Interplay between Cu diffusion and bonding anisotropy on the thermoelectric performance of double cation chalcohalides $CuBiSeX_{2} (X = Cl, Br)$

Abstract: Double cation chalcohalide have recently been emerged as the 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 mechanism, and thermoelectric properties of $CuBiSeX_{2} (X = Cl, Br)$ 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 suitable electronic band structure promotes the electron transport properties. Comprehensively, the delocalized Cu atom enhancing the phonon scattering process and the off-centred displacement of cations leading to bonding anharmonicity results ultra-low lattice thermal conductivity $(\kappa_L)$. Among these systems, $CuBiSeCl_2$ exhibits low $\kappa_L$ (0.24 $W m^{-1} K^{-1}$ at 300 K) and superior thermoelectric performance (zT = 1.18 at 600 K), whereas $CuBiSeBr_2$ ($\kappa_L$ = 0.65 $W m^{-1} K^{-1}$ at 300 K, zT = 0.68 at 600 K) demands further optimization. Overall, the study sheds light into the interplay between the Cu diffusion and bonding anisotropy in phonon propagation and establishes the potential of double-cation chalcohalides for mid-temperature thermoelectric applications.