# Twin (diffraction pattern of)

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 Revision as of 15:08, 17 May 2017 (view source)m (Style edits to align with printed edition)← Older edit Revision as of 13:31, 22 May 2017 (view source)m (Edits to align with printed edition)Newer edit → Line 4: Line 4: The diffraction pattern of a [[twin]] is the superposition of the diffraction pattern of the individuals building the twin. 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 individual is mapped on to 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), 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 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 [[Twin obliquity|obliquity]]. At higher angles, the reflections are separated and can be measured independently. + *In '''[[twinning by pseudomerohedry]]''' the overlap concerns a sub-set of the reflections, at small Bragg angles; this sub-set depends on the [[Twin obliquity|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 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. + *In '''[[twinning by reticular pseudomerohedry]]''' the reflections forming the common reciprocal sublattice are only approximately overlapped; they are actually separated at higher Bragg angles. [[Category:Twinning]] [[Category:Twinning]]

## Revision as of 13:31, 22 May 2017

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.