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Pseudo symmetry

From Online Dictionary of Crystallography

Revision as of 14:11, 20 September 2016 by MassimoNespolo (talk | contribs) (typo (wrong sinograph))

Pseudo symétrie (Fr); Pseudo simmetria (It); 擬対称 (Ja)


A crystal space can in general be divided in N components S1 to SN. When a coincidence operation φ(Si)→Sj brings the i-th component Si to coincide with the j-th component Sj, for any i and j, φ is a symmetry operation of the space group.

Sometimes, φ brings Si close to, but not exactly on, the position and orientation of Sj: in this case the operation mapping Si onto Sj is not crystallographic but the linear and/or rotational deviation from a space group operation is limited. For this reason, it is preferable to describe the crystallographic operation φ as a pseudo symmetry operation.

Pseudo symmetry operations for the lattice play an important role in twinning, namely in the case of twinning by pseudomerohedry and twinning by reticular pseudomerohedry.