From Online Dictionary of Crystallography
Revision as of 10:12, 17 November 2017 by BrianMcMahon (talk | contribs) (Tidied translations and added German and Spanish (U. Mueller))
Pseudo symétrie (Fr). Pseudosymmetrie (Ge). Pseudo simmetria (It). 擬対称 (Ja). Seudosimetría (Sp).
A crystal space can in general be divided in N components S1 to SN. When a coincidence operation φ(Si)→Sj brings the ith component Si to coincide with the jth 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.