Thermoelectric Properties of the As/P-Based Zintl Compounds EuIn ₂ As _{2– x} P _x ( x = 0–2) and SrSn ₂ As ₂

Zintl compounds containing Sb have been studied extensively because of their promising thermoelectric properties. In this study, we prepared As/P-based Zintl compounds, EuIn2As2–xPx (x = 0–2) and SrSn2As2, and examined their potential for use as thermoelectric materials. These compounds show p-type polarity with Hall carrier concentrations of ∼1019 cm–3 for EuIn2As2–xPx and ∼1021 cm–3 for SrSn2As2 at 300 K. The high carrier concentration of SrSn2As2 is likely due to self-doping by hole-donating Sn vacancies. The electrical power factor reaches ∼1 mW m–1 K–2 at ∼600 K for EuIn2As2–xPx with x = 0.1 and 0.2, which is almost twice that of the end-member compounds (x = 0 and 2). The lattice thermal conductivity κl is determined to be 1.6–2.0 W m–1 K–1 for EuIn2As2 and SrSn2As2 and 2.8 W m–1 K–1 for EuIn2P2 at 673 K. The dimensionless figure of merit reaches ZT = 0.29 at 773 K for EuIn2As2–xPx with x = 0.2, owing to the optimized carrier concentration and/or electronic structure, as well as reduced lattice thermal conductivity in the solid solution. First-principles calculations show that EuIn2As2 and SrSn2As2 are topologically nontrivial materials with band inversion, while EuIn2P2 is a conventional semiconductor with a band gap. The present study demonstrates that As/P-based Zintl compounds can also show promising thermoelectric properties, thus expanding the frontier for efficient thermoelectric materials.