Origin of high hardness and optoelectronic and thermo-physical properties of boron-rich compounds B6X (X = S, Se): A comprehensive study via DFT approach

Abstract

In the present study, the structural and hitherto uninvestigated mechanical (elastic stiffness constants, machinability index, Cauchy pressure, anisotropy indices, brittleness/ductility, Poisson's ratio), electronic, optical, and thermodynamic properties of novel boron-rich compounds B 6 X (X = S, Se) have been explored using density functional theory. The estimated structural lattice parameters were consistent with the prior report. The mechanical and dynamical stability of these compounds have been established theoretically. The materials are brittle in nature and elastically anisotropic. The value of fracture toughness, K IC for the B 6 S and B 6 Se are ~ 2.07 MPam 0.5 , evaluating the resistance to limit the crack propagation inside the materials. Both B 6 S and B 6 Se compounds possess high hardness values in the range 31-35 GPa, and have the potential to be prominent members of the class of hard compounds. Strong covalent bonding and sharp peak at low energy below the Fermi level confirmed by partial density of states (PDOS) resulted in the high hardness. The profile of band structure, as well as DOS, assesses the indirect semiconducting nature of the titled compounds. The comparatively high value of Debye temperature (Θ D ), minimum thermal conductivity (K min ), lattice thermal conductivity (k ph ), low thermal expansion coefficient, and low density suggest that both boron-rich chalcogenides might be used as thermal management materials. Large absorption capacities in the mid ultraviolet region (3.2-15 eV) of the studied materials and low reflectivity (~16 %) are significantly noted. Such favorable features give promise to the compounds under investigation to be used in UV surface-disinfection devices as well as medical sterilizer equipment applications. Excellent correlations are found among all the studied physical properties of these compounds.

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

“…These values are higher than those of B 6 S (2.070 MPam 0.5 ) and B 6 Se (2.072 MPam 0.5 ). 6 Moreover, Mazhnik and Oganov 64 have also reported the K IC of superhard diamond (6.3 MPam 0.5 ), WC (5.4 MPam 0.5 ), and c-BN (5.4 MPam 0.5 ), which are higher than those of B 12 S and B 12 Se. Overall, the fracture hardness values of boron-rich B 12 S and B 12 Se are reasonably high.…”

“…Similar results were also reported in the B-rich chalcogenide B 6 X (X = S and Se) compounds. 6 Furthermore, the top of the VB was shifted from the M point to the Γ point when the S element was substituted by the Se atom to form the compound B 12 Se. It is also seen that the bands along H–K and H–L directions for both compounds are much less dispersive than in any other direction.…”

“… 52 − 54 The required expressions for these calculations can be found elsewhere. 6 , 55 , 56 The obtained values are listed in Table 4 from which it is seen that the B , G , and Y are 187, 148, and 351 GPa and 171, 96, and 243 GPa for B 12 S and B 12 Se, respectively, obtained using the GGA functional. As B , G , and Y measure the volume deformation resistance, shear deformation resistance, and stiffness of solids, B 12 S exhibits strong volume and shear deformation resistances as well as strong stiffness compared to B 12 Se.…”

“… 19 The choice of the pseudopotential is quite important in view of optimization of the crystal structure and its electronic structure, especially of the semiconductor materials. 6 , 20 , 21 The exchange–correlation potentials are evaluated by using the functional form of Perdew–Burke–Ernzerhof (PBE) type within the generalized gradient approximation (GGA) and also of Ceperly and Alder–Perdew and Zunger (CA-PZ) type within the local density approximation (LDA). 18 , 22 , 23 The optimizations for both chalcogenide crystal structures are done by the Broyden–Fletcher–Goldfarb–Shanno (BFGS) method 24 using the following optimization input parameters: a plane wave basis set kinetic energy cutoff of 550 eV, a Monkhorst–Pack k -point mesh size 25 of 6×6×3, an energy convergence threshold of 5 × 10 –6 eV/atom, a maximum force of 0.01 eV/Å, a maximum stress of 0.02 GPa, and a maximum atomic displacement of 5 × 10 –4 Å.…”

“…Boron (B)-rich compounds having structural units of B 12 closed cage clusters are novel systems attracting significant interest of the research community. 1 − 6 Many interesting physical properties such as high hardness, useful electronic properties, low mass density, a very high melting point, and excellent thermal stability and chemical inertness of B-rich compounds have gained extra attention of the materials science and engineering research community in view of the potential technical applications. 1 , 6 Among the B-rich compounds, the B-rich solids with icosahedral clusters 1 , 6 , 7 structurally derived from α-rhombohedral boron (α-rh boron) offer some industrial potential candidate materials similar to boron carbide, boron sub-oxide and boron sub-phosphide, REB 15.5 CN, REB 22 C 2 N, and REB 28.5 C 4 (RE = heavy rare-earth element, Y, and Sc).…”