Crystal Structure and Thermoelectric Properties of the ^7,7L Lillianite Homologue Pb₆Bi₂Se₉

Pb₆Bi₂Se₉, the selenium analogue of heyrovsyite, crystallizes in the orthorhombic space group Cmcm (#63) with a = 4.257(1) Å, b = 14.105(3) Å, and c = 32.412(7) Å at 300 K. Its crystal structure consists of two NaCl-type layers, A and B, with equal thickness, N₁ = N₂ = 7, where N is the number of edge-sharing [Pb/Bi]Se₆ octahedra along the central diagonal. In the crystal structure, adjacent layers are arranged along the c-axis such that bridging bicapped trigonal prisms, PbSe₈, are located on a pseudomirror plane parallel to (001). Therefore, Pb₆Bi₂Se₉ corresponds to a ^7,7L member of the lillianite homologous series. Electronic transport measurements indicate that the compound is a heavily doped narrow band gap n-type semiconductor, with electrical conductivity and thermopower values of 350 S/cm and -53 μV/K at 300 K. Interestingly, the compound exhibits a moderately low thermal conductivity, ∼1.1 W/mK, in the whole temperature range, owing to its complex crystal structure, which enables strong phonon scattering at the twin boundaries between adjacent NaCl-type layers A and B. The dimensionless figure of merit, ZT, increases with temperature to 0.25 at 673 K.