Atomistic Mechanisms for Contrasting Stress-Strain Relations of B13CN and B13C2.

Boron-rich compounds comprise intricate bonding structures and possess excellent mechanical properties. Here, we report on a comparative study of B13CN and B13C2, which are isostructural but differ in electron fillings, with the former being electron-precise and the latter electron-deficient. Our results show that the different electron fillings in B13CN and B13C2 have profound effects on the bonding features despite their shared crystal structure, generating distinct structural deformation modes and the accompanying stress responses under diverse loading strain conditions. The most striking phenomena include a creeplike stress response under a tensile strain and superior strength under the vast majority of loading conditions for B13CN compared to B13C2. Such enhanced stability of the B12 icosahedra in B13CN by N-induced electron compensation may be effective for structural and mechanical enhancement of other boron-rich compounds and offers improved understanding of a broader class of covalent crystals with complex bonding networks.