Full-potential Density Functional Theory Calculations Research Articles

First principle study on the mechanical response of ZrC and ZrN at high-pressure conditions: anisotropy perspective

ABSTRACT We perform full-potential density functional theory calculations to study the mechanical properties of ZrC and ZrN ceramics in the pressure range of 0-100 GPa. Birch-Murnaghan (B-M) isothermal equation of state was used to estimate energy-volume relationship and related structural parameters. We use Viogt-Reuss-Hill (VRH) approximation to account for the polycrystalline elasticity of the compounds. We calculate elastic constants as a function of pressure and cover bulk, shear, and Young’s moduli, hardness, and Poisson’s ratio. For each property, we report the yields of Voigt, Reuss, and Hill approximations as a function of pressure. The pressure-dependent absolute and universal anisotropy have also been reported. We follow our calculations by exploring the brittleness/ductility of ZrN and ZrC by calculating Cauchy pressure and Pugh’s ratio. Among thermal properties, we calculate minimum thermal conductivity and Debye temperature. The calculated results indicate that ZrN has higher bulk modulus; however, the shear and Young’s modulus are comparable for ZrC and ZrN at all pressures. ZrN has a higher Poisson ratio but, ZrC shows higher hardness in the calculated pressure range. Excluding Poisson’s ratio, pressure has an increasing effect on these properties, where the rate of increase is almost identical for both compounds.

Enhanced optical and thermoelectric properties of ZnS monolayer and stacked bilayer compared with bulk

In this study, we report the structural, electronic, optical and thermoelectric properties for three different forms of ZnS : bulk (ZnS-B), stacked bilayer (ZnS-BL) and monolayer (ZnS-ML), using first-principles of full-potential density functional theory calculations and semiclassical Boltzmann transport theory. Our results show that ZnS-BL and ZnS-ML with relatively low formation energies of about 152 meV/atom and 209 meV/atom respectively could be exfoliated from ZnS bulk and stabilized in a structure like that of graphene. The optimized ZnS-ML gives a planar structure whereas AB stacking bilayer gives low buckling effect of about 0.003 Å. The calculated electronic band structures and optical properties using TB-mbj approximation of the above structures reveal that their relatively large and direct electronic band gap (4.31 eV for ZnS-ML and 3,94 for stacked ZnS-BL) and their optical performances like low reflectivity and high transparency in the visible light with a widening of the optical window make them a candidate for future opto-electronic devices. Based on electron transport coefficients, we found a larger power and merit factors values, for ZnS-ML and ZnS-BL than ZnS-B, which imply an inherent advantage in their electrical transport properties and make them a potential two-dimensional material comparable to other known good thermoelectric materials.

Structural, Electronic and Magnetic Properties of Silicene Substituted with Monomers and Dimers of Cr, Fe and Co

Extensive full-potential density functional theory calculations were performed to investigate the structural, electronic and magnetic properties of silicene substituted with monomers and dimers of Cr, Fe and Co atoms. Different concentrations of monomers between 1 and 12 percent were considered. The structures substituted with all concentrations of Fe were metal and showed magnetic properties. The structures substituted with Co and Cr were half-metal or metal and, surprisingly, silicene substituted with 1.4 percent Co did not show a magnetic moment. Homonuclear and heteronuclear dimers of the above atoms were investigated for two different substitution positions in silicene. The structures showed metallic or half-metallic properties and a range of magnetic moments from 0 to 6 . This variety of electronic and magnetic properties suggest that metal-substituted silicene materials could play essential roles in nano-electronic devices in fields such as spintronics.

Tailoring the structural and electronic properties of a graphene-like ZnS monolayer using biaxial strain

Our first-principles full-potential density functional theory calculations show that a ZnS monolayer (ML-ZnS), which is predicted to adopt a graphene-like planar honeycomb structure with a direct band gap, undergoes strain-induced modifications in its structure and band gap when subjected to in-plane homogeneous biaxial strain (δ). ML-ZnS gets buckled for compressive strain greater than 0.92% ; the buckling parameter Δ(= 0.00 Å for planar ML-ZnS) linearly increases with increasing compressive strain (Δ = 0.435 Å at δ = −5.25%). A tensile strain of 2.91% turns the direct ML-ZnS band gap into indirect. Within our considered strain values of |δ| < 6%, the band gap shows linearly decreasing (non-linearly increasing as well as decreasing) variation with tensile (compressive) strain. These predictions (based on our calculations with two atoms per unit cell) may be exploited in future for potential applications in strain sensors and other nano-devices such as nano-electromechanical systems and nano-optomechanical systems.

Errata: Graphene and Some of its Structural Analogues: Full-potential Density Functional Theory Calculations [WJOE 10(1) (2013) 39-48

List of Corrections: 1. Page No. 41, Left Column, Paragraph No.1, Line No. 11, Read “ 2 μμeV/ato.” as “ 2 μeV/atom.” 2. Page No. 41, Right Column, Paragraph No. 2, Line No. 11, Read “Mukhopadhyay, 2012)” as “Mukhopadhyay 2010)” 3. Page No. 43, Table 2, Second Row for MLBN, Read “183.91” as “181.91” 4. Page No. 44, Left Column, Read Equation No. 1 as “E± = ± VF hk” 5. Page 44, Left Column, Read Line No. 2 as “charge carriers in graphene and (h k) is the” 6. Page 45, Table 3, Row No. 8 and Column 2 for ML-SiC, Read “2.39 (K→M)” as “2.39” 7. Page 45, Table 3, Row No. 15 and Column 2 for ML-BN, Read “4.35 (K→Γ)” as “4.35” 8. Page 45, Table 3, Row Nos. 22-26 and Column 2 for ML-ZnO, ML-ZnS and MLZnSe, Read “(K→Γ)” as “(Γ→Γ)” 9. Page 46, Left Column, Paragraph 1 (References), Last Line, Read “Phys. Rev. Lett. 96, 66-102” as “Phys. Rev. Lett. 96, 066102.” Errata: Graphene and Some of its Structural Analogues: Full-potential Density Functional Theory Calculations [WJOE 10(1) (2013) 39-48]

Graphene and some of its structural analogues: full-potential density functional theory calculations

Using full-potential density functional calculations we have investigated the structural and electronic properties of graphene and some of its structural analogues, viz., monolayer (ML) of SiC, GeC, BN, AlN, GaN, ZnO, ZnS and ZnSe. While our calculations corroborate some of the reported results based on different methods, our results on ZnSe, the two dimensional bulk modulus of ML-GeC, ML-AlN, ML-GaN, ML-ZnO and ML-ZnS and the effective masses of the charge carriers in these binary mono-layers are something new. With the current progress in synthesis techniques, some of these new materials may be synthesized in near future for applications in nano-devices.

Diffraction refinement of localized antibonding at the Si(111)7×7surface

We report an experimental refinement of the local charge density at the Si(111) $7\ifmmode\times\else\texttimes\fi{}7$ surface using a combination of x-ray and high-energy electron diffraction. This method can be generally applied to the charge-density refinement at surfaces of other materials. By perturbing about a bond-centered pseudoatom model, we find experimentally that the adatom electrons occupy antibonding-like backbond states with the atoms below. We are also able to refine a charge transfer of $0.26\ifmmode\pm\else\textpm\fi{}0.04\text{ }{e}^{\ensuremath{-}}$ from each adatom to the underlying layers, in agreement with full-potential density-functional theory calculations.

Role of carbon inAlCNi3andGaCNi3: A density functional theory study

In this paper we report first-principles calculations on the ternary antiperovskites $\mathrm{Al}\mathrm{C}{\mathrm{Ni}}_{3}$ and $\mathrm{Ga}\mathrm{C}{\mathrm{Ni}}_{3}$. We find that in contrast to recent experimental results $\mathrm{Al}\mathrm{C}{\mathrm{Ni}}_{3}$ and $\mathrm{Ga}\mathrm{C}{\mathrm{Ni}}_{3}$ are nonmagnetic metals. Taking into account data from the literature as well as our own calculations, we have evidence that some of the recent experiments on $\mathrm{Al}\mathrm{C}{\mathrm{Ni}}_{3}$ and $\mathrm{Ga}\mathrm{C}{\mathrm{Ni}}_{3}$ may have been performed on samples with carbon deficiencies. We believe that the conclusions which have been drawn from these experiments caused several misinterpretations. First, neither in our density functional theory (DFT) calculations for $\mathrm{Al}\mathrm{C}{\mathrm{Ni}}_{3}$ nor in those for $\mathrm{Ga}\mathrm{C}{\mathrm{Ni}}_{3}$ do we find indications for a magnetic ground state $(\mathrm{Al}\mathrm{C}{\mathrm{Ni}}_{3})$ or the presence of strong electron-electron correlations $(\mathrm{Ga}\mathrm{C}{\mathrm{Ni}}_{3})$. Second, we find that the local density approximation (LDA) and in particular the generalized gradient approximation (GGA) are well suited for a proper description of these compounds. The unusually large volume differences between the LDA calculations and some experiments are thus rather due to a comparison with substoichiometric samples. By employing accurate full-potential DFT calculations using both the LDA and the GGA we performed fixed spin moment calculations in order to estimate the Stoner enhancement factors $S$. In addition we investigated $T{\mathrm{C}}_{0.5}{\mathrm{Ni}}_{3}$ with $T=\mathrm{Al}$ and Ga and found that the density of states at the Fermi energy is expected to increase about linearly upon lowering the carbon concentration. This explains the tendencies toward magnetism found in experiment, since for the carbon-free border compounds $\mathrm{Al}{\mathrm{Ni}}_{3}$ and $\mathrm{Ga}{\mathrm{Ni}}_{3}$ their proximity to ferromagnetic order is well established.

Electronic Structure and Excited-state Properties of Perovskite-like Oxides

The role of structural distortion, magnetic ordering, and Coulomb-correlation effect on the electronic structure of perovskite-like oxides is analyzed. The density-functional theory (DFT) is originally devised to describe the ground-state properties of materials. However, during recent years it has been seen that the DFT can also be used to study excited state properties succesfully. We have recently calculated the electronic structure and linear optical properties for the series LaXO 3 (X=Sc–Cu) and found that the gradient-corrected DFT describes correctly the insulating behavior of the ionic insulator LaScO 3 and the charge-transfer insulators LaCrO 3, LaFeO 3, and LaMnO 3 although the band gaps are systematically underestimated. For example, the good agreement between experimental and theoretical reflectivity spectra for LaCrO 3 clearly demonstrates that accurate full-potential DFT calculations not only describe the occupied and unoccupied states of the bands well, but also reproduce their characters. We have also calculated XPS, XANES, and magneto-optical spectra for perovskite-like oxides. For Mott–Hubbard insulators such as LaTiO 3 and LaVO 3 the DFT failed to predict insulating behavior and here the LDA+ U method is applied to describe the electronic structure correctly.