| Summary: | We report the synthesis and thermoelectric transport properties of As-doped layered pnictogen oxyselenides NdO<sub>0.8</sub>F<sub>0.2</sub>Sb<sub>1−<i>x</i></sub>As<i><sub>x</sub></i>Se<sub>2</sub> (<i>x</i> ≤ 0.6), which are predicted to show high-performance thermoelectric properties based on first-principles calculation. The crystal structure of these compounds belongs to the tetragonal <i>P</i>4/<i>nmm</i> space group (No. 129) at room temperature. The lattice parameter <i>c</i> decreases with increasing <i>x</i>, while <i>a</i> remains almost unchanged among the samples. Despite isovalent substitution of As for Sb, electrical resistivity significantly rises with increasing <i>x</i>. Very low thermal conductivity of less than 0.8 Wm<sup>−1</sup>K<sup>−1</sup> is observed at temperatures between 300 and 673 K for all the examined samples. For As-doped samples, the thermal conductivity further decreases above 600 K. Temperature-dependent synchrotron X-ray diffraction indicates that an anomaly also occurs in the <i>c</i>-axis length at around 600 K, which may relate to the thermal transport properties.
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