Guoying Gao

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 1.28u2009–u20093.13u2009eV.

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.

Mechanically ductile 3D sp – sp 2 microporous carbon

A new sp–sp2-hybridized tetragonal carbon allotrope, namely Tetra-carbon, is predicted through the evolutionary particle swarm structural search. Tetra-carbon has a 3D framework composed of sp2 carbon helixes connected by linear sp carbon chains, similar to the interconnected network of propadienyl groups, which forms the well-proportioned microporous structure. Tetra-carbon is thermodynamically more stable than known graphdiyne and carbyne carbon and also shows mechanical and dynamic stabilities at ambient pressure. Tetra-carbon is a semiconductor with an indirect band gap of 3.27xa0eV and has anisotropic tensile strengths with an unexpected large tensile strain of 0.64 along the [001] direction. Base on the analysis of Poisson’s ratios as well as the tensile strains, it is significantly revealed that Tetra-carbon is a mechanically ductile microporous carbon allotrope in contrast with the known brittle carbons such as diamond, potentially applied in the fields where the ductile metals are available.

3D hybrid carbon composed of multigraphene bridged by carbon chains

The element carbon possesses various stable and metastable allotropes; some of them have been applied in diverse fields. The experimental evidences of both carbon chain and graphdiyne have been reported. Here, we reveal the mystery of an enchanting carbon allotrope with sp-, sp2-, and sp3-hybridized carbon atoms using a newly developed ab initio particle-swarm optimization algorithm for crystal structure prediction. This crystalline allotrope, namely m-C12, can be viewed as braided mesh architecture interwoven with multigraphene and carbon chains. The m-C12 meets the criteria for dynamic and mechanical stabilities and is energetically more stable than carbyne and graphdiyne. Analysis of the B/G and Poisson’s ratio indicates that this allotrope is ductile. Notably, m-C12 is a superconducting carbon with Tc of 1.13 K, which is rare in the family of carbon allotropes.

New hexagonal boron nitride polytypes with triple-layer periodicity

Regular hexagonal boron nitride (h-BN) samples present a few of intrinsic stacking faults, which result in a long-standing controversy about their electronic properties. To resolve this controversy, we designed eight possible BN polytypes with triple-layer periodicity. Under ambient pressure, the energies of all the proposed polytypes are between those of observed AA and Aa (h-BN) structures. Two proposed polytypes with direct bandgaps might be responsible for the direct bandgap observed in the h-BN samples. A model was proposed to show how the proposed structures might exist in the h-BN samples by analyzing the stacking characteristics and the previous experimental micrographs of h-BN samples.