Strain-Induced Suppression of Coherent Phonon Transport and Thermoelectric Enhancement in CuBiSeCl ₂ with Strong Quartic Anharmonicity

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 κ_<i>L</i> = 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 <i>b</i> 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.