Newly Synthesized Three-Dimensional Boron-Rich Chalcogenides B12X (X = S and Se): Theoretical Characterization of the Physical Properties for Optoelectronic and Mechanical Applications.

Abstract

Boron-rich chalcogenides have been predicted to have excellent properties for optical and mechanical applications in recent times. In this regard, we report the electronic, optical, and mechanical properties of recently synthesized boron-rich chalcogenide compounds B12X (X = S and Se) using density functional theory for the first time. The effects of exchange and correlation functionals on these properties are also investigated. The consistency of the obtained crystal structure with the reported experimental results has been checked in terms of lattice parameters. The considered materials are mechanically stable, brittle, and elastically anisotropic. Furthermore, the elastic moduli and hardness parameters are calculated, which show that B12S can be treated as a prominent member of the hard materials family compared to B12Se. The origin of differences in hardness is explained on the basis of density of states near the Fermi level. Reasonably good values of fracture toughness and the machinability index for B12X (X = S and Se) are reported. The melting point, Tm, for the B12S and B12Se compounds suggests that both solids are stable, at least up to 4208 and 3577 K, respectively. Indirect band gaps of B12S (2.27 eV) and B12Se (1.30 eV) are obtained using the HSE06 functional. The energy gaps using local density approximation (LDA) and generalized gradient approximation (GGA) are found to be significantly lower. The electrons of the B12Se compound show a lighter average effective mass than that of the B12S compound, which signifies a higher mobility of charge carriers in B12Se. The optical properties such as the dielectric function, refractive index, absorption coefficient, reflectivity, and loss function are characterized using GGA-PBE and HSE06 methods and discussed in detail. These compounds possess bulk optical anisotropy, and excellent absorption coefficients in the visible-light region along with very low static values of reflectivity spectra (range of 7.42–14.0% using both functionals) are noted. Such useful features of the compounds under investigation show promise for applications in optoelectronic and mechanical sectors.