Thermoelectric Material SnPb₂Bi₂S₆: The ^4,4L Member of Lillianite Homologous Series with Low Lattice Thermal Conductivity

Although the binary sulfides Bi₂S₃, PbS, and SnS have attracted extensive interest as thermoelectric materials, no quaternary sulfides containing Sn/Pb/Bi/S elements have been reported. Herein, we report the synthesis of a new quaternary sulfide, SnPb₂Bi₂S₆, which crystallizes in the orthorhombic space group Pnma with unit cell parameters of a = 20.5458(12) Å, b = 4.0925(4) Å, c = 13.3219(10) Å. SnPb₂Bi₂S₆ has a lillianite-type crystal structure consisting of two alternately aligned NaCl-type structural motifs separated by a mirror plane of PbS₇ monocapped trigonal prisms. In the lillianite homologous series, SnPb₂Bi₂S₆ can be classified as ^4,4L, where the superscripted numbers indicate the maximum numbers of edge-sharing octahedra in the two adjacent NaCl-shaped slabs along the diagonal direction. The obtained SnPb₂Bi₂S₆ phase exhibited good thermal stability up to 1000 K and n-type degenerate semiconducting behavior, with a power factor of 3.7 μW cm^–1 K^–2 at 773 K. Notably, this compound exhibited a very low lattice thermal conductivity of 0.69–0.92 W m^–1 K^–1 at 300–1000 K. Theoretical calculations revealed that the low thermal conductivity is caused by the complex crystal structure and the related elastic properties of a low Debye temperature, low phonon velocity, and large Grüneisen parameters. A reasonable figure of merit (ZT) of ∼0.3 was obtained at 770 K.