Investigation of electronic, mechanical, dynamic and optical properties of some B12 -based compounds under pressure: AB initio calculation

The structural, electronic, elastic, Born effective charges, vibration and optical properties of some B12-based boron-rich rhombohedral solid compounds (α-B12, B12O2, B12P2, B12As2, B12C3, B13C2 and B13N2) under pressure (P=0-350 GPa) were investigated by ab initio method within density functional theory. The lattice parameters of all compounds tended to decrease with increasing pressure. The calculated bond lengths between boron atoms and interstitial atoms decreased under pressure. Born effective charge tensors and optical dielectric constants determined by non-analytical contribution to the dynamic matrix in the q→0 limit were calculated. Electronic properties were calculated with two different functionals, GGA-PW91 and quasi-particle GW-GGA-PE. While the forbidden band gap of α-B12, B12P2, B12As2 and B12C3 compounds decreases with pressure, the band gap of B12O2 compounds increases with pressure. While B13N2 and B13C2 exhibited metallic properties, other compounds showed semiconductor properties. The elastic constants of all compounds tend to increase under pressure. Elastic constants provide the mechanical stability condition at ambient pressure. Bulk, shear and Young's modulus, sound velocity, Poisson's ratio, crystal anisotropy, Vickers hardness and Debye temperatures of the compounds were derived with the help of elastic constants. The phonon dispersion curves and density of phonon states of the compounds were calculated using the direct method. The variation of entropy, heat capacity, free energy and internal energy with temperature of the compounds were obtained. Optical calculations were calculated with three different methods: IPA, BSE and mBSE. Optical constants such as absorption coefficient, energy loss function, refractive index, extinction coefficient, optical conductivity and reflectivity are obtained from the imaginary and real part of the dielectric function. All calculated physical quantities were compared with available experimental and theoretical values.