Twin (diffraction pattern of)
From Online Dictionary of Crystallography
Macle (cliché de diffraction d'une) (Fr). Geminato (spettro di diffrazione di un) (It), 双晶（回折図形） (Ja)
The diffraction pattern of a twin is the superposition of the diffraction pattern of the individuals building the twin.
- In twinning by merohedry, the whole diffraction of one individuals is mapped of that of the other(s) individual(s). Because the twin operation overlaps independent reflections (reflections that are not related by the symmetry operations of the space group), he measured intensities are actually the sum of the intensities from each individual, scaled by their volume fraction, without a phase relation.
- In twinning by pseudomerohedry the overlap concerns a sub-set of the reflections, at small Bragg angles: this sub-set depends on the obliquity. At higher angles, the reflections are separated and can be measured independently.
- In twinning by reticular merohedry a reciprocal sublattice is in common to the individuals forming the twin: the fraction of the of reflection overlapped corresponds to the twin index and does not change with the Bragg angle.
- In twinning by reticular pseudomerohedry the reflections forming the common reciprocal sublattice are only approximately overlapped: they are actually separated at higher Bragg angles.