A fully tetrahedrally bonded boron nitride (BN) allotrope with an orthorhombic structure (O-BN) was investigated through first-principles calculations. O-BN has a bulk modulus of 371.8 GPa and a hardness of 66.4 GPa, thereby making it a superhard material with potential technological and industrial applications. O-BN becomes thermodynamically more stable than layered hexagonal BN (h-BN) at pressure above 1.5 GPa and is more favorable than the recently reported Pct-BN at any pressure. The phase transformations from h-BN and BN nanotubes to O-BN were respectively simulated, indicating the feasible synthesis of this superhard phase.
Understanding the direct transformation from graphite to diamond has been a long-standing challenge with great scientific and practical importance. Previously proposed transformation mechanisms1-3, based on traditional experimental observations that lacked atomistic resolution, cannot account for the complex nanostructures occurring at graphite-diamond interfaces during the transformation4,5. Here, we report the identification of coherent graphite-diamond interfaces constituted with four structural motifs in partially transformed graphite samples recovered from static compression, using high-angle annular dark-field scanning transmission electron microscope. These observations provide vital insight into possible pathways of the transformation. Theoretical calculations confirm that transformation through these coherent interfaces is energetically favored to those through other paths previously proposed1-3. The graphite-to-diamond transformation is governed by the formation of nanoscale coherent interfaces (diamond nucleation), which, under static compression, advance to consume the remaining graphite (diamond growth). These results also shed light on transformation mechanisms of other carbon materials and boron nitride under different synthetic conditions.
A series of Ca-doped MgB2 superconducting compounds has been synthesized under high pressure and high temperature with a calcium source of Ca3B2N4. Their structure and superconducting properties have been investigated, revealing that the lattice parameters of a and c for the Ca-doped MgB2 are enlarged and the superconducting transition temperature Tc is suppressed from 37.8 K for pure MgB2 to 36.5 K for Mg0.92Ca0.08B2 by the Ca doping. In addition to the main phase of the Ca-doped MgB2, the MgO and CaB6 phases have been observed with the MgO and CaB6 grains being of the size of several hundred nanometres and being embedded randomly in the MgB2 matrix. In comparison to the undoped MgB2, the rapid decrease in Jc observed in the Ca-doped samples is mainly due to the relatively large grain sizes of the second phases, which result in the poor connectivity between grain boundaries.
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