硅酸盐学报

具有超硬性和导电性的材料在极端条件下具有广阔的应用前景，尤其是在多功能器件领域。本工作设计了2种新型BCN结构——P2/m-BCN和Pm-BCN。这2种结构在常压下可作为亚稳相存在，并具备优异的弹性与动力学稳定性，其维氏硬度分别为43.7 GPa和49.9 GPa。2种结构均呈现超硬材料中较为罕见的金属性。拉伸计算结果显示：P2/m-BCN在[100]张应力方向表现出少见的双峰应力–应变特征。电子局域函数分析表明，该异常行为源于分步断裂机制：首先[100]方向上的B—N键发生断裂，载荷随后转移至C—C共价网络，直至最终完全失效。这一研究成果不仅丰富了BCN化合物的结构多样性，还为设计具有可调各向异性性能的超硬导电材料提供了理论依据。

Introduction Ultra-hard materials,characterized by a hardness exceeding 40 GPa,showcase remarkable mechanical properties such as superior wear resistance and compressive strength,outperforming traditional hard materials.The mid-20th-century breakthroughs in techniques like powder metallurgy and high-pressure synthesis have established ultra-hard materials as a distinct and advancing field within materials science.Among these materials,diamond and cubic boron nitride (c BN) stand out as prime examples,finding extensive application across various industries including resource extraction,precision processing,electronics,aerospace,and scientific research.Despite their exceptional properties,diamonds are limited by their low oxidation temperature and reactivity with iron,while c BN,although chemically stable and effective for processing ferrous metals,has a lower hardness compared to diamond.These limitations highlight the need for the development of new ultra-hard materials that can combine the strengths of both diamond and c BN.B-C-N compounds,composed of boron,carbon,and nitrogen,benefit from the strong covalent bonds formed due to their close proximity in the periodic table,endowing them with exceptional chemical stability and mechanical strength.Their robust lattice structures contribute to their durability and toughness,making them highly suitable for applications that demand high stability and strength.Recent research has delved into various B-C-N materials:Nakano et al.(1994) synthesized cubic BCN phases under high-pressure and high-temperature conditions,Komatsu et al.(1996) produced c BC_2.5N with notable nano-scale characteristics and a bulk modulus greater than that of c BN,and Solozhenko et al.(2001) synthesized cubic BC_2N blocks with a remarkable hardness of 76GPa,surpassing c BN.The advent of advanced computer technology has seen first-principles calculations based on density functional theory play a significant role in materials design.Researchers have proposed various BCN structures,such as orthogonal BC_2N(Mattesini et al.,2001) and zinc blende BC_2N (Sun et al.),which have shown potential as ultra-hard materials.Gao et al.(2018)introduced BC_6N structures with mixed hybridization,demonstrating one-dimensional conductive properties.This study presents two novel BCN structures that are stable under ambient pressure and possess both ultra-hard properties and excellent conductivity.This represents a significant advancement in the research of metallic ultra-hard materials and provides essential theoretical foundations for their future practical applications in materials science.Methods The structural models were constructed in the Materials Visualizer module of Materials Studio.The property calculations for the two different structures were performed using the CASTEP software package on the basis of density functional theory.The calculations employed ultrasoft pseudopotentials,and the exchange-correlation functional (GGA) was chosen to be the Perdew–Burke–Ernzerhof (PBE) form of the generalized gradient approximation.After a series of convergence tests,the plane wave cutoff energy was set to 630 e V;the first Brillouin zones of all structures were meshed using the Monkhorst-Pack method,with the spacing of the K-point grid set to 2π×0.04Å^-1.The convergence criteria for the structural relaxation process were for the total energy of the structure,the maximum ionic displacement,the maximum internal stress,and the change in the Hellmann–Feynman force for the ions to be less than 5.0×10^-6 e V/atom,5.0×10^-4Å,0.02 GPa,and 0.01 e V/Å,respectively.The elastic constants and electronic band structures of the structures were calculated using the primitive cell.The phonon spectra of the two structures were calculated using the finite displacement method.Results and discussion The results show that both P2/m-BCN and Pm-BCN structures are elastic and dynamically stable.Their thermodynamic stability was assessed by calculating formation enthalpies at various pressures.Both structures have positive formation enthalpies,indicating higher energy than graphite and h BN,necessitating external forces like high pressure or catalysts for formation.Beyond a critical pressure,the enthalpy becomes negative,suggesting these BCN structures can be synthesized from graphite and h BN.Using the HSE06 mixed functional method,we studied their electronic properties under atmospheric pressure,revealing a certain degree of conductivity.Projected density of states (PDOS) calculations indicate that the conductivity is primarily due to the 2p orbitals of carbon and boron atoms,highlighting the role of orbital hybridization.Mechanical property analysis shows that P2/m-BCN and Pm-BCN have bulk and shear moduli slightly lower than c BN,indicating high incompressibility and shear resistance.Vickers hardness calculations yield 43.7 GPa and 49.9 GPa for P2/m-BCN and Pm-BCN,respectively,confirming their potential as superhard materials.Stress-strain studies reveal a high tendency for cleavage on the (001) plane,suggesting brittle fracture along this direction.Notably,P2/m-BCN exhibits a unique bimodal stress–strain curve along the[100]direction,indicating a staged fracture process:initial selective bond breaking,partial stress release,and subsequent network failure under higher strain.This provides insights into the mechanical stability and fracture toughness of these structures.Conclusions This study successfully designs two novel BCN structures,P2/m-BCN and Pm-BCN,via first-principles calculations.These structures exhibit exceptional elastic and dynamical stability as metastable phases at ambient pressure,establishing a theoretical basis for potential applications.Structural analysis indicates that P2/m-BCN has sp²-hybridized carbon atoms,while Pm-BCN contains boron–nitrogen units,endowing them with unique electronic structures and metallic properties.Mechanical assessments reveal high strength and rigidity,with the lowest tensile strengths along the[001]direction (39.7 GPa for P2/m-BCN and 42.4 GPa for Pm-BCN),suggesting a strong cleavage tendency on the (001) plane.Notably,P2/m-BCN exhibits a distinctive double-peak stress–strain behavior along the[100]direction.Electron localization function (ELF) analysis reveals a stepwise fracture mechanism:initial B–N bond rupture leads to partial stress release,followed by load transfer to the C–C network,which fails at higher strain.This highlights the transient load-bearing capacity of heterogeneous bonding networks.Overall,these metastable metallic BCN phases enhance the structural diversity of superhard materials,combining high mechanical strength,metallicity,and tunable anisotropic fracture behavior.These findings offer new theoretical insights for designing superhard multifunctional materials,with promising applications in cutting,wear resistance,and extreme environments.