Ultralow thermal conductivity and anharmonic rattling in two-dimensional WB4 monolayer

Abstract

Two-dimensional (2D) WB4 monolayer is a typical graphene analog with high electrical conductivity and structural stability. Yet, its thermal transport properties are not available. By using first-principles calculations and iteratively solving the linearized Boltzmann transport equation, we predict an ultralow in-plane lattice thermal conductivity (κlat) of 0.28 W/m K at T = 300 K. Such an ultralow κlat is attributed to WB4 monolayer's predominantly large phonon scattering rates and flat acoustic phonon dispersion caused by strong anharmonicity. By analyzing the vibrational patterns and bonding environment, we confirm the origin of the strong anharmonicity to be tungsten atom rattling inside the framework of two boron sheets. Such a mechanism fulfills the concept of phonon glass-electron crystal, making the WB4 monolayer an outstanding 2D thermoelectric material. The rich formation mechanism, including multiple interactions in the WB4 monolayer, provides us inspiration for searching for materials with ultralow κlat.