Illustration of the "missing piece" method of assignment for a complex sandwich SiC crystal J13 .

(a) A full plate topograph showing the contributions from the basic 6H and 4H polytypes, the pattern is useful in recognising the overlap of the 3144 4H reflection with the 3146 6H reflection.

(b) An enlargement of the central 4150 (h, k, l = 0) reflection showing the total crystal profile containing all the polytypes with a measured thickness of 2280 m m. Bands of high defect density can be seen at the boundaries between changing morphology and may indicate the prescence of 1DD layers.

(c) An enlargement of part of the 314l row showing the spatial displacement of different polytypes along the row. The two parts of the 3144 6H reflection show a "missing" piece which may be accounted for by the displaced 3142 4H reflection immediately below it. However the individual sizes of the 6H+4H+6H reflections have been measured as 350+170+1680 m m respectively totalling 2200 m m which leaves 80 m m unaccounted for.

(d) An enlargement of the same row containing simultaneously the 3146 6H and 3144 4H reflections which overlap. In this case (l ¹ 0) unlike the 4150 (l = 0) reflection where the whole crystal provides diffraction contrast and there are clearly visible two abscences of contrast corresponding to polytype layers with either a long repeat or with one dimensional disorder. Since the layers are relatively thin » 20 m m and 60 m m, and there are no feint closely spaced reflections suggesting LPP's, the streaking due to disorder would be negligible and the layers have been assigned 1DD status.

(e) A model to scale of the polytypic content of the crystal which for the purposes of this survey has been termed an American club sandwich .

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