Low lattice thermal conductivity in Pb5Bi6Se14, Pb3Bi2S6, and PbBi2S4: promising thermoelectric materials in the cannizzarite, lillianite, and galenobismuthite homologous series

The thermoelectric properties of Pb5Bi6Se14, a member of the cannizzarite homologous series; Pb3Bi2S6, a member of the lillianite homologous series; and PbBi2S4, a member of the galenobismuthite homologous series were investigated over the temperature range of 300 K to 723 K. The samples were synthesized by a solid state reaction of the binary precursors PbQ and Bi(2)Q(3) (Q = S and Se) in evacuated and sealed quartz tubes, followed by pulsed electric current sintering. The crystal structure of Pb5Bi6Se14 consists of alternating two-dimensional infinite layers of PbSe and Bi2Se3. In the Pb5Bi6Se14 sintered compacts, the ab-plane was preferentially oriented perpendicular to the pressing direction, resulting in highly anisotropic electrical and thermal transport properties. The crystal structure of Pb3Bi2S6 is formed by stacking NaCl-type (Pb/Bi) S layers with a mirror as twinning operation, while that of PbBi2S4 consists of the NaCl-type and Bi2S3-type strips (broken layers) of finite widths. The crystal grains of Pb3Bi2S6 and PbBi2S4 were grown randomly, leading to nearly isotropic electrical and thermal transport properties in the sintered compacts. For all the samples, an n-type degenerate semiconductor-like behavior was found, providing a notable thermoelectric power factor of similar to 3.0 mu W K-2 cm(-1) at 705 K for Pb5Bi6Se14, similar to 2.4 mu W K-2 cm(-1) at 715 K for Pb3Bi2S6, and similar to 2.6 mu W K-2 cm(-1) at 515 K for PbBi2S4 in a direction perpendicular to the pressing direction. Moreover, these materials exhibited effective phonon scattering, presumably at the interfaces between the layers, leading to extremely low lattice thermal conductivity in the range of 0.29 W K-1 m(-1) to 0.80 W K-1 m(-1) over the temperature range of 300 K to 723 K. The highest ZT of similar to 0.46 at 705 K was observed in Pb5Bi6Se14 for the ab-plane direction.