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 < T < 320K. Both parent materials exhibit a resistive transition peak, T-p approximate to 80K for HfTe5 and T-p approximate to 145K for ZrTe5. Each display a large positive (p-type) thermopower (alpha greater than or equal to + 125 mu V/K) around room temperature, which undergoes a change to a large negative (n-type) thermopower (alpha less than or equal to -125 mu V/K) below the peak temperature. The magnitude of this resistive anomaly is typically 3-7 times the room temperature value of approximate to 1 m Omega.cm. Through isoelectronic substitution of Zr for Hf (Hf1-xZrxTe5), a systematic shift is observed in T-p as the Zr concentration increases. Small Ti substitution for Hf and Zr affects the electronic properties strongly, producing a substantial reduction in T-p 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 approximate to 20% while decreasing the resistivity by as much as 25%. These effects double the power factor, alpha(2)sigma T, 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.