Thermoelectric properties of layered Bi₂YO₄Br: a cageless rattler host structure

Thermoelectric (TE) materials serve as a promising renewable energy source by harvesting the waste energy and enabling efficient direct conversion between electricity and heat. The layered mixed-anion compounds have recently emerged as pivotal candidates in a spectrum of technological domains as well as for TE applications. In the present work, we predicted the TE performance of a layered mixed-anion oxide, Bi₂YO₄Br, using the first principles method based on density functional theory. Unlike single or multi-filled caged rattlers, which typically exist in crystal systems, this structure acts as a host to the cageless rattling atom Br. The ultralow lattice thermal conductivity (∼0.6 W m^-1 K^-1 at 900 K) originates from weak interlayer coupling, strong lattice anharmonicity, bonding heterogeneity, and rattling-induced phonon scattering. The phonon dispersion also features topological optical phonons, which can also contribute to the anharmonicity in the system. Bi₂YO₄Br is an indirect semiconductor with a band gap of 2.17 eV. Electronic structure properties hint at promising transport properties benefiting from the emergence of flat bands at the top of the valence bands, resulting in higher hole effective mass leading to a higher power factor and Seebeck coefficient for the p-type Bi₂YO₄Br. The ultralow lattice thermal conductivity and favourable electronic transport properties yield a desirable figure of merit of ∼0.8 in p-type Bi₂YO₄Br at 900 K, making it a promising candidate for TE applications.