High <i>ZT</i> in p‑Type (PbTe)<sub>1–2<i>x</i></sub>(PbSe)<sub><i>x</i></sub>(PbS)<sub><i>x</i></sub> Thermoelectric Materials
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Abstract
Lead chalcogenide thermoelectric
systems have been shown to reach
record high figure of merit values via modification of the band structure
to increase the power factor or via nanostructuring to reduce the
thermal conductivity. Recently, (PbTe)<sub>1–<i>x</i></sub>(PbSe)<sub><i>x</i></sub> was reported to reach high
power factors via a delayed onset of interband crossing. Conversely,
the (PbTe)<sub>1–<i>x</i></sub>(PbS)<sub><i>x</i></sub> was reported to achieve low thermal conductivities
arising from extensive nanostructuring. Here we report the thermoelectric
properties of the pseudoternary 2% Na-doped (PbTe)<sub>1–2<i>x</i></sub>(PbSe)<sub><i>x</i></sub>(PbS)<sub><i>x</i></sub> system. The (PbTe)<sub>1–2<i>x</i></sub>(PbSe)<sub><i>x</i></sub>(PbS)<sub><i>x</i></sub> system is an excellent platform to study phase competition
between entropically driven atomic mixing (solid solution behavior)
and enthalpy-driven phase separation. We observe that the thermoelectric
properties of the PbTe–PbSe–PbS 2% Na doped are superior
to those of 2% Na-doped PbTe–PbSe and PbTe–PbS, respectively,
achieving a <i>ZT</i> ≈2.0 at 800 K. The material
exhibits an increased the power factor by virtue of valence band modification
combined with a very reduced lattice thermal conductivity deriving
from alloy scattering and point defects. The presence of sulfide ions
in the rock-salt structure alters the band structure and creates a
plateau in the electrical conductivity and thermopower from 600 to
800 K giving a power factor of 27 μW/cmK<sup>2</sup>. The very
low total thermal conductivity values of 1.1 W/m·K of the <i>x</i> = 0.07 composition is accounted for essentially by phonon
scattering from solid solution defects rather than the assistance
of endotaxial nanostructures