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 M _n+1 (Bi/Sb) ₂ Q _n+5 (n = 2-7) were synthesized by direct reactions of the elements at high temperatures. The compounds include InPbBi ₃ S ₇ (n = 2), In _0.5 Mn ₂ Bi _3.5 Se ₈ (n = 3), CdPb ₂ Bi ₄ S ₉ (n = 4), Ag _1.5 CdBi _5.5 Se ₁₀ (n = 5), Ag ₂ CdBi ₆ Se ₁₁ (n = 6), and Ag _2.5 CdSb _6.5 Se ₁₂ (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 CdPb ₂ Bi ₄ S ₉ and CdAg ₂ Bi ₆ Se ₁₁ 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 CdAg ₂ Bi ₆ Se ₁₁ and 0.73 W·m ^-1 ·K ^-1 for CdPb ₂ Bi ₄ S ₉ 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 CdPb ₂ Bi ₄ S ₉ and CdAg ₂ Bi ₆ Se ₁₁ , respectively, were attained at 775 K.