Thermoelectric Properties and Nanostructuring in the p-Type Materials NaPb18-xSnxMTe20 (M = Sb, Bi)

The thermoelectric properties of materials with nominal compositions NaPb18-xSnxMTe20 (M = Sb, Bi) were investigated in the temperature range 300-650 K. All the members of NaPb18-xSnxMTe20 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., NaPb15Sn3BiTe20 and NaPb13Sn5SbTe20, show high carrier concentrations of similar to 1 x 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 NaPb18BiTe20 reaching a ZT approximate to 1.3 at 670 K. The NaPb18-xSnxMTe20 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 NaPb18-xSnxSbTe20. The lowest lattice thermal conductivity, similar to 0.4 W/(m K), was observed for the composition NaPb2Sn16BiTe20 at 650 K. High-resolution transmission electron microscopy on several members of the NaPb18-xSnxMTe20 series reveal that they are inhomogeneous on the nanoscale with widely dispersed nanocrystals embedded in a Pb1-ySnyTe matrix. Also observed are lamellar features in these materials associated with compositional fluctuations and significant strain at the nanocrystal/matrix interface.