High thermoelectric figure of merit in nanostructured p-type PbTe–MTe (M = Ca, Ba)

We demonstrate that by endotaxially embedding nanocrystals of the alkaline earth tellurides CaTe or BaTe in PbTe doped with Na2Te heat carrying phonons can be strongly scattered with little effect on the power factor. An intriguing observation is that the insertion of CaTe in PbTe increases the solubility of Na2Te, thereby allowing high levels of carrier concentration. Powder X-ray diffraction patterns reveal that all samples crystallize in the NaCl-type structure without noticeable secondary phase, while scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) demonstrate the presence of endotaxially coherent/semi-coherent nanostructures in the PbTe–CaTe/BaTe samples. We show that this is the principal factor for the very low thermal conductivity. Electron energy loss spectroscopy (EELS) confirms the presence of Ca in the nanoprecipitates. We present a comparison in the lattice thermal conductivity in going from nanocrystals of CaTe to SrTe to BaTe in PbTe. Temperature dependent Hall coefficient measurements of PbTe–CaTe materials suggest that the carrier population in the so-called heavy hole valence band in PbTe becomes important above 450 K and is responsible for the high power factors. The highest thermoelectric figure of merit, ZT, value achieved is ∼1.5 at 765 K for 6% CaTe containing sample, while 3% BaTe containing sample shows the ZT of ∼1.3 at 760 K.