Improvement in the Thermoelectric Figure of Merit by La/Ag Cosubstitution in PbTe

The thermoelectric properties of La-doped and Ag/La codoped PbTe were investigated in the temperature range of 300 to ∼720 K. All samples crystallize in the NaCl-type structure without noticeable secondary phase and exhibit narrow bandgaps of 0.26−0.30 eV, typical of PbTe. La doping (≤5 at %) in PbTe significantly enhances the room temperature electrical conductivity to >5000 S/cm, indicating that La is an efficient electron donor. Hall coefficient data confirm the significant increase in electron carrier concentration from ∼2 × 10 18 cm −3 at 300 K for PbTe to ∼5.3 × 10 19 cm −3 for Pb 0.99 La 0.01 Te, and ∼1.7 × 10 20 cm −3 for Pb 0.95 La 0.05 Te. Ag doping in Pb 0.99 La 0.01 Te has an opposite effect on the electron carrier concentration (∼2.0 × 10 19 cm −3 at 300 K for Ag 0.05 Pb 0.99 La 0.01 Te and ∼1.5 × 10 19 cm −3 for Ag 0.1 Pb 0.99 La 0.01 Te), consistent with the role of Ag as electron acceptor. The temperature dependence of mobility is discussed in detail using the physics of electron scattering and possible scattering mechanisms. For Ag 0.05 Pb 0.99 La 0.01 Te, a high power factor of ∼22 μW/(cm K 2 ) at ∼720 K was achieved with the optimal total thermal conductivity of ∼ 1.3 W/(m K) at ∼720 K, giving a maximum figure of merit ZT ≈ 1.2 at ∼720 K.