REFINEMENT STRATEGIES FOR TWINNED CRYSTAL STRUCTURES

Michael Bolte

Institut für Organische Chemie, J.-W.-Goethe-Universität, Marie-Curie-Str. 11, 60439 Frankfurt/Main, Germany. E-mail: bolte@chemie.uni-frankfurt.de

Keyword: Twinning

Twinning is a crystal growth anomaly in which the specimen is composed of separate crystal domains whose orientations differ in a specific way. This leads to an overlap of the reciprocal lattices of the different components, which complicates the structure determination, because one has to find out the geometrical relationship of the twin components, which is in other words the twin law.

Twins can be classified according to the kind of overlap: Merohedral twins are characterized by an exact overlap of the reciprocal lattices of the different domains. Pseudo-merohedral twins show only a partial overlap of the different reciprocal lattices, whereas in non-merohedral twins only some points of the reciprocal lattices are affected by overlap.

With three examples of merohedral, pseudomerohedral and non merohedral twinning some hints are given for the detection of twinning, and it will be shown how a twin law can be derived and how to set up the refinement instructions for SHELXL-97 [1].

Whereas it is possible to refine some twinned structures neglecting the twinning (when only few reflections are affected), it is in other cases indispensible to take the twinning into account. The virtue of correct treatment of twinning will be demonstrated by comparing the twin refinement with other procedures.

1. G. M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal Structures. Univ. of Göttingen, Germany (1997)