Large thermal conductivity of boron suboxides despite complex structures

Boron suboxides (B6O) were identified to be a superhard material, and their stiff bonds are expected to give large thermal conductivity. But their complex crystal structures suggest otherwise. Using first-principles calculations, we show that both α- and β-B6O have unusually high lattice thermal conductivities of 284.9 and 207.1 W/(m K), respectively, at room temperature, despite their complex structures. Our detailed phonon analysis attributed the dominant factor of its large thermal conductivity to the strong bond strength. Their large group velocities result from the strong bonding and light atomic mass, while their large phonon lifetimes can be explained by small anharmonicity and limited scattering phase space. Our results show that materials with complex unit cells like α- and β-B6O can still have high thermal conductivity. The combination of large thermal conductivity and an excellent physical hardness makes B6O a promising material for lightweight, multifunctional thermal management applications.