The crystal structure of stage-2 4H-Ag0.22NbS2

Second stage AgxNbS2 occurs in two different modifications, 6R-AgxNbS2 and 4H-AgxNbS2, both built of sandwiches, NbS2, with the niobium atoms in trigonal-prismatic coordination by sulfur atoms, and with the gaps between sandwiches NbS2 alternately empty and occupied by silver. 6R-AgxNbS2 (0.22 ≤ x ≤ 0.30) of which the structure is known from powder diffraction, a = 3.325 A, c = 39.59 A, space group R3-barm, has silver atoms in trigonally distorted tetrahedra of sulfur atoms. The structure of 4H-Ag0.22NbS2, prepared by vapour transport, a = 3.334 A, c = 26.09 A, space group P63mc, is determined by single crystal X-ray diffraction. In 4H-Ag0.22NbS2 the silver atoms occupy a fraction (0.43) of the octahedral sites between sandwiches NbS2; the silver atoms are ordered two-dimensionally on a √3a x √3a superlattice. A hypothetical phase 4H(c)-NbS2 (known for TaSe2) may be considered to be the host for intercalation. Crystals of both modifications show lattice disorder due to stacking faults. (author)