Inorganic Chemistry, Vol.58, No.2, 1339-1348, 2019
Thermoelectric Material SnPb2Bi2S6: The L-4,L-4 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) angstrom, b = 4.0925(4) angstrom, c = 13.3219(10) angstrom. SnPb2Bi2S6 has a lillianite-type crystal structure consisting of two alternately aligned NaCl-type structural motifs separated by a mirror plane of PbS, monocapped trigonal prisms. In the lillianite homologous series, SnPb2Bi2S6 can be classified as L-4,L-4, 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 mu W 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 Griineisen parameters. A reasonable figure of merit (ZT) of similar to 0.3 was obtained at 770 K.