Rhombohedral boron nitride (rBN), distinguished from its hexagonal counterpart (hBN) by a unique interlayer stacking sequence, has recently garnered considerable attention owing to its unusual optical nonlinearity and interfacial ferroelectricity. However, thermal transport in rBN remains unexplored, despite the critical role for heat dissipation in functional devices. Here, we present a combined experimental and theoretical investigation of heat conduction in bulk rBN. The measured in-plane and out-of-plane thermal conductivities align closely with our first-principles calculations incorporating three-phonon, four-phonon, and phonon-isotope scatterings. Compared to the theoretical thermal conductivities of hBN, about 15% suppression is observed. This is attributed to the lower crystal symmetry and folding of the Brillouin zone induced by the stacking configuration of rBN, which leads to more phonon scattering channels and stronger anharmonicity. In particular, thermal transport via the transverse acoustic phonon modes is primarily impeded. Our work offers a benchmark for the anisotropic thermal conductivity of rBN, and provides insights into phonon transport in commensurate metastable structures of layered materials.

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• Thermal conductivity
• Boron nitride
• Density functional calculations
• Thermoreflectance