Design of a Series of Metallic B_xN_x+1 with Tunable Mechanical Properties

Here a series of sp²–sp³ B_xN_x+1 (x = 1, 2, 3, 4, 5, 6) structures was constructed. These structures can be viewed as diamond-like BN blocks connected by single N–N bonds. Elastic constants and phonon dispersion curves confirm that all of the proposed structures are mechanically and dynamically stable. These structures all possess metallicity originating from the conductive channels formed by sp²-hybridized N atoms and adjacent sp³-hybridized B and N atoms. These structures exhibit tunable mechanical properties with a regular change in the sp²/sp³ ratio. The theoretical Vickers hardness increases and the ductility decreases as the number of diamond-like BN blocks increases, gradually approaching those of c-BN. Moreover, the convex hull at ambient pressure and 50 GPa indicates that high pressure is beneficial in the synthesis of these B–N phases. The simulated X-ray diffraction patterns of these structures were also calculated to provide more information for further experiments.