The incommensurate misfit layer structure of (SnS)1.17NbS2, `SnNbS3':I. A study by means of X-ray diffraction

(SnS)1.17NbS2, Mr = 334.92, is a compound with misfit layer structure consisting of two-atom-thick layers of SnS and three-atom-thick sandwiches of NbS2 which alternate along the c axis. The lattices of SnS and NbS2 both have C-centered orthorhombic unit cells which match along the b and c axes but not along the a axes. The unit cells and space groups are a = 5.673 (1), b = 5.750 (1), c = 11.760 (1) Å, space group C2mb (no. 39), Z = 4, for SnS; a = 3.321 (1), b = 5.752 (1), c = 11.763 (1) Å, space group Cm2m (no. 38), Z = 2, for NbS2. From the ratio of the lengths of the a axes of the two parts of the complete structure (5.673/3.321 = 1.708 being irrational) one obtains a composition (SnS)1.17NbS2. The structure determination consisted of three parts: the structures of the SnS and NbS2 parts separately and their relative position. Intensities were measured with Mo K{\bar \alpha} radiation ([lambda] = 0.71073 Å) at T = 293 K, [mu] = 102.3 cm-1. For the SnS part RF = 0.088 for 306 unique reflections; for the NbS2 part RF = 0.031 for 329 unique reflections (for both cases 0kl reflections excluded). The relative position of these two lattices was determined from the common reflections 0kl: RF = 0.072 for 98 reflections. The SnS part of the structure consists of deformed slices of SnS with a thickness of half the cell edge of (hypothetical) NaC1-type SnS. Each Sn atom is in this way coordinated by five sulfur atoms; four sulfur atoms are in a plane perpendicular to the c axis with SnS distances 2.74 (1) (1×), 2.913 (1) (2×) and 3.01 (1) Å (1×), whereas the fifth Sn-S bond with length 2.695 (9) Å is approximately along the c axis. The NbS2 part of the structure is that of NbS2-2H with Nb in trigonal prisms of sulfur; the Nb-S distances are 2.473 (1) Å. From refinement of the common 0kl reflections the relative y and z positions of the two sublattices were found; along the common a direction the lattices of SnS and NbS2 are incommensurate