Heterogeneous C C bonding induced anomalous phonon transport in superhard BC 2 N polymorphs

In this work, the effect of chemical bonding on thermal transport in superhard BC 2 N polymorphs has been comparatively evaluated by the first-principles calculations. Compared with the high-symmetry phase, a cliff-like drop of lattice thermal conductivity occurs in the C 121 phase with the monoclinic lattice. The findings can be attributed to the heterogeneous C C bonding induced suppression of thermal transport via the transverse phonon modes. The crystal orbital Hamilton population analysis shows that there exists an extremely weak C C bond in the C 121 phase of BC 2 N. Despite the presence of stronger B N and C N bonds in the C 121 phase, the weak C C bond predominantly results in softened acoustic phonon branches, strong lattice anharmonicity, and low elastic moduli. These factors enhance the phonon–phonon scattering and are thereby responsible for the suppression of phonon thermal conductivity. This study gives fundamental insights into structure and chemical bonding effects on phonon transport and provides guidance toward designing superhard materials with tunable thermal conductivity under extreme conditions.