Natural Superlattice Material: TiS ₂ -Based Misfit-Layer Compounds

Layered titanium disulfide has been reported to show a high power factor due to its two-dimensional electronic state. However, its high thermal conductivity makes the conversion efficiency too small for application. Our strategy is to intercalate a layer of BiS, SnS or PbS into the van der Waals gap of the TiS ₂ layers to form natural superlattices with a general formula (MS) 1+x (TiS 2)n (M= Bi, Sn, Pb; n = 1, 2). It has been found that the lattice thermal conductivity was significantly reduced after intercalation, which is close to or even lower than the calculated minimum thermal conductivity. Measurement of sound velocities shows that the ultra-low thermal conductivity partially originates from the softening of the transverse modes of lattice wave due to the low shear modulus between the hetero-layers. Furthermore, various planer defects including translational displacement and stacking faults are found in those misfit layer compounds and further reduce the lattice thermal conductivity. Meanwhile, electron transfer from the MS layer to the TiS ₂ layer deteriorates the thermoelectric performance by reducing the power factor and increasing the electronic thermal conductivity. The SnS intercalation compound (SnS) _1.2 (TiS ₂) ₂ shows the least electron transfer and the ZT value reaches 0.37 at 700 K. Reduction in the carrier concentration in these misfit layer compounds is required to achieve higher ZT value. Moreover, we propose a large family of misfit layer compounds (MX) _{1 + x} (TX ₂) _n (M=Pb, Bi, Sn, Sb, Rare earth elements; T=Ti, V, Cr, Nb, Ta, X=S, Se; n= 1, 2, 3) with natural superlattice structures for possible candidates for high-performance thermoelectric materials, including both n-type and p-type.