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 similar 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 similar to 2 x 10(18) cm(-3) at 300 K for PbTe to similar to 5.3 x 10(19) cm(-3) for Pb0.99La0.01Te, and similar to 1.7 x 10(20) cm(-3) for Pb0.95La0.05Te. Ag doping in Pb0.99La0.01 Te has an opposite effect on the electron carrier concentration (similar to 2.0 x 10(18) cm(-3) at 300 K for Ag0.05Pb0.99La0.01Te and similar to 1.5 x 10(19) cm(-3) for Ag0.1Pb0.99La0.01Te), 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 Ag0.05Pb0.99La0.01Te, a high power factor of similar to 22 mu W/(cm K-2) at similar to 720 K was achieved with the optimal total thermal conductivity of similar to 1.3 W/(m K) at similar to 720 K, giving a maximum figure of merit ZT approximate to 1.2 at similar to 720 K.