“Missing piece ?” Edge Method
An illustration of the “missing piece ” method to deduce the coalescence of polytype neighbours from edge profiles. The example shown is from the overlapping contributions of the 6H and 4H polytypes in sample J15. The dark contrast of a defect band in the central 5270 reflection suggests the possible presence of a 1DD layer. The edge may be built up from the two parts of the 6H 3142 reflection at (a) which total 40 µm + 990 µm respectively and the 4H 3142 reflection at (b) which measures 90 µm yielding a total polytype thickness of 1120 µm. However the total measured crystal thickness is 1180 µm leaving 60 µm of the crystal unaccounted for by diffraction contrast alone.
The polytype diffraction patterns are spatially displaced along the row: at (b) the 3144 6H reflection is immediately above the 3142 4H reflection while at (c) the 3146 6H and 3144 4H reflections overlap. At (c) there is a clear absence marked ? corresponding to the exact location of the “missing piece” of the crystal which measures 60 µm and has been assigned as a 1DD layer in the model.
6H+4H+1DD+6H+1DD+6H+1DD+6H