WYCKSPLIT -- A COMPUTER PROGRAM FOR DETERMINATION OF THE RELATIONS OF WYCKOFF POSITIONS FOR A GROUP - SUBGROUP PAIR.

E. Kroumova¹, J.M. Perez-Mato², M.I. Aroyo³

¹ Departamento Fisica Aplicada II, Universidad del Pais Vasco, Apto. 644, 48080 Bilbao, Spain, ² Departamento de Fisica de la Materia Condensada, Universidad del Pais Vasco, Apto. 644, 48080 Bilbao, Spain,
³ Faculty of Physics, University of Sofia, 1164-Sofia, Bulgaria

Keywords: Space groups, Wyckoff positions, group-subgroup chains.

During a continuous or nearly continuous phase transitions the symmetry of high- and low-symmetry phases are described by group-subgroup related space groups G>H. Atoms which are symmetrically equivalent under G, i.e. belong to the same orbit of G, may become non equivalent under H, ( i.e. the orbit splits) and/or their site-symmetries can be reduced. The structural relation between the group-subgroup related phases follows from the relations of the occupied orbits, and this behaviour is the same for all orbits belonging to a Wyckoff position. The aim of the program is the determination of the relations of the Wyckoff positions (known assplitting of Wyckoff positions (cf. Wondratschek H., Mineralogy and Petrology 48 (1993), 87-96.)) for a group-subgroup symmetry break G>H.

Consider a group-subgroup pair G>H and a transformation matrix relating the group and subgroup bases. Given an orbit O^G of G the program finds the Wyckoff positions of the group H into which this orbit splits. The orbit O^G can be specified either by the corresponding Wyckoff position of G, or by one of its representative coordinate triplets. WYCKSPLIT can treat group or subgroup data in unconventional settings, once the transformation matrices to the corresponding conventional settings are known.

The program can be used without local installation from any computer with www-browser via Internet. The URL of the documentation and the program itself is http://lcdx00.wm.lc.ehu.es/cryst/wpsplit.html