Hard three-dimensional BN framework with one-dimensional metallicity

Carbon can be metal, semiconductor or insulator depending on its structures, and its conductivity can be one-dimensional (1D), 2D, and 3D correspondingly. Boron Nitride (BN) is isoelectronic to carbon, but one prominent difference between the two systems is their electronic properties. For many years, the synthesized and predicted BN allotropes were always insulators, irrespective of their structures. In this paper, with the help of structures searching based on first-principles calculations, a sp2/sp3-hybridized metallic monoclinic 3D BN (M-BN) structure is proposed. M-BN can be viewed as puckered hexagonal-BN (h-BN) layers bucked by partial sp3 BN bonds. The electronic property analysis revealed that the metallicity of M-BN is attributed to the delocalized 2p electrons of the sp2 B and N atoms. The metallic B atoms and N atoms are aligns in two arrays along a 1D axial direction, resulting in the unusual 1D dual-threaded conduction in 3D M-BN structure. The enthalpy calculation have revealed that M-BN is the most energetically favorable structure among the predicted metallic BN structures so far, and it might be obtained via compressing layered h-BN precursor theoretically. Due to the 3D strong framework, M-BN has an estimated Vickers Hardness of 33.7 GPa, indicating it is a potential hard material with novel 1D conduction.