An approach to the structure of incommensurately modulated NbS3 type II

NbS3 type I is commensurately modulated and the structure is known. NbS3 type II is incommensurately modulated and the structure has not been determined. Starting from a superspace-group description of NbS3 type I, a proposal is presented for the modulation unit cell and superspace group for NbS3 type II which meets the following criteria: (i) the basic structure unit cell is approximately a simple multiple of the unit cell of the NbS3 type I basic structure; (ii) the Fermi level, deduced for a simple bonding electron count, is in agreement with the observed incommensurate superlattice spot positions; and (iii) the modulation superspace group and the corresponding dualistic notation is as given, and leads to extinction rules that are the same as those deduced from electron diffraction patterns. A normal-mode analysis, which is independent of the above, has also been carried out. This approach leads to extinction rules and modulation symmetry conditions that are identical to those deduced from the superspace-group analysis. A phase transition that occurs in the type-II modulation structure at about 350 K is interpreted on the basis of the crystallographic description.