A superspace group description of the misfit layer structure of (SnS)1.17(NbS2)

The misfit layer compound (SnS) 1.17 NbS 2 belongs to the class of intergrowth compounds. In the simplest approximation, the structure can be described as that of two interpenetrating sublattices. The true structure consists of two interpenetrating, incommensurately modulated structures. In this paper the theory of superspace groups is applied to (SnS) 1.17 NbS 2 . It is shown that the symmetry is fully characterised by the (3+1)-dimensional superspace group P 11s Cm2m ( alpha 00). Definitions of the component structure space groups and superspace groups are given. For the NbS 2 part they are found to be G 1 =Cm2m and G s 1 =P 11s Cm2m . For the SnS part the authors obtain G 2 =Cm2a and G s 2 =P 111 Cm2a . It is shown that this unified approach gives more information about the symmetry than when the subsystems are considered independently. One result is that in a structure refinement fewer possibilities have to be considered. Finally, the superspace group is used to derive the symmetry restrictions on the modulation functions of both subsystems.