A superspace-group description of the commensurately modulated structure of TaTe4

Abstract

A superspace-group description of the commensurately modulated structure of TaTe4 is given on the basis of X-ray data reported by Bronsema, van Smaalen, de Boer, Wiegers, Jellinek & Mahy [Acta Cryst. (1987), B43, 305-313]. The structural model, given in that work, easily explains in the present approach all the observed extinction rules, including those which are not systematic limitations of space group P4/ncc. The proposed superspace group, eP4/ncc (00y), with = ~ and a (2a x 2a x c) basic unit I T l l Y cell, corresponds to two possible space groups, P4/ncc and P4cc, and gives a new insight into the problem of centroversus noncentrosymmetricity of the modulated structure of TaTe4. It is also shown that the superspace-group description gives practically the same R factor with fewer structural parameters as compared with the space-group method. The refinements also show that the choice of space group P4/ncc (Bronsema et al., 1987) is justified on the basis of the reduced number of parameters needed for the structure description. This reduction does not, at least significantly, increase the discrepancy factor R.