Upon cooling, ${\mathrm{Ag}}_{0.6}$${\mathrm{NbS}}_2$ shows a transition near ${\mathit{T}}_{\mathit{c}}$=124 K from an Ag-disordered phase to an incommensurately ordered phase. From single-crystal x-ray-diffraction data, the cell parameters of the basic hexagonal unit cell are determined as a=3.355(1) Å and c=14.310(1) Å; the two wave vectors describing the modulation have components with respect to the basic reciprocal-lattice vectors of the average structure of (0.175, 0.175, 0.000) and (-0.175, 0.350, 0.000), respectively. The superspace group characterizing the incommensurately modulated structure is found to be P:P$6_3$mc:6mm. Atomic coordinates and temperature factors of the basic structure as well as the fundamental (first harmonic) of the modulation functions are determined by least-squares refinement against the observed intensities. The refinements on 304 main reflections and 729 first-order satellites, with I>2.5σ(I), converged to a weighted R factor of 0.057, with partial R factors of 0.056 for the main reflections and 0.057 for the satellite reflections. It is shown that an important contribution to the modulation comes from the incommensurate ordering of Ag atoms on tetrahedrally coordinated sites of a puckered honeycomb lattice between ${\mathrm{NbS}}_2$ sandwiches. The accompanying displacements of the Nb and S atoms strongly correlate with the Ag density waves. The distance of S to occupied Ag sites is large and to unoccupied Ag sites small; the opposite is valid for the Nb-Ag distances.

• Received 19 November 1990

DOI:https://doi.org/10.1103/PhysRevB.43.9420