Theoretical Study On The Stress Modulation Effect Of The Structure And Physical Properties Of BC₂ Compounds

Crystal structure, electronic properties, elastic mechanics and lattice dynamical properties of several BC₂ N compound under external force（including biaxial and uniaxial strain and hydrostatic pressure） are systematically investigated via first principles calculations.At the same time, in order to overcome the deficiency of the generalized gradient approximation（GGA）, we also use the density functional theory（HSE06） method to improve the accuracy of band gap calculation. the main work and results obtained are presented as follows:（1） The electronic properties, elastic mechanics and lattice dynamical properties of t-BC₂ N and z-BC₂ N under the biaxial strain are investigated using both GGA and HSE06 method. We found that the elastic constants Cij, bulk modulus BH, shear modulus of GH and Young’s modulus EH of t-BC₂ N and z-BC₂ N compounds decrease（increase） almost linearly with increasing biaxial tensile（compressive） stress. The band gap energy Eg of the two compounds exhibits highly asymmetric and nonlinear characteristics with the change of biaxial stress. t-BC₂ N undergoes from an indirect band gap to direct band gap transition at-=%36.1xxe, whereas z-BC₂ N is always a direct band gap in the range ofxxe we have applied. The crystal field splitting energyΔcf is reduced from positive to negative under a certain tensile biaxial strain. The phonon frequency of infrared activity exhibits a large red（blue） shift with the increase of biaxial tensile（compressive） strain. In addition, we also analyzed the atomic displacement mode of the typical strong infrared active vibration mode.（2） The GGA and HSE06 methods are used to study the structure, electron,elastic and lattice dynamics of w-BC₂ N under different external forces（including biaxial strain, uniaxial strain and hydrostatic pressure）. The study found that the four different bond lengths changes of w-BC₂ N are not consistent with different external forces. Biaxial strain mainly affectes the B-N and C1-C₂ bond length in the ab plane,while the uniaxial strain mainly affects the B-C1 and C₂-N bond lengths along the c axis, and the effect of hydrostatic pressure on all four bond lengths is almost uniform.It is worth noting that, when the uniaxial strain 3%5.11zze is applied, the w-BC₂ N has been changed from the structure of wurtzite to the graphite-like structure. The calculated energy band change trend of w-BC₂ N using GGA and HSE06 two methods is consistent. Different forces cause different change of band gap value. Band gap of w-BC₂ N is changed from indirect to direct at the uniaxial strain =%12.6zze, which isalways indirect in the range of biaxial strain and hydrostatic pressure. In addition, the change of the band gap caused by biaxial and uniaxial strain is highly asymmetric and nonlinear, and with the increase of the hydrostatic pressure, the band gap decrease almost linearly. In addition, the band gap of the graphite like phase is close to zero.We also analyze the changing tendency of the phonon mode frequencies at Γ point with the changing biaxial strain and uniaxial strain and the corresponding atomic displacement pattern.