Thermoelectric properties and nanostructuring in the p-type materials NaPb _18-xSn _xMTe ₂₀ (M = Sb, Bi)

The thermoelectric properties of materials with nominal compositions NaPb _18-xSn _xMTe ₂₀ (M = Sb, Bi) were investigated in the temperature range 300-650 K. All the members of NaPb _18-xSn _xMTe 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 ₁₅Sn ₃BiTe ₂₀ and NaPb ₁₃Sn ₅SbTe 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 ₁₈BiTe ₂₀ reaching a ZT ≈ 1.3 at 670 K. The NaPb _18-xSn _xMTe 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-xSn _xSbTe ₂₀. The lowest lattice thermal conductivity, ̃0.4 W/(m K), was observed for the composition NaPb ₂Sn ₁₆BiTe ₂₀ at 650 K. High-resolution transmission electron microscopy on several members of the NaPb _18-xSn _xMTe ₂₀ series reveal that they are inhomogeneous on the nanoscale with widely dispersed nanocrystals embedded in a Pb _1-ySn _yTe matrix. Also observed are lamellar features in these materials associated with compositional fluctuations and significant strain at the nanocrystal/matrix interface.