This lecture/exercise will illustrate how the use of the programs of the Bilbao Crystallographic Server can facilitate the structure determination of twinned crystal structures. In addition, it will be shown, how the resulting structure can be related to other isotypical compounds using group-subgroup relationships.

Starting from the symmetry of the archetype structure and from the diffraction pattern of a twinned crystal, it will be shown how the programs MAXSUB, CELLSUB and SUBGROUPGRAPH can be of help to identify the underlying domain structure and to derive the corresponding twin laws.

After successful determination of the structure of the twinned crystal, the analysis of its pseudosymmetry using the programs PSEUDO and DOPE will reveal the close relationship between twinning and pseudosymmetry.

Finally, the determined structure is related to isotypical compounds with the help of the programs SUBGROUPGRAPH, EQUISTRU, TRANSTRU, CELLTRAN and WYCKSPLIT. In particular, it will be shown, how the program WYCKSPLIT can be used to understand symmetry changes related to order/disorder processes.

Supplementary Material