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Register a new userOrigin of high hardness and optoelectronic and thermo-physical properties of boron-rich compounds B6X (X = S, Se): a comprehensive study via DFT approach
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Added by
Md. Mukter Hossain PhD
Publication date
2021
fields
Physics
and research's language is
English
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In the present study, the structural and hitherto uninvestigated mechanical (elastic stiffness constants, machinability index, Cauchy pressure, anisotropy indices, brittleness/ductility, Poissons ratio), electronic, optical, and thermodynamic properties of novel boron-rich compounds B6X (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, KIC for the B6S and B6Se are ~ 2.07 MPam0.5, evaluating the resistance to limit the crack propagation inside the materials. Both B6S and B6Se 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 ({Theta}D), minimum thermal conductivity (Kmin), lattice thermal conductivity (kph), 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.
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New boron-rich sulfide B6S and selenide B6Se have been discovered from high pressure - high temperature synthesis combined with ab initio evolutionary crystal structure prediction, and studied by synchrotron X-ray diffraction and Raman spectroscopy at ambient conditions. As it follows from Rietveld refinement of powder X-ray diffraction data, both chalcogenides have orthorhombic symmetry and belongs to Pmna space group. All experimentally observed Raman bands have been attributed to the theoretically calculated phonon modes, and the mode assignment has been performed. Prediction of mechanical properties (hardness and elastic moduli) of new boron-rich chalcogenides have been made using ab initio routines, and both compounds were found to be members of a family of hard phases (Hv ~ 31 GPa).
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.
In the present study, the structural, electronic, optical and thermoelectric properties of two isostructural chalcogenide materials, NaInS2 and NaInSe2 with hexagonal symmetry (R-3m) have been studied using the first principles method. A very good agreement has been found between our results with the available experimental and theoretical ones. The studied materials are semiconducting in nature as confirmed from the electronic band structure and optical properties.The strong hybridizations among s orbitals of Na, In and Se atoms push the bottom of the conduction band downward resulting in a narrower band gap of NaInSe2 compared to that of NaInS2 compound. Different optical (dielectric function, photoconductivity, absorption coefficient, reflectivity, refractive index and loss function) and thermoelectric (Seebeck coefficient, electrical conductivity, power factor and thermal conductivity) properties of NaInX2 (X = S, Se) have been studied in detail for the first time. It is found that all these properties are significantly anisotropic due to the strongly layered structure of NaInX2 (X = S, Se). Strong optical absorption with sharp peaks is found in the far visible to mid ultraviolet (UV) regions while the reflectivity is low in the UV region for both the compounds. Such features indicate feasibility of applications in optoelectronic sector.The calculated thermoelectric power factors at 1000 K for NaInS2 and NaInSe2 along a-axis are found to be 151.5 micro Watt /cmK2 and 154 micro Watt/cmK2, respectively and the corresponding ZT values are ~0.70. The obtained thermal conductivity along a-axis for both compounds is high (~22 W/mK).This suggests that the reduction of such high thermal conductivity is important to achieve higher ZT values of the NaInX2(X = S, Se) compounds.
By using solid-state reactions, we successfully synthesize new oxyselenides CsV$_2$Se$_{2-x}$O (x = 0, 0.5). These compounds containing V$_2$O planar layers with a square lattice crystallize in the CeCr$_2$Si$_2$C structure with the space group of $P4/mmm$. Another new compound V$_2$Se$_2$O which crystallizes in space group $I4/mmm$ is fabricated by topochemical deintercalation of cesium from CsV$_2$Se$_2$O powder with iodine in tetrahydrofuran(THF). Resistivity measurements show a semiconducting behavior for CsV$_2$Se$_2$O, while a metallic behavior for CsV$_2$Se$_{1.5}$O, and an insulating feature for V$_2$Se$_2$O. A charge- or spin-density wave-like anomaly has been observed at 168 K for CsV$_2$Se$_2$O and 150 K for CsV$_2$Se$_{1.5}$O, respectively. And these anomalies are also confirmed by the magnetic susceptibility measurements. The resistivity in V$_2$Se$_2$O exhibits an anomalous log(1/$T$) temperature dependence, which is similar to the case in parent phase or very underdoped cuprates indicating the involvement of strong correlation. Magnetic susceptibility measurements show that the magnetic moment per V-site in V$_2$Se$_2$O is much larger than that of CsV$_2$Se$_{2-x}$O, which again suggests the correlation induced localization effect in the former.
We explain the nature of the electronic band gap and optical absorption spectrum of Carbon - Boron Nitride (CBN) hybridized monolayers using density functional theory (DFT), GW and Bethe-Salpeter equation calculations. The CBN optoelectronic properties result from the overall monolayer bandstructure, whose quasiparticle states are controlled by the C domain size and lie at separate energy for C and BN without significant mixing at the band edge, as confirmed by the presence of strongly bound bright exciton states localized within the C domains. The resulting absorption spectra show two marked peaks whose energy and relative intensity vary with composition in agreement with the experiment, with large compensating quasiparticle and excitonic corrections compared to DFT calculations. The band gap and the optical absorption are not regulated by the monolayer composition as customary for bulk semiconductor alloys and cannot be understood as a superposition of the properties of bulk-like C and BN domains as recent experiments suggested.
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