Optimizing Thermoelectric Performance in Phase‐Stabilized n‐Type BiSbSe ₃

Orthorhombic n‐type BiSbSe 3 possesses a theoretically promising thermoelectric performance in single‐crystalline form. However, orthorhombic BiSbSe 3 is stable only at high temperatures but thermodynamically unstable at room temperature. While orthorhombic BiSbSe 3 polycrystals can be synthesized via rapid quenching to kinetically preserve this metastable phase, single‐crystal growth demands slow cooling to enable near‐equilibrium atomic incorporation at the growth front. Resolving the room‐temperature thermodynamic instability of orthorhombic BiSbSe 3 is an essential precondition for unlocking its thermoelectric potential. In this work, we successfully realized thermodynamic stabilization of the orthorhombic BiSbSe 3 at room temperature through chemical composition optimization. Alloying with 15% sulfur (S) enables reproducible, quench‐free synthesis of pure orthorhombic BiSbSe 3 , opening a viable pathway toward single‐crystal growth and full realization of its thermoelectric potential. Building on this, we conducted a systematic investigation into the effects of halogen (Cl, Br, and I) doping on thermoelectric transport of n‐type phase‐stabilized orthorhombic BiSbSe 3 polycrystals. The optimized BiSbSe 2.52 S 0.45 Br 0.03 achieves a peak ZT of ∼1.0 at 773 K and a single‐leg conversion efficiency of ∼6.1% at a temperature difference of ∼474 K, demonstrating that our phase‐stabilization approach does not compromise the promising thermoelectric properties of orthorhombic BiSbSe 3 , thereby paving the way for anticipated high performance in high‐quality single crystals.