A new layered chalcogenide of the misfit type in the Pb-Fe-Nb-S system: (Pb2FeS3)(0.58)NbS2

A new layered chalcogenide of the misfit type has been obtained in the system Pb-Fe-Nb-S. Preliminary studies (electron probe microanalysis, X-ray diffraction and electron microscopy) indicate that this phase is a new misfit homologue, '1.5Q/1H', with the regular alternation of one pseudo-hexagonal NbS2 layer (H) and a three-atom-thick, pseudo-quadratic layer (Q) close to Pb2FeS3. Unit cell parameters are (Angstrom): a(Q)=5.773(1), a(H)=3.3461(7), b(Q)=b(H)=5.803(1), c(Q)=c(H)=14.106(2), alpha = beta = gamma = 90 degrees. Iron fills the inner atomic sheer of the Q layer giving the simplified formula (Pb2FeS3)(0.58)NbS2. Iron plays a specific crystal chemical role that permits the stabilization of this new homologue type, like antimony for the franckeite type, '2Q/1H'. This specific three-atom-thick Q layer has been recognized in other chalcogenides and oxychalcogenides.