Six Quaternary Chalcogenides of the Pavonite Homologous Series with Ultralow Lattice Thermal Conductivity

Six new quaternary chalcogenides belonging to the pavonite sulfosalt mineral family with the general formula Mn+1(Bi/Sb)2Qn+5 (n = 2–7) were synthesized by direct reactions of the elements at high temperatures. The compounds include InPbBi3S7 (n = 2), In0.5Mn2Bi3.5Se8 (n = 3), CdPb2Bi4S9 (n = 4), Ag1.5CdBi5.5Se10 (n = 5), Ag2CdBi6Se11 (n = 6), and Ag2.5CdSb6.5Se12 (n = 7) and crystallized in the monoclinic space group C2/m. The pavonite structure consists of two alternating slabs, a thinner slab composed of pairs of monocapped trigonal prisms separated by octahedra and a thicker slab with a galena-like structure motif. In the general formula, n corresponds to the number of octahedra along the diagonal direction of the galena-like slab. The complex compositions, mixed occupancies of the cations, and quasi-two-dimensional structures endow compounds of this family with extremely low thermal conductivity. The charge and thermal transport properties of CdPb2Bi4S9 and CdAg2Bi6Se11 were characterized from 300 to 810 K, and both materials exhibited n-type semiconductor behaviors and ultralow lattice thermal conductivities of less than 0.35 W·m–1·K–1 for CdAg2Bi6Se11 and 0.73 W·m–1·K–1 for CdPb2Bi4S9 in the measured temperature range. Density functional theory calculations revealed the origins of this low lattice thermal conductivity to be a combination of the low Debye temperature, small phonon velocities, and large Grüneisen parameters. Thermoelectric properties were measured, and the highest ZT values of 0.53 and 0.95 for undoped CdPb2Bi4S9 and CdAg2Bi6Se11, respectively, were attained at 775 K.