Thermoelectric Properties of n-Type Bi₄O₄SeX₂ (X = Cl, Br)

The multiple anion superlattice Bi₄O₄SeCl₂ has been reported to exhibit extremely low thermal conductivity along the stacking c-axis, making it a promising material for thermoelectric applications. In this study, we investigate the thermoelectric properties of Bi₄O₄SeX₂ (X = Cl, Br) polycrystalline ceramics with different electron concentrations by adjusting the stoichiometry. Despite optimizing the electric transport, the thermal conductivity remained ultra-low and approached the Ioffe-Regel limit at high temperatures. Notably, our findings demonstrate that non-stoichiometric tuning is a promising approach for enhancing the thermoelectric performance of Bi₄O₄SeX₂ by refining its electric transport, resulting in a figure of merit of up to 0.16 at 770 K.