Exploration and Optimization of Tellurium‐Based Thermoelectrics

The power factor for these compounds is displayed on the right side of Figure 12.2. All measured
power factors rise in a similar fashion to the Seebeck coefficients with Bi‐heavy compounds typically
achieving the higher values which is exemplified by Tl9Sn0.2Bi0.8Te6 with a room temperature PF of 4.23
µW∙cm ‐1 K ‐2 which increases to 8.08 µW∙cm ‐1 K ‐2 at 587 K. Since , increases in S can be quite
significant, as is the case here. With Tl9Sn0.2Bi0.8Te6 having S = ~80 W∙cm ‐1 K ‐2 at room temperature –
double the value of Tl9Sn0.8Bi0.2Te6 – S 2 therefore overpowers its lower σ value.
12.3.3.2. Tl10‐x‐ySnxBiyTe6
The Seebeck coefficient values of compounds with composition Tl8.67Sn1.33‐yBiyTe6 increase in a
similar fashion as the previous set of p‐type semiconductors. Like its counterparts, these gently
increasing Seebeck coefficient values have the lowest coefficient encountered with a larger [Sn] – in this
case 68 V∙K ‐1 at room temperature increasing to 116 V∙K ‐1 at 563 K – and, following the pattern of
increasing [Bi], leads to 113 V∙K ‐1 at room temperature to 175 V∙K ‐1 at 614 K for Tl8.67Sn0.50Bi0.83Te6.
With the exception of the slightly higher‐than‐expected numbers for Tl8.67Sn0.70Bi0.63Te6, (based on the
earlier‐mentioned single crystal and EDX study of this composition leading to “Tl8.67Sn0.67Bi0.66Te6”, this
slight break in the trend can be attributed to synthetic errors leaving both samples with near‐identical
compositions,) the same behaviour trends are observed in this case – and displayed in Figure 12.3 (left).
Upon studying the Tl8.33Sn1.67‐yBiyTe6 phase, the same observations can be made as this sample contains
a larger quantity of Sn versus Bi and therefore sits at a similar magnitude to the lower y values in
Tl8.67Sn1.33‐yBiyTe6. For reference, Tl8Sn2Te6 from the previous chapter is shown here as this series has a
lower Tl:Sn ratio than Tl9SnTe6: The higher quantity of Sn (versus Tl or Bi) places this value at the
bottom of the figure with 50 V∙K ‐1 (300 K), following the general trend in this graph.
Figure 12.3 ZEM measurements for Tl 10‐x‐ySn xBi yTe 6:
solid Seebeck coefficient and dashed electrical conductivity (left), and power factor (right).
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