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Effects of merohedric twinning on the diffraction pattern
Author(s) -
Nespolo Massimo,
Ferraris Giovanni,
Souvignier Bernd
Publication year - 2014
Publication title -
acta crystallographica section a
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.742
H-Index - 83
ISSN - 2053-2733
DOI - 10.1107/s2053273313029082
Subject(s) - crystal twinning , reflection (computer programming) , crystallography , diffraction , extension (predicate logic) , group (periodic table) , space (punctuation) , materials science , crystal (programming language) , space group , condensed matter physics , physics , x ray crystallography , optics , chemistry , computer science , microstructure , quantum mechanics , programming language , operating system
In merohedric twinning, the lattices of the individuals are perfectly overlapped and the presence of twinning is not easily detected from the diffraction pattern, especially in the case of inversion twinning (class I). In general, the investigator has to consider three possible structural models: a crystal with space‐group type H and point group P , either untwinned ( H model ) or twinned through an operation t in vector space ( t ‐H model ), and an untwinned crystal with space group G whose point group P ′ is obtained as an extension of P through the twin operation t ( G model ). In 71 cases, consideration of the reflection conditions may directly rule out the G model; in seven other cases the reflection conditions suggest a space group which does not correspond to the extension of H by the twin operation and the structure solution or at least the refinement will fail. When the twin operation belongs to a different crystal family (class II B twinning: the crystal has a specialized metric), the presence of twinning can often be recognized by the peculiar effect it has on the reflection conditions.