Ambiguity in indexing electron diffraction patterns of R3 crystals.

The R3 space group inherently lacks 2, m and m_{{\bar 1}10} symmetry operations. However, in crystals with R3 symmetry, these transformations produce `pseudoplanes' with the same interplanar spacing and angles as the original crystallographic planes, causing a lack of uniqueness in the electron diffraction (ED) pattern. The difference in atomic arrangements of pseudoplanes and original planes is reflected in the intensities of diffraction spots; it is hard to differentiate in standard ED patterns, frequently causing wrong assignment of the zone axes. The implications of this ambiguity in analysis of crystal orientations are discussed in detail and a suitable routine to follow while indexing R3 ED patterns is proposed.