From Se Incorporation to ZrSe3 Formation: Concentration-Dependent Transport Tuning in n-Type ZrCoSb

Half-Heusler ZrCoSb offers an Hf-free platform for medium-to-high-temperature thermoelectric power generation, but its performance is constrained by high lattice thermal conductivity and the limited choice of effective donor dopants. Here, nominal Se additions of 0–6 at.% were investigated to clarify the transition from matrix incorporation to secondary-phase formation. At 2 at.% Se (X2), the Seebeck coefficient reaches −285 μV·K−1 and the power factor reaches 22.9 × 10−4 W·m−1·K−2 at 900 K. At higher addition levels (≥4 at.%), ZrSe3 formation deteriorates electrical transport but further suppresses heat conduction, reducing the thermal conductivity to 3.5 W·m−1·K−1 at 6 at.% Se (X6) at 900 K. The X2 sample achieves a peak ZT of 0.5 at 900 K and an average ZT enhancement of 168% relative to pristine ZrCoSb over 300–900 K. These results identify Se as a low-cost, Hf-free donor strategy that couples electronic-structure modulation with phonon scattering in n-type ZrCoSb.