The Sialon Polytypes

The way in which these materials change their structures as the anion to cation ratio is varied is of interest. The structures are all variants of the hexagonal wurtzite structure of AIN in which we have alternate sheets of A1 and N atoms packed normal to the hexagonal c-axis. The stacking sequence can be represented by drawing the atom positions on the (110) planes, as all possible atom sites lie on such planes. In 

The basic features of the idealized structures of the 12H and 15R phases show why the n layer blocks are displaced one from another. In the MX2 layer we have to insert an extra anion. In the close packing of the N metal atoms we have 2N tetrahedra, N of which point upwards while the other N point downwards. The non-metal atoms sit in these tetrahedra. In the wurtzite structure possessed by the normal MX layers, only one set of tetrahedra is occupied but this is filled completely. In an MX2 layer we are forced to put the extra cation into a tetrahedron of the opposite unfilled set. This means that these two filled tetrahedra must share a face. This energetically unfavourable situation can be avoided if the packing of the metal atom layers is changed locally from hexagonal to cubic. 

The structures so described for these polytypes are idealized. More recent structure determinations suggest that midway between the MX2 layers we have a sheet of AlOg octahedra. This, if proven, will provide a very attractive example of a chemically 

Thompson, Nitrogen Ceramics , ed. F. L. Riley, Proc. NATO Advanced Study Institute, Noordhof, Leyden, 1977, p. 129. 

In addition to these sialons, similar phases occur in many related systems. For details, the original papers can be studied.  

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