Allotwinning: oriented crystal association of polytypes – some warnings on consequences

The oriented association of two or more polytypes of the same compound enters neither in the Friedel's definition of twinning nor in that of epitaxy/syntaxy; for it the termallotwinningis here introduced and the relevant laws of formation are established. The appearance of allotwins is in principle recognizable from the diffraction pattern, but allotwins involving polytypes with the same number of layers and the same lattice symmetry may produce complete overlap of the lattice nodes of the individuals,i.e.as in case of twinning by merohedry. Differently from that case, however, the observed intensities are the sum of non-equivalent diffractions. This characteristic, if not recognized, may in principle lead to mistake the allotwin for a single crystal and possibly to find a reasonable, but wrong, stacking sequence.