Mixed-anion semiconductors exhibit ultralow lattice thermal conductivity, yet conventional Boltzmann transport, including only three-phonon scattering, cannot capture their strong anharmonicity. Here, first-principles simulations incorporating four-phonon interactions and coherent phonon transport reveal strain-driven thermoelectric enhancement in CuBiSeCl2. Four-phonon scattering strengthens mode coupling and slightly enhances phonon coherence, while tensile strain suppresses it through vibrational mode decoupling, reducing the coherent contribution by 17% and yielding κL = 0.33 W m-1 K-1 at 300 K. Tensile strain notably enhances quartic anharmonicity and weakens Cu-Cl bonding. Simultaneously, it narrows the band gap to 1.22 eV and lowers the electron effective mass along the b axis to 0.211 m0, improving the power factor to 0.29 mW m-1 K-2 at 700 K. The combined thermal and electronic optimization produces a peak ZT ≈ 0.8 at 800 K for n-type doping, highlighting that four-phonon scattering, phonon coherence, and strain engineering jointly govern heat and charge transport in strongly anharmonic thermoelectric materials.
Zeng et al. (Wed,) studied this question.