Increasing Seebeck Coefficients and Thermoelectric Performance of Sn/Sb/Te and Ge/Sb/Te Materials by Cd Doping

Ge and Sn in (GeTe)(n)Sb2Te3 (GST) and (SnTe)(n)Sb2Te3 compounds, respectively, are partially substituted by Cd. These layered compounds (n = 1) consist of NaCl-type slabs that are separated by van der Waals gaps. Resonant X-ray diffraction yields different Cd-atom distributions in Cd0.2Ge0.8Sb2Te4 and Cd0.2Sn 0.8Sb2Te4, which indicate how van der Waals gaps in metastable cubic GeTe-rich or SnTe-rich compounds (n = 7) are surrounded. The latter exhibit promising thermoelectric properties with figures of merit ZT approximate to 0.8 at 450 degrees C. Alloying with Sb2Te3 increases the ZT value of SnTe by a factor of up to two because of an increased Seebeck coefficient and a reduced thermal conductivity; the hole concentration is significantly reduced. A further improvement in ZT by approximate to 20% is possible by Cd-doping, most likely due to a more covalent bonding character, yielding a ZT of approximate to 1.1 at 450 degrees C for Cd1.2Sn10.8Sb2Te15. In order to understand the electronic transport properties of these p-type semiconductors, the single parabolic band model is applied. By contrast, Cd doping in GST materials reduces the Hall mobility and therefore decreases the power factor compared to undoped GST materials.