Substitutional effects on the thermoelectric properties of transition metal pentatellurides

The thermoelectric properties (resistivity and thermopower) of single crystals of the low-dimensional pentatelluride materials, HfTe5 and ZrTe5, have been measured as a function of temperature from 10K p 80K for HfTe5 and Tp 145K for ZrTe5. Each display a large positive (p-type) thermopower (α ≥ +125μV/K) around room temperature, which undergoes a change to a large negative (n-type) thermopower (α &le -125μV/K) below the peak temperature. The magnitude of this resistive anomaly is typically 3-7 times the room temperature value of 1 mΩ&middotcm. Through isoelectronic substitution of Zr for Hf (Hf1-xZrxTe5), a systematic shift is observed in Tp as Zr concentration increases. Small Ti substitution for Hf and Zr affects the electronic properties strongly, producing a substantial reduction in Tp for either parent compound. However, the large values of thermopower and the magnitude of the resistive peak remain essentially unchanged. Substitutions of Se or Sb on the Te sites greatly affects the electronic behavior of the parent materials. Se doping increases the thermopower values by 20% while decreasing the resistivity by as much as 25%. These effects double the power factor, α2δT, of the parent materials. Small Sb substitutions appear to completely suppress the resistive anomaly. These features in the resistivity and thermopower signal a large degree of tunability in the temperature range of operation. The potential of these materials as candidates for low temperature thermoelectric applications will be discussed.