Anomalous bond softening mediated by strain-induced Friedel-like oscillations in a BC2N superlattice

The crystal structure of BC2N and the origin of its superhardness remain under constant debate, hindering its development. Herein, by evaluating the x-ray diffraction pattern, the thermodynamic stability at normal and high pressures of a series of BC2N candidates, the (111) BC2N2x2 superlattice (labeled R2u-BC2N) is identified as the realistic crystal structure of the experimentally synthesized BC2N. We further reveal that the strain-induced Friedel-like oscillations dominates the preferable slip systems of R2u-BC2N by drastically weakening the heterogenous bonds across the slip plane and thus leads to its ultralow dislocation slip resistance, which originates from the metallization triggered by the reduction in energy separation between bonding and antibonding interactions of the softened bonds. Our results rule out R2u-BC2N as the intrinsic superhard material surpassing c-BN, whereas the experimentally determined extreme hardness can be attributed to the nanocrystalline grains glued by interfacial amorphous carbon which provides a strong barrier for plastic deformation. These findings provide a view of the longstanding issue of the possible structure of experimentally observed BC2N, and establish a mechanism underlying the strain-driven electronic instability of superlattice structures, providing guidance towards rational design of superhard materials.