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Effect of Isovalent Substitution on the Electronic Structure and Thermoelectric Properties of the Solid Solution α‑As<sub>2</sub>Te<sub>3–<i>x</i></sub>Se<sub><i>x</i></sub> (0 ≤ <i>x</i> ≤ 1.5)
We report on the influence of Se
substitution on the electronic band structure and thermoelectric properties
(5–523 K) of the solid solution α-As<sub>2</sub>Te<sub>3–<i>x</i></sub>Se<sub><i>x</i></sub> (0
≤ <i>x</i> ≤ 1.5). All of the polycrystalline
compounds α-As<sub>2</sub>Te<sub>3–<i>x</i></sub>Se<sub><i>x</i></sub> crystallize isostructurally
in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12, <i>Z</i> = 4). Regardless of the Se content,
chemical analyses performed by scanning electron microscopy and electron
probe microanalysis indicate a good chemical homogeneity, with only
minute amounts of secondary phases for some compositions. In agreement
with electronic band structure calculations, neutron powder diffraction
suggests that Se does not randomly substitute for Te but exhibits
a site preference. These theoretical calculations further predict
a monotonic increase in the band gap energy with the Se content, which
is confirmed experimentally by absorption spectroscopy measurements.
Increasing <i>x</i> up to <i>x</i> = 1.5 leaves
unchanged both the p-type character and semiconducting nature of α-As<sub>2</sub>Te<sub>3</sub>. The electrical resistivity and thermopower
gradually increase with <i>x</i> as a result of the progressive
increase in the band gap energy. Despite the fact that α-As<sub>2</sub>Te<sub>3</sub> exhibits very low lattice thermal conductivity
κ<sub>L</sub>, the substitution of Se for Te further lowers
κ<sub>L</sub> to 0.35 W m<sup>–1</sup> K<sup>–1</sup> at 300 K. The compositional dependence of the lattice thermal conductivity
closely follows classical models of phonon alloy scattering, indicating
that this decrease is due to enhanced point-defect scattering