Twin (diffraction pattern of)

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 pattern of one individual is mapped on to that of the other individual(s). Because the twin operation overlaps independent reflections (reflections that are not related by the symmetry operations of the space group), the 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 common to the individuals forming the twin: the fraction of the 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.