Ultralow Lattice Thermal Conductivity and Colossal Thermoelectric Figure of Merit of the Room-Temperature Antiferromagnet CsMnBi

We study the experimentally synthesized layered material CsMnBi using first-principles calculations and the linearized electron and phonon Boltzmann transport equations. CsMnBi is found to be a semiconductor with an indirect bandgap of 0.76 eV and to realize C-type antiferromagnetism, which is energetically favorable by 0.19 eV per formula unit over ferromagnetism. Energetical overlap between the acoustic and low-frequency optical phonon modes enhances the phonon-phonon scattering. Combined with low group velocities and high lattice anharmonicity this results in an ultralow lattice thermal conductivity of 0.07 W m^-1 K^-1 at 300 K. A high thermoelectric figure of merit of 2.2 (1.7) is achieved at 300 K at a hole (electron) density of 6.0 × 10¹⁸ (1.0 × 10¹⁸) cm^-3.