The Structures And Properties Of New Boron Nitride Under High Pressure Studied In Theory

As the nearest neighbor of the carbon atom,boron nitride(BN)possessing a similar structure and properties to carbon,has the ability to form a variety of allotropes and compounds due to their diverse hybridizations.A broad family of BN materials such as hexagonal boron nitride(h-BN),cubic boron nitride(c-BN),wurtzite boron nitride(w-BN)and amorphous boron nitride(a-BN),etc.is obtained which play important roles in scientific development and life/industrial fields.Thus,searching for BN materials with excellent properties and novel structures is always the important topic in Material science,Physics.High pressure as an efficient method can regulate the electron distribution,interaction and bonding style of atoms.The application of high pressure to the research of BN materials has become an important way to explore the structural transformation law of BN materials under extreme conditions and search for novel BN structures.It has been found that the sp~2hybridized BN materials can undergo the structural transformation from sp~2to sp~3under shear high pressure,hydrostatic high pressure and high temperature and high pressure,forming sp~3 hybridized BN materials.However,the mechanisms of pressure-induced BN bonding transition are still not very clear.In this paper,we are focued on the mechanism of typical sp~2 hybridized h-BN and BN nanotubes under high pressure.Revealing the behaviors of sp~2 hybridized h-BN transition as applied shear pressure.On the other hand,exploring the polymerization behavior and obtaining new BN structures by compressing BN nanotubes with different diameters and helicity under hydrostatic pressure.1.Using the shear deformation program designed by the research group build different shear structure models.Molecular dynamics simulation was used to study the polymerization behavior of h-BN structures under different shear stresses.The result of the research shows that the h-BN will transform to lots of metastable high-pressure phases under shear pressure,such as layered sp~2/sp~3 hybridized BN,sp~3hybridized Cm-BN,sp~2/sp~3 hybridized P-1-BN.Interestingly,the metastable high-pressure BN structure obtained under specific shear stress transforms to w-BN structure,and other metastable high-pressure boron nitride structures transform into layered boron nitride structures as releasing the pressure.The study theoretically revealed the transformation mechanism of sp~2 hybridized h-BN to w-BN under shear pressure.2.Using the first principle based on density functional theory,we studied the polymerization behavior of single-walled boron nitride nanotubes(SWBNNTs)with different diameters and helicities under high pressure.Eight stable and novel BN structures were obtained.Calculation and analysis of electronic structures show that the band gaps of eight new BN structures range from 2.18 e V to 6.18 e V.The analysis of electron localization function shows that hybrid sp~2/sp~3 m P44-BN,o I28-BN,m P60-BN,m P72-BN,m P76-BN and m P84-BN are obtained,while the sp~3 hybrid h P24-BN and h P48-BN are obtained.The analysis of mechanical properties shows that the six hybrid sp~2/sp~3 BN structures are superhard phases(H_v:44.73-52.55 GPa),while the m P76-BN is the hardest hybrid sp~2/sp~3 BN structures by comparing with the previous reports.This study reveals the high-pressure transition mechanism of SWBNNTs,which provides a theoretical reference for construction of new superhard BN structures.