Intercalation: building natural superlattice for better thermoelectric performance in layeredchalcogenides

A natural superlattice with a composition of (SnS)1.2(TiS2)2 is built by intercalating an SnSlayer into the van der Waals gap of layered TiS2, as has been directly observed by HRTEM. Inthe direction parallel with the layers, the thermoelectric performance is found to be improved,as the electron mobility was maintained while the phonon transport is significantly suppressed,which was identified due to softening of the transverse sound velocites because of theweakened interlayer bonding. In the direction perpendicular to the layers, the lattice thermalconductivity of (SnS)1.2(TiS2)2 is even lower than the predicted minimum thermalconductivity and phonon localization due to the translational disorder of the SnS layers in thedirection parallel with the layers possibly accounts for it. We propose a large family of misfitlayer compounds (MX)1+x(TX2)n (M = Pb, Bi, Sn, Sb, Rare earth elements; T = Ti, V, Cr, Nb,Ta, X = S, Se; n = 1, 2, 3) with a natural superlattice structure as possible candidates of highperformancethermoelectric materials.