Misfit layer compounds family (MS) TS (M = Sn, Pb, Bi, rare earth element; T = Nb, Ta; n = 1.08-1.19)

Compounds (MS) n TS 2, where M is a rare earth element, tin, lead or bismuth and T is a transition metal, constitute a new family of incommensurate materials. They are built up from a succession of| MS| and| TS 2 | slabs. Each| MS| slab is a distorted NaCl type double layer, where M surrounding is a distorted square pyramid of S atoms. Tcoordination is trigonal prismatic in| TS 2 | sandwich, which is very similar to those of 2H-TS 2 structures. X-ray diffraction of each type of slab shows that one intralayer lattice parameter of| MS| slab is incommensurate with the corresponding one of a| TS 2 | slab. Both layers are then commensurate along the stacking axis and one perpendicular direction, and misfitted in any other direction. This misfit is directly responsible for the formulation, e.g. (LaS) 1.14 NbS 2 or (SnS) 118 NbS 2. As is shown for (LaS) NbS, three different approaches were used for the structural determination of these compounds by X-ray diffraction: (i) composite approach, which separates both sublattices (ii) supercell approach, in a commensurate approximation (iii) superspace groups approach. The very anisotropic electrical transport and magnetic properties can be interpreted by considering that| MS | slabs are intercalated in a| TS 2 | structure with some charge transfer. These compounds are low-Tc (some K) superconductors. Nevertheless, these members of the very wide misfit layer compounds family are very interesting as a tool for investigations of interactions leading to incommensurability. © 1991, Taylor & Francis Group, LLC. All rights reserved.