X-RAY STRUCTURE DETERMINATION AND SUPERCONDUCTIVITY OF A NEW LAYERED MISFIT COMPOUND WITH A FRANCKEITE-LIKE STACKING, [(PB,SB)S](2.28)NBS2

The crystal structure of [(Pb,Sb)S](2.28)NbS2 has been determined. Thi s synthetic compound belongs to the layer misfit family with the struc ture type of franckeite. Its structure is built up from two [(Pb,Sb)S] layers (noted ''Q'') alternating with one [NbS2] layer (noted ''H'') along the stacking direction ((c) over right arrow axis). The cell par ameters of this composite structure are: b=5.8285(9) Angstrom, c=17.64 9(6) Angstrom, alpha=86.41(2)degrees, beta=86.55(3)degrees, gamma=89.9 7(2)degrees: Q sublattice a=5.864(2) Angstrom: H sublattice a=3.33 Ang strom. Antimony atoms are specifically located at the interface within the two ''Q'' layers as in natural franckeite. Stereochemical activit y of Sb3+ (with its lone electron pair) stabilizes the double ''Q'' en tity which has the SnS structure, giving the following structural form ulation: [PbS(Pb0.6Sb0.4)S](1.14)NbS2. A superconducting transition is observed at T-c=1.05 K, a temperature lower than that observed for th e corresponding 1Q/1H misfit derivative. This feature may be related t o a decrease of the oxidation state for Nb, from +4 in the stoichiomet ric binary 2H-NbS2 compound (T-c =6.2 K) to partially +3 in this compo und. Crystal chemical constraints which govern the stabilization of fr anckeite-type compounds are discussed. (C) 1997 Elsevier Science S.A.