Ultralow Lattice Thermal Conductivity in Bulk and Monolayer TlCuSe: a Comparative Study from First-Principles

Layered TlCuSe was experimentally found to possess ultralow lattice thermal conductivity due to the weak chemical bond and the strong anharmonicity, however, there is an imaginary frequency in the calculated phonon spectrum based on density functional theory (DFT) (Lin et al 2021 Adv. Mater. 33 2104908). Herein, using DFT + U (Coulomb interaction) and phonon Boltzmann transport theory, we demonstrate that the Coulomb interaction can effectively eliminate the imaginary frequency of the phonon spectrum for both bulk and monolayer TlCuSe. The lattice thermal conductivity can be further decreased from bulk (0.43 W m(-1)K(-1) in-plane at 300 K) to monolayer (0.35 W m(-1)K(-1) at 300 K), which comes from the competition between the increased phonon group velocity and the decreased phonon relaxation time. The larger Gruneisen parameters and phase space volume of the monolayer compared to the bulk indicate an enhanced anharmonicity, leading to a low phonon relaxation time and dominating the decreasing lattice thermal conductivity. The present work highlights the indispensability of Coulomb interaction when exploring the phonon transport. The ultralow lattice thermal conductivity of TlCuSe, especially in the form of monolayers, suggests promising thermoelectric applications.