Interplay between Cu Delocalization and Bonding Anisotropy on the Thermoelectric Performance of Double-Cation Chalcohalides CuBiSeX ₂ (X = Cl, Br)

Double-cation chalcohalides have recently emerged as interesting candidates for sustainable energy conversion applications owing to their intrinsic chemical tunability, suitable band gap, and low thermal conductivity. With this motivation, the current study is designed to explore the structural, electron, and phonon transport mechanisms and the thermoelectric properties of CuBiSeX2 (X = Cl, Br) through density functional theory-based computations. The experimental feasibility of the compounds is ensured, and they are predicted to be thermally, dynamically, and mechanically stable. The distinct structural attributes coupled with a suitable electronic band structure promote the electron transport properties. Comprehensively, the delocalized Cu atom enhances the phonon scattering process and the off-centered displacement of cations, leading to bonding anharmonicity, and results in ultralow lattice thermal conductivity (κL). Among these systems, CuBiSeBr2 exhibits low κL (0.17 W m–1 K–1) and superior thermoelectric performance (zT = 1.06) at 600 K, whereas CuBiSeCl2 (κL = 0.33 W m–1 K–1, zT = 0.57 at 600 K) demands further optimization. The glimpse of the anisotropic thermoelectric studies proposes electron doping accompanied by growth along the b-axis as a feasible strategy to improve the thermoelectric performance of CuBiSeCl2. Overall, the study sheds light on the interplay between Cu delocalization and bonding anisotropy in phonon propagation and establishes the potential of double-cation chalcohalides for midtemperature thermoelectric applications.