The crystal structure of ${\mathrm{BC}}_2\mathrm{N}$ 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 ${\mathrm{BC}}_2\mathrm{N}$ candidates, the (111) $\mathrm{B}{\mathrm{C}}_2{\mathrm{N}}_{2\times 2}$ superlattice (labeled $\mathrm{R}2u-\mathrm{B}{\mathrm{C}}_2\mathrm{N}$) is identified as the realistic crystal structure of the experimentally synthesized ${\mathrm{BC}}_2\mathrm{N}$. We further reveal that the strain-induced Friedel-like oscillations dominates the preferable slip systems of $\mathrm{R}2u-\mathrm{B}{\mathrm{C}}_2\mathrm{N}$ 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 $\mathrm{R}2u-\mathrm{B}{\mathrm{C}}_2\mathrm{N}$ as the intrinsic superhard material surpassing $c\text{−}\mathrm{BN}$, 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 ${\mathrm{BC}}_2\mathrm{N}$, and establish a mechanism underlying the strain-driven electronic instability of superlattice structures, providing guidance towards rational design of superhard materials.

中文翻译：

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由应变诱导的 aBC2N 超晶格中的类似弗里德尔振荡介导的异常键软化

晶体结构${\mathrm{公元前}}_2\mathrm{ñ}$而其超硬度的起源一直处于争论之中，阻碍了它的发展。在此，通过评估 X 射线衍射图，一系列的在常压和高压下的热力学稳定性${\mathrm{公元前}}_2\mathrm{ñ}$候选人，(111)$\mathrm{乙}{\mathrm{C}}_2{\mathrm{ñ}}_{2\times 2}$超晶格（标记$\mathrm{R}2你-\mathrm{乙}{\mathrm{C}}_2\mathrm{ñ}$) 被确定为实验合成的真实晶体结构${\mathrm{公元前}}_2\mathrm{ñ}$. 我们进一步揭示了应变引起的类似弗里德尔振荡支配了优选的滑移系统$\mathrm{R}2你-\mathrm{乙}{\mathrm{C}}_2\mathrm{ñ}$通过大大削弱滑移平面上的异质键，从而导致其超低位错滑移阻力，这源于软化键的键合和反键相互作用之间的能量分离减少引发的金属化。我们的结果排除了$\mathrm{R}2你-\mathrm{乙}{\mathrm{C}}_2\mathrm{ñ}$作为本征超硬材料超越$C\text{-}\mathrm{国阵}$，而实验确定的极端硬度可归因于由界面无定形碳粘合的纳米晶粒，这为塑性变形提供了强大的屏障。这些发现提供了对实验观察到的可能结构的长期问题的看法${\mathrm{公元前}}_2\mathrm{ñ}$，并建立超晶格结构的应变驱动电子不稳定性的潜在机制，为超硬材料的合理设计提供指导。