Theoretical And Experimental Research Of Novel Structures Of Boron-nitride Compounds

Boron Nitride（BN）compounds are improtant inorganic materials with widely applications.The applications of materials are depend on its structure,the novel structure and its potential applications would bring about advance of science and technology,so it is important to desigin and synthsize novel structures of materials.In this work,with first-principles calculation,the properties of novel BN allotropes have been desigined and predicted in theoretically,the high pressure high temperature phase transition of multi-walled Boron Nitride Nanotubes have been studied in experimently.With the help of structures searching program CALYPSO which based on first-principles calculation,a sp²/sp³-hybridized metallic monoclinic 3D BN（M-BN）structure have been proposed.The electronic properties analysis revealed that the metallicity of M-BN is attributed to delocalization electrons of the sp² B and N atoms,which formed 1D conduction feature.The enthalpy calculation have revealed that M-BN is the most energetically favorable structure among the predicted metallic BN structures so far,and it might be obtained via compressing layered hBN precursor theoretically.Due to the 3D strong framework,M-BN has an estimated Vickers Hardness of 33.7 GPa,indicating it is a potential hard material with novel 1D conduction.Three N-rich Boron Nitride compounds,which are named by c I-B₅N₆,tP-B₃N₅ and oA-B₃N₅ have been predicted via CALYPSO program.cI-B₅N₆ and tP-B₃N₅ structure are indirect bandgap structures.oA-B₃N₅ is a metallic structure and the metallicity is attributed to the sp² hybridized N-N bonds.The ELF calculation show these structure are covalent structures with strong covalent bonds.Structure evolutions of single-walled boron nitride nanotube bundles under high pressure were investigated via CASTEP model in Material Studio program.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 the semiconductive feature,superhard and high tensile strength for these novel boron nitride polymorphs.The high-pressure behaviors of large-diameter single-walled BNNTs have been studied by first-principles method,and four BN allotropes have been obtained,named as oP26-BN,tP28-BN,m P28-BN and tP24-BN.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 Mulliken’s population analysis have been indicated the covalent nature of the B-B and N-N dimers.tP24-BN shows a local conducting feature,which attributing to the B atoms forming B-B bonds.The other three structures are superhard semiconductors.The phase transition of multi-walled BNNTs under high pressure and high temperature（HPHT）conditions have been studied.Results of ex-situ X-ray diffraction of samples under ambient pressure show,under the HPHT conditions,multi-walled BNNTs would transform into wurtzite BN and cubic BN,or transform into cBN completely,which is depending on the extra pressure and temperature.The HPHT phase transition states can be invidied into four area:（1）the BNNTs phase;（2）the transition state BNNTs,wBN and c BN phases;（3）wBN and cBN phases;（4）cBN phase.During the phase transition process,wBN are accompanied with cBN since the beginning of pahse transition until the multi-walled BNNTs transform into cBN completely.The temperature of phase transition is lower than pyrolytic BN,which is useful for synthesized cBN.