Thermoelectric Properties and Nanostructuring in the p-Type Materials NaPb_18−xSn_xMTe₂₀ (M = Sb, Bi)

The thermoelectric properties of materials with nominal compositions NaPb 18− x Sn x MTe 20 (M = Sb, Bi) were investigated in the temperature range 300−650 K. All the members of NaPb 18− x Sn x MTe 20 have a cubic rock-salt (NaCl) type structure and exhibit p-type charge transport behavior between 300−650 K. The relative fraction of Sn strongly affects the physical, structural, and transport properties of the materials. Independent of the nature of the pnicogen atom (M), the electrical conductivity increases with decreasing Pb:Sn ratio, whereas the thermopower decreases. Hall effect data for selected samples, e.g., NaPb 15 Sn 3 BiTe 20 and NaPb 13 Sn 5 SbTe 20, show high carrier concentrations of ∼1 × 10 20 cm −3 at room temperature. Comparing corresponding members from the antimony and bismuth series, we observed that the Sn-free compositions ( x = 0) exhibit the highest power factors, and as a consequence, the highest ZT, with NaPb 18 BiTe 20 reaching a ZT ≈ 1.3 at 670 K. The NaPb 18− x Sn x MTe 20 series exhibit increasing total thermal conductivity with increasing fraction of Sn with room temperature values between 1.37 W/(m K) for x = 3 and 3.9 W/(m K) for x = 16 for NaPb 18− x Sn x SbTe 20 . The lowest lattice thermal conductivity, ∼0.4 W/(m K), was observed for the composition NaPb 2 Sn 16 BiTe 20 at 650 K. High-resolution transmission electron microscopy on several members of the NaPb 18− x Sn x MTe 20 series reveal that they are inhomogeneous on the nanoscale with widely dispersed nanocrystals embedded in a Pb 1− y Sn y Te matrix. Also observed are lamellar features in these materials associated with compositional fluctuations and significant strain at the nanocrystal/matrix interface.