  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 functional
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 is likely to be a
prominent member of hard materials family compared to B12Se. The origin of
different in hardness is explained on the basis of density of states near the
Fermi level. Reasonably good values of fracture toughness and 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 gap of B12S (2.27 eV) and B12Se (1.30
eV) are obtained using the HSE06 functional.The electrons of B12Se compound
show lighter average effective mass compared to that of B12S compound, which
signifies higher mobility of charge carriers in B12Se. The optical properties
are characterized using GGA-PBE and HSE06 method and discussed in detail. These
compounds possess bulk optical anisotropy and excellent absorption coefficients
in visible light region along with very low static value of reflectivity
spectra (range: 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.
