Addition of Co, Ni, Fe and their role in the thermoelectric properties of colusite Cu26Nb2Ge6S32

We have investigated the effect of Co, Ni, and Fe addition on the thermoelectric properties in colusite Cu26Nb2Ge6S32. The samples of Cu26Nb2Ge6TxS32 with T = Co, Ni, Fe for x <= 1.0 were prepared by melting mixtures of their constituent elements in evacuated quartz tubes at 1323 K followed by hot pressing at 973 K. The addition of Co, Ni, and Fe does not significantly affect the lattice parameter; the additive atoms partially substitute for Cu, Nb and Ge atoms. The expelled Cu, Nb and Ge atoms form microscale precipitates. While the addition of Co increases the hole carrier concentration and decreases the electrical mobility, the addition of Ni has a small effect on the electrical transport properties. A low electrical resistivity and high thermoelectric power factor are maintained with the addition of Co and Ni. On the other hand, the addition of Fe decreases both the hole carrier concentration and electrical mobility, leading to high electrical resistivity and low power factor. A little effect of the addition of Co, Ni, and Fe on the lattice thermal conductivity was found; all the samples exhibit low lattice thermal conductivity (similar to 0.4 W K-1 m(-1) at 665 K) because of the complex crystal structure of colusites. High p-type ZT of approximately 0.7 at 665 K is achieved in Cu26Nb2Ge6S32, Cu26Nb2Ge6CoxS32, and Cu26Nb2Ge6Ni0.5S32. (C) 2017 Elsevier B.V. All rights reserved.