Disorder and thermal motion of Ag in Ag0.6NbS2

Refinement of single crystal X-ray diffraction data of the two-dimensional silver conductor Ag0.6NbS2, measured in the temperature range 295-695 K, shows significant anharmonicity in the thermal motion of Ag. The plane intermediate between the two sublattices of Ag (alpha and beta), forming together a puckered honeycomb lattice, plays an important role in the conduction process. The alpha- and beta-sites are equally occupied for 0.30 already at room temperature. The diffusion path between the approximately tetrahedrally coordinated alpha- and beta-sites goes via the octahedron formed by sulfur atoms of neighbouring sandwiches NbS2, but the center of this octahedron is avoided. The behaviour of the thermal expansion is anisotropic, but disagrees with data known for comparable 2-dimensional ionic conductors. The expansion along the unique axis is much larger than perpendicular to it, and seems not related to anharmonic thermal vibrations in that direction.