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 Bi₂Q₃ (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 ∼3.0 μW K^-2 cm^-1 at 705 K for Pb₅Bi₆Se₁₄, ∼2.4 μW K^-2 cm^-1 at 715 K for Pb₃Bi₂S₆, and ∼2.6 μW 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 ∼0.46 at 705 K was observed in Pb₅Bi₆Se₁₄ for the ab-plane direction.