High-resolution electron microscopy of a Sr-containing sialon polytypoid phase

A new type of Sr-containing sialon polytypoid phase with the structural for mula SrSi10-xAl18+xN32-xOx (x approximate to l) has been found in the Sr-Si -Al-O-N system. The phase was characterised by X-ray powder diffraction (XR PD), and its structure was investigated by electron diffraction (ED) and hi gh resolution electron microscopy (HREM). It is considerably disordered, bu t the average structure has a rhombohedral unit cell with a=5.335(5)approxi mate to root 3.a(AIN) and c= 79.1(1)Angstrom approximate to 30.c(AIN). The Sr atoms ave located in layers M-Sr-M, M=(Si/Al), at the origin of the unit cell with 12 X= (O,N) atoms around it, at distances of similar to 3 Angstr om, forming a cubo-octahedron. The X atoms that form a hexagon around the S r atom in the ab plane are corner shared by M = (Si/Al) tetrahedra with opp osite polarity in adjacent layers in which 2/3 of the tetrahedra are occupi ed. The M-Sr-M layers alternate with normally eight-layer-thick AIN type bl ocks, although the thickness of these blocks frequently varies. The structu ral model obtained from the HREM images includes a polarity reversal of the tetrahedra in the AIN blocks, similar to that proposed to occur in Si-Al-O -N polytypoid phases. The model with one Sr layer and 10 M = (Si,Al) layers per 1/3 of the repeat unit agrees with the composition of the phase and ex perimental HREM images. (C) 1999 Elsevier Science Ltd. All rights reserved.