Effect of rare earth doping on the thermoelectric and electrical transport properties of the transition metal pentatelluride HfTe5

The transition metal pentatellurides HfTe₅ and ZrTe₅ have been observed to possess high thermoelectric power factors and anomalous electrical transport behavior. The temperature dependence of the resistivity is semimetallic except for a large resistive peak as a function of temperature at around 75 K for HfTe₅ and 145 K for ZrTe₅. At a temperature corresponding to this peak, the thermopower crosses zero as it moves from large positive values to large negative values. Previous doping studies have shown profound and varied effects on the anomalous transport. In this study we investigate the effect on the electrical resistivity, thermopower, and magnetoresistance of doping HfTe₅ with rare-earth elements. Doping with rare-earth elements of increasing atomic number leads to a systematic suppression of the anomalous transport behavior and large magnetoresistive effect observed in the parent compound. Rare-earth doping also leads to an enhancement of the thermoelectric power factor over previously studied pentatellurides. For nominal Hf_0.75Nd_0.25Te₅ and Hf_0.75Sm_0.25Te₅, values more than a factor of 2 larger than that of the commonly used thermoelectric material Bi₂Te₃ were observed.