Mei Xiong

Compressed carbon nanotubes: A family of new multifunctional carbon allotropes

The exploration of novel functional carbon polymorphs is an enduring topic of scientific investigations. In this paper, we present simulations demonstrating metastable carbon phases as the result of pressure induced carbon nanotube polymerization. The configuration, bonding, electronic, and mechanical characteristics of carbon polymers strongly depend on the imposed hydrostatic/non-hydrostatic pressure, as well as on the geometry of the raw carbon nanotubes including diameter, chirality, stacking manner, and wall number. Especially, transition processes under hydrostatic/non-hydrostatic pressure are investigated, revealing unexpectedly low transition barriers and demonstrating sp2→sp3 bonding changes as well as peculiar oscillations of electronic property (e.g., semiconducting→metallic→semiconducting transitions). These polymerized nanotubes show versatile and superior physical properties, such as superhardness, high tensile strength and ductility, and tunable electronic properties (semiconducting or metallic).

First-principles study of O-BN: A sp3-bonding boron nitride allotrope

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.

Novel three-dimensional boron nitride allotropes from compressed nanotube bundles

Phase transitions of single-walled boron nitride nanotube bundles under high pressure were investigated. Multiple three-dimensional boron nitride allotropes were identified from the transition products, which highly depended on factors including compressing mode, nanotube geometry, and intertube orientation. Sequential theoretical calculations indicated a variety of electronic and mechanical properties for these novel boron nitride polymorphs, such as tunable band gap, ductility, high tensile strength, and superhardness. Especially, a direct band gap superhard phase, 3D (10,0)-III, and a ductile superhard phase, 3D (4,4)-I, were revealed. This study should stimulate further theoretical and experimental work on novel boron nitride allotropes with unique electronic and mechanical properties.

Pressure-induced boron nitride nanotube derivatives: 3D metastable allotropes

The high-pressure behaviors of large-diameter single-walled boron nitride nanotubes (BNNTs) are studied by the first-principles method. One sp3-hybridized and three sp2/sp3-hybridized BN allotropes are obtained via compressing large diameter BNNTs. Due to the restricted movement of nonequivalent B and N atoms, the large BN nanotubes have a chance to form B-B and N-N bonds between intertubes under pressure, in addition to the common B-N bonds. The electron localization function and Mullikens population analysis indicate the covalent nature of the B-B and N-N dimers. The electronic band structure and density of state calculations show a local conducting feature of tP24-BN and superhard semiconducting character of the other three allotropes with indirect band gaps of 1.28 – 3.13 eV.

Superhard sp2-sp3 hybridized BC2N: A 3D crystal with 1D and 2D alternate metallicity

A novel sp2-sp3 hybridized orthorhombic BC2N (o-BC2N) structure (space group: Pmm2, No. 25) is investigated using first-principles calculations. O-BC2N is constructed from multi-layers of C sandwiched between two layers of BN along the c axis; this structure contains sp2- and sp3-hybridized B-C, C-C, and C-N bonds. The structural stability of o-BC2N is confirmed based on the calculation results for elastic constants and phonon dispersions. On the basis of the semi-empirical microscopic model, we speculate that the o-BC2N compound is a potential superhard material with a Vickers hardness of 41.2 GPa. Calculated results for electronic band structures, density of states (DOS) and partial DOS (PDOS) show that the o-BC2N crystal is metallic. The conducting electrons at the Fermi level are mostly from the 2p orbits of sp2-hybridized B4, N1, and Ci (i = 2, 3, 4, 6, 7, 8) atoms, with slight contribution from the sp3-hybridizd B2 atoms. Furthermore, the calculated electron orbits of the o-BC2N crystal demonstrate ...

Superhard three-dimensional B3N4 with two-dimensional metallicity

As the stable compound in boron nitrides, stoichiometric BN is a well-known insulator, irrespective of its structure and dimensionality. The exploration of novel B–N compounds with various stoichiometric ratios can lead to the discovery of unexpected electrical and mechanical properties. To the best of our knowledge, previously reported graphite-like or diamond-like B–N compounds obtained from experimental synthesis and theoretical prediction are mostly insulators or semiconductors. In this paper, a sp2–sp3 hybridised tetragonal phase of B3N4 (t-B3N4) possessing unique two-dimensional (2D) metallicity in a 3D ultra-strong framework has been predicted through an unbiased swarm structure search. The structure of t-B3N4 can be considered as sp3-hybridised cubic BN blocks interlinked by sp2 N–N bonds. Noticeably, t-B3N4 is metastable at ambient pressure, but becomes stable under high pressure. The transition pressure from layered B3N4 to t-B3N4 is 14.7 GPa, and the calculated formation enthalpies of t-B3N4 with respect to h-BN and N2 become negative at pressures above 20 GPa, indicating its viability under pressure. Its structure stability has been confirmed by the criteria of both elastic constants and phonon frequency dispersions. The analyses of the band structure, density of states, and electron orbitals show that the metallic behaviour of t-B3N4 mainly originates from the N 2p electrons, and that the conduction is interrupted by the insulated boron sheets stacked along the c axis, giving rise to the 2D metallicity of the material. The theoretical Vickers hardness of t-B3N4 is estimated to reach 42.5 GPa, which is the highest among all proposed B3N4 polymorphs. Furthermore, t-B3N4 exhibits ultra-high axial incompressibility even beyond that of diamond, due to the existence of strong short N–N bonds.