First-Principles Study The Stability Of Various Hexagonal BN Phases

Being one of the most interesting solids among the III-V covalent compounds, boron nitride has tremendous amounts of outstanding fundamental chemical and physical properties.With different hybridism, atom B and N can form to various BN crystal among which h-BN is the most ordinary and easy-making structure. Belonging to the P63/mmc space group, h-BN consists of sp2-hybridized BN rings held together by van der Waals bonds and electrostatic interactions. An important feature of h-BN is its stacking sequence. Adjacent basal planes of h-BN can slide and rotate in numerous ways as confirmed by different symmetries found in x-ray diffraction study.Thus,The structural constants,electronic properties,elastic constants and phonon spectra of five hexagonal boron nitride(h-BN) stacking sequences(A,B,C,D,E, respectively) were investigated thoroughly using first-principles calculations based on the density-functional theory local-density approximation. Three of five possible h-BN structures were found to be stable or substable through energy aspect. The two reminders were unstable which both had large structural lattice c. Four structures with indirect band gap and one substable structure B with a direct band gap were predicted,the band gap are 3.9685eV,3.4047eV,3.5167eV,4.1550eV and 3.4810eV for five structures respectively,and it is the substable structure B that changed the electronic properties of h-BN, bringing the transform from indirect band gap to direct band gap. Furthermore, trough DOS,PDOS and Mulliken distribution we found that different stacking behavior contributed to the change of atom bond and charge. Meanwhile, we found three structures A,B and D were elastic stable and C,E were elastic unstable using Born stable-principles by calculating elastic constants of five different h-BN structures. Finally, through phonon spectra, we found the tendency of acoustic branches of three structures A,B and D were positive which indicated that the three structures were stable, while structure C and E were unstable because of the negative frequencies appeared at Brillouin zoneΓand A area, respectively. As a result, we can concluded that the structure A,B,D were stable and C,E were unstable.Limited by the time and research level, the temperature and pressure effects on h-BN properties would be studied in the future.