedra occur as double layers in an otherwise similar arrangement. Direct observation of the structures by high resolution electron microscopy gives results in excellent agreement with those obtained by calculation from the X-ray structures.