GGA Exchange Research Articles

Alkali-based half metals as sustainable materials for spin electronics and energy harvesting application — Materials computation

The structural, mechanical, electronic, magnetic, and thermoelectric properties of alkali-based half-Heusler alloys (LiCrGe, LiCrSn, and LiCrPb) are investigated using the Wien2k code. These alloys exhibit half-metallic behavior in both ferromagnetic and antiferromagnetic phases. A phase transition from a stable ferromagnetic phase to a more stable antiferromagnetic phase indicates ductility. In the antiferromagnetic phase, using GGA exchange–correlation functional with 10,000 K-points, a band gap is observed in the spin-up state, with band gap energies of 0.9367 eV, 0.7762 eV, and 0.7913 eV, respectively. The alloys have a spin magnetic moment of −3μB, consistent with the Slater–Pauling rule. They also exhibit high Curie temperatures and 100% spin polarization in the spin-down state. The alloys LiCrZ (Z = Ge, Sn, Pb) shows promising thermoelectric behavior. Using the BolzTrap package, we observed, high Seebeck coefficient, electrical conductivity, low thermal conductivity, and enhanced power factor. The p-type LiCrZ (Z = Ge, Sn, Pb) compounds show thermoelectric figure of merit of 0.52, 0.76, and 0.84 at 1200 K, while n-type compounds have figure of merit of 0.56, 0.65, and 0.80 at the same temperature. These properties make these materials promising for spintronic and thermoelectric applications.

Probing the structural, electronic and optical properties of pure and B, N, or Li substituted Cyclo-18 ring: density functional theory investigations

Employing the quantum computational approach by using the Density Functional Theory along with GGA exchange correlation functional, we have investigated the structural, electronic, and optical properties of Cyclo-18 ring containing 18 sp hybridized carbon atoms and substituted Cyclo-C17X (X = B, N, and Li) ring. The Cyclo-18 ring has two opposite π electron system that can be organized as a D9h polyynic and D18h cumulene form. Our computational simulations suggest that D9h polyynic structure is minimum energy structure. Alkali metal doping makes C18 metallic by lowering the band gap when compared to the pure C18 (5.02eV). The strength of the chemical bonding analyzed using average binding energies for the Li, B, and N substituted Cyclo-C18 ring which are −4.58 eV, −4.65 eV, and −2.83 eV respectively. The positive charges on B, N and Li and negative charges on the Cyclo-18 ring demonstrate the partial Coulomb interactions and also charge transfer from B, N, and Li to Cyclo-18 ring. It is also found that the dominant adsorption IR peak at 2049 cm−1, 1329 cm−1, and 1011 cm−1 for B, N, and Li substituted C18 ring. There is an enhancement in optical absorption in the visible region due to doping which makes the system suitable for photo-catalytic applications.

A comprehensive investigation on the physical properties of SiC polymorphs for high-temperature applications: A DFT study based on GGA and hybrid HSE06 exchange correlation functionals

This investigation reports the structural, electronic, bonding, elastic, mechanical, thermodynamic, thermal and optical characteristics of SiC polymorphs by DFT based GGA and hybrid HSE06 ab-initio computational methodology. SiC exhibits a multitude of structural orientations, with cubic (3C SiC) and hexagonal (4H SiC and 6H SiC) being the most prevalent. In my research, I aim to gain a comprehensive understanding of these SiC polymorphs and their potential suitability for high-temperature applications, especially as a cladding material for accident-tolerant fuel (ATF) in new-generation nuclear power plants (NPPs). The structural stability is ensured by geometry optimization and the relaxed parameters are agreed well with the previously reported findings. All the studied SiC polytypes reveal the semiconducting electronic property. By utilizing the hybrid HSE06 functional, the obtained electronic band gap meets the experimental results more closely than that obtained by the GGA approximation. The mechanical stability of all studied SiC polymorphs is evident from their elastic and mechanical properties, showcasing a brittle nature. The analysis of bond populations reveals the coexistence of both ionic and covalent bonds within the SiC structure. The thermoelectric and thermal characteristics of mechanically stable SiC polymorphs revealing that, all the compounds possess high lattice thermal conductivity and high melting point. Among these, 3C SiC stands out with the highest thermal conductivity of 172 Wm-1K−1 at 300 K and a melting temperature of 2840 K. The relatively low dielectric constant, high absorption coefficient and broad reflectivity spectra with > 90 % reflection in the UV region suggest that, SiC polymorphs can also be the potential candidates for optoelectronic devices. All the extreme physical stabilities make SiC polymorphs suitable for practical applications in high temperature environments.

Ultrahigh-Density Superhard Hexagonal BN and SiC with Quartz Topology from Crystal Chemistry and First Principles

Based on superdense C6 with a quartz (qtz) topology, new ultrahigh-density hexagonal binary phases, qtz BN and qtz SiC, were identified via full geometry structure relaxations and ground state energies using calculations based on the quantum density functional theory (DFT) with a gradient GGA exchange–correlation XC functional. Like qtz C6, with respect to diamond, the resulting binary qtz BN and qtz SiC were found to be less cohesive than cubic BN and cubic SiC, respectively, but were confirmed to be mechanically (elastic constants) and dynamically (phonon band structures) stable. Higher densities of the new phases correlate with higher hardness values compared to cubic BN and cubic SiC. In contrast to the regular tetrahedra that characterize the cubic BN and SiC phases, the corner-sharing tetrahedra in the new phases are distorted, which accounts for their exceptional density and hardness. All three qtz phases were found to be semiconducting to insulators, with reduced band gaps compared to diamond, cubic BN, and cubic SiC.

On the possibility of low-symmetry structures in Zr2CoSi

First principles calculation shows that the energy difference between the cubic L21 and Hg2CuTi inverse structure for Zr2CoSi is 1.454 eV/f.u., indicating than the L21 structure is more stable. The density of states and band structure of Zr2CoSi in the L21 structure do not support the half–metallic state. Moreover, van Hove singularity involving partially occupied Co 3d eg states occurs in minority states at the Fermi level and the Zr2CoSi in L21 structure satisfies the preconditions to exhibit a cubic-to-tetragonal instability according to a band-Jahn-Teller mechanism. The computed total energy landscapes of the system shows that there are two tetragonal and one orthorhombic distorted phases of Zr2CoSi that are energetically favored over the undistorted cubic L21 structure. However, only one tetragonal phase of Zr2CoSi with c/a > 1.0 fulfills necessary and sufficient elastic stability conditions for both GGA and LDA exchange–correlation potentials.

Electronic properties of Y-247 ceramic via computational method

Y2Ba4Cu7Ox (Y-247) is one of the phases that came from the YBCO superconductor family. The superconductor has been studied through the computational method by applying the Density Functional Theory (DFT) via the first principles study. The structural of the pure Y-247 have been constructed in the Material Studios software and the electronic properties of the material were calculated and optimized using Generalized Gradient Approximation Perdrew Burke Ernzerhof (GGA PBE) and Local Density Approximation (LDA) exchanged correlation with OTFG Ultrasoft. From the band structure analysis, the pure Y-247 pseudogap observe above the fermi level and overlapped from the valance band to the conduction band. It is also shown that the conduction band and valence band overlapped each other for other phases, showing the conducting properties of the structure. Different pseudogap shown for both of the exchange correlation due to overbinding in GGA exchange correlation. Different phases for YBCO family shown difference in pseudogap due to the temperature dependance for each phases. For density of states, copper 3d orbital state and oxygen 2p state shows important role to maintaining the superconducting properties with electron-hole migration concept.

Toward Site-Specific Interactions of nH2 (n = 1–4) with Ga12As12 Nanostructured for Hydrogen Storage Applications

While hydrogen combustion generates a lot of energy and can be done in a variety of ways, the primary challenge in utilizing hydrogen energy is obtaining an efficient hydrogen storage material. Herein, the potential of Ga12As12 as a hydrogen adsorbent and storage material was investigated within the framework of density functional theory (GGA-DFT) computations at the B3LYP-GD3BJ/def2tzvp level of theory. The study was systematically conducted by increasing the number of molecular hydrogen adsorptions (n = 1–4) at Ga- and As- sites of the Ga12As12 adsorbent material. Results showed that adsorption on the As site is preferred as the hydrogen binding on this site is closer to the DoE requirement. Via DFT-GGA with the incorporation of D3 dispersion, we demonstrated that the Ga12As12 nanocluster can store up to four molecular hydrogens with a calculated gravimetric wt % of 5.71%, closer to the 6.5 wt % proposed by the DoE. Average binding energies for both As and Ga adsorption sites were observed to be −0.49 and −0.84 eV, respectively, which is within the range of H2 adsorption energy according to DoE. The electronic properties, thermodynamics, and the density of state disclosed a linear relationship with the increase in H2 adsorption. This trend is also seen in the adsorption energy, which shows a higher adsorption range as the number of hydrogen molecules on the Ga12As12 nanocage increases. Ab initio molecular dynamics simulations divulged that the studied system is considerably stable both at room temperature and at extreme temperatures. Based on the utilization of GGA exchange correlations, confirmation of stability via ab initio MD simulations, high desorption temperature (1454 K), and the computed gravimetric wt % (5.71), which is close to the DoE standard (6.5%), we strongly believe that proper surface engineering of the studied Ga12As12 nanocluster could further improve the overall properties and suitability toward hydrogen storage applications.

Investigation of the optoelectronic properties of cubic metal halide perovskite compound RbSiCl3

Lead free halide metal perovskite compounds have been exploring more since last decades due to their wide potential applications as well as their environmental friendly nature. Presented work agreements with the density functional investigation of the material properties (optoelectronic) of lead free halide metal perovskite compound RbSiCl3 by using WIEN2k code that is rely on FP-LAPW: full potential-linearized augmented plane wave method. The optoelectronic response are well explored by the Perdew Burke Ernzergof GGA exchange correlation functional. The computed energy band gap of this compound is 0.96 eV and results reveal that it is direct band gap semiconductor material. In addition, the integrated absorption coefficient also computed through absorption spectra, which gives the value 123.39 (×104 eV/cm). All computation proves the utility of the compound for photovoltaic and optoelectronic applications.

First-principles study of half-metallic properties in X2CrAl (X = Co and Mn) FullHeusler and their quaternary MnCoCrAl and CoMnCrAl compounds

The full potential linearized augmented plane waves (FP- LAPW) approach is used to investigate the structural, electronic, and magnetic properties of full-Heusler alloys X2CrAl (X = Co and Mn) and their quaternary compounds MnCoCrAl and CoMnCrAl. The density of states (DOS) of Co2CrAl, Mn2CrAl, and related quaternary compounds reveal the existence of energy band gaps with half-metallic behavior based on GGA and mBJ exchange–correlation potential.All full-Heusler materials and their quaternary compounds have total magnetic moments that agree well with the Slater Pauling rule. Mechanical properties of these compounds are also researched, including bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Pugh's ratios, and all studied compounds display mechanical stability under the mechanical stability criterion. Our findings provide experimental researchers with suggestions for synthesizing promising materials for future spintronic applications.

Band engineering of modified rhombohedral Cu4Mn2Te4: ab initio approach

ABSTRACT Systematic band structure calculations are reported for the first time on the ‘modified rhombo (R-3 m) structured Cu4Mn2Te4’ compound by adapting the Lotgering model. Wyckoff positions of all the atoms are estimated. There is a good agreement between the present computed lattice parameter values with the already reported. Mechanism of phase transition from Fd-3m structure (3Cu added spinel CuMn2Te4) to R-3m structure (distorted rhombo Cu4Mn2Te4) at room temperature is discussed. Spin-polarised calculations reveal Cu4Mn2Te4 energetically prefers the AFM phase at ambient conditions. Band structures predict the compound to be of metallic nature at its NM and FM phases whereas a band gap opening occurs at its AFM phase. Band gap value is estimated as ∼ 0.2 eV and ∼0.7 eV using GGA and mBJ exchange–correlation schemes, respectively. Origin of band gap opening is discussed. Total spin magnetic moment (µB ) per formula unit of Cu4Mn2Te4 compound is calculated as 9.878 and −5.001, respectively, at FM and AFM phases. Electrical conductivity is enhanced by a factor of 102 from 100 K to 300 K at minority spin states. Maximum optical conductivity occurs at 6500 Ω−1cm−1 which lies in the visible region of the electromagnetic spectrum.

Surface half metallicity and thermodynamic stability of 001-plane Ti2XSi (X=Mn, Co) Heusler alloys (HAs): A DFT approach

We report the surface stability and surface half-metallicity of Ti2XSi (X=Mn/Co) [001] slab with natural TiSi and TiX(X=Mn/Co) terminals from the first principles calculation. We started our calculation from the bulk optimization followed by the electronic and magnetic properties by adopting GGA exchange correlation for treating all electrons interaction. We have also analyzed the surface stability by calculating the surface energies as a function of constituents chemical potentials within the framework of ab-initio thermodynamics. Within the allowed chemical potentials range, TiSi(Ti2MnSi) terminated surface found to be the most energetically favorable thin film while the TiX terminated surfaces show strong molecular attraction. The bulk half metallicity is preserved in TiSi(Ti2MnSi) terminated surface with 100% spin polarization while the other terminal surfaces are metallic. The results of atomic site partial magnetic moments in the surface states along with their corresponding values in the bulk structure are also presented.

Theoretical DFT studies of Cu2HgSnS4 absorber material and Al:ZnO/ZnO/CdS/Cu2HgSnS4/Back contact heterojunction solar cell

We investigated a single heterojunction solar cell with stannite phase Cu2HgSnS4 as a new potential absorber material using the hybrid density functional theory for materials parameter and macroscopic device simulation studies for the photovoltaic response. The lattice constants are optimized using PBEsol and GGA exchange–correlation approach with mBJ parameterization and considering spin–orbit coupling effect on heavy element Hg, while strong correlation of Cu and Hg 3d electrons are taken into account by Hubbard parameter U = 0.52 Ry. The computed bandgap is ~1.33 eV. The effective mass of electrons and holes in the respective band edges (electron in conduction band and the hole in the valence band) is 0.25 m0 and 0.91 m0, respectively. Further, the computed materials optoelectronic parameters are used to optimize the device performance by introducing a variation in minority carrier lifetime, defect concentration in Cu2HgSnS4 absorber, Cu2HgSnS4/CdS interface, and the absorber thickness to achieve a realistic photovoltaic response. The optimal conversion efficiency is 11.6% after taking more realistic parameters in the considered single-junction solar cell. However, the maximum photovoltaic response >17% can be achieved by controlling absorber and interface defects together with optimal carrier concentration.

First-principle study of structural and electronic properties of V2Se

First-principles calculation for anti-fluorite type cubic V2Se compound has been performed to study its structural, electronic and magnetic properties using WIEN2k. We used LDA and GGA as exchange-correlation energy functional for our calculation. For the calculation of ground state electronic properties, we implemented DFT + U method with the Hubbard parameter of 5.5 eV. Geometry optimization for the ferromagnetic, anti-ferromagnetic and non-magnetic states were done and results indicate that the material is more stable in ferromagnetic state. The equilibrium lattice constant and bulk modulus were found for both LDA and GGA exchange correlation energy functional which is in agreement with the experimental lattice constant. Electronic band structure and density of states have been studied. The electronic properties indicate that the material exhibit metallic behavior.

A computational study of intercalation of streptozotocin (STZ) into DNA base pairs.

Deoxyribonucleic acid (DNA) drug intercalation is a well-known phenomenon for the treatment of cancer. Streptozotocin (STZ) is a drug agent containing toxic properties that make it good in the pancreatic cancer. The main objective of this study is the intercalation of the anticancer drug into the stacked base pair of DNA sequence with ATGC using a density functional theory (DFT) code named as ADF-Molecule. ADF code implements DFT using the Slater-type orbitals (STO) for computational analysis of atomic and molecular structures. All the calculations were carried out with the GGA and hybrid exchange correlation functional with TZ2P basis sets. It was captivatingly studied that during the intercalation process, the bonds between the DNA base pairs broken. Moreover, during the process of intercalation, the free radicals are considered responsible for disturbance in the base configurations. It was determined that the disturbances that occurred in the base pairs lead to discontinuity in the replication of that particular sequence in the DNA strand.

Electronic structures of trivalent cations doped bulk and cubic La2Mo2O9 oxide ion conductors

Effect of intrinsic oxygen vacancies and different dopants such as Bi, Er and Y on the electronic structure of La2Mo2O9 have been studied in the framework of density functional theory using plane-wave pseudopotential method. First principles electronic structure calculations are performed using GGA exchange correlation functional to estimate the density of states and bands structures. Our calculated electronic density of states and band structures show that these systems are semiconductors with an indirect band gap ~ 2 ​eV. Electronic band gap increases with the increasing concentration of Bi, unlikely with that of Er and Y. O-2p and Mo-4d orbitals are mainly responsible for tuning the band gap. Bi remains the most electron donating character, whereas both Er and Y play the role of electron acceptors as evident from partial charge distribution plots and Bader charge analyses. Ionic bonds are observed for La, Bi, Er, and Y with O, but the bonds between Mo and O are covalent in nature for all these compounds.

Cation modified A2(Ba, Sr and Ca) ZnWO6 cubic double perovskites: A theoretical study

The cubic double perovskites A2ZnWO6 (A = Ba, Sr and Ca) are studied to understand the effect of A cation site, using the density functional theory (DFT) based full potential augmented plane wave method (FP-LAPW) with GGA and mBJ exchange correlation potentials. The structural robustness and stability are investigated using the bond lengths and the total energy. The band structure and density of states suggest that all these cubic double perovskites are indirect wide band gap semiconductors. The band gap varies from 3.90 eV (2.97 eV) for Ba2ZnWO6 system to 3.40 eV (2.8 eV) for Ca2ZnWO6 system using mBJ (GGA) exchange correlation potentials. Our studies suggest that A cation site modification has a strong effect on physical and electronic properties, in contrast to the structural robustness. The lattice parameter decreases from 8.19 Å to 7.9 Å from Ba to Ca at alkali cation site and the electronic band gap variation follows the common cation rule. The charge densities show enhanced localization of charges near the zinc and oxygen sites with increasing alkali cation atomic radii. In addition, we discuss the impact of A cation site modification on the dielectric and optical properties for A2ZnWO6 double perovskites.

Quantifying confidence in density functional theory predictions of magnetic ground states

The success of descriptor-based material design relies on eliminating bad candidates and keeping good candidates for further investigation. While DFT has been widely successfully for the former, often times good candidates are lost due to the uncertainty associated with the DFT-predicted material properties. Uncertainty associated with DFT predictions has gained prominence and has led to the development of exchange correlation functionals that have built-in error estimation capability. In this work, we demonstrate the use of built-in error estimation capabilities within the BEEF-vdW exchange correlation functional for quantifying the uncertainty associated with the magnetic ground state of solids. We demonstrate this approach by calculating the uncertainty estimate for the energy difference between the different magnetic states of solids and compare them against a range of GGA exchange correlation functionals as is done in many first principles calculations of materials. We show that this estimate reasonably bounds the range of values obtained with the different GGA functionals. The estimate is determined as a post-processing step and thus provides a computationally robust and systematic approach to estimating uncertainty associated with predictions of magnetic ground states. We define a confidence value (c-value) that incorporates all calculated magnetic states in order to quantify the concurrence of the prediction at the GGA level and argue that predictions of magnetic ground states from GGA level DFT is incomplete without an accompanying c-value. We demonstrate the utility of this method using a case study of Li and Na-ion cathode materials and the c-value metric correctly identifies that GGA level DFT will have low predictability for NaFePO$_4$F.

Study of Structural and Electronic Properties of Fluoride Perovskite KCaF3 using FP-LAPW Method

To study the structural and electronic properties of cubical perovskite KCaF3, the first principles calculation within the full potential linearized augmented plane wave (FP-LAPW) method is applied. The exchange correlation effects are included through the GGA exchange potential. The calculated structural properties such as equilibrium lattice constant, the bulk modulus and its pressure derivative are in agreement with the published results of other authors. From our study we have found that the band gap of KCaF3 is 6.4 eV which is the indication of insulating behavior.The Himalayan Physics Vol. 6 & 7, April 2017 (100-103)

Band gap engineering in ruthenium-based Heusler alloys for thermoelectric applications

In this work, systematic electronic structure calculations are performed on Ru2TiZ alloys to examine their structural and thermoelectric related transport properties. The electronic structural properties are analyzed using the GGA and GGA+U as exchange correlational potential. The calculated lattice parameters agree very well with the existing experimental data, and the percentage of error is less than 1%. The electronic structural properties as analyzed, using the GGA exchange correlation scheme, reveal that these alloys can be semimetals. In the band structure, it is observed that ruthenium-d and titanium-d states are lying very close to the Fermi level. Hence, computations are performed again by including the Hubbard potential for d states of ruthenium and titanium. The calculated electronic structure in GGA+U reveals that all the 3 alloys are semiconductors with the indirect energy gap of 0.209, 0.175, and 0.259 eV, respectively, for Ru2TiSi, Ru2TiGe, and Ru2TiSn. The electrical transport coefficients are calculated in both the GGA and GGA+U exchange correlational potential and reported. If experimentalists prove that these alloys are semiconductors, then all 3 alloys will be potential thermoelectric materials. If by experiment they are semimetals, only then Ru2TiSn can be a good thermoelectric material. Doping of electron and hole for various concentrations is studied for Ru2TiSn, and optimum doping level is reported.

Role of the Crystal Lattice Constants and Band Structures in the Optoelectronic Spectra of CdGa2S4 by DFT Approaches

The electronical and optical properties of CdGa2S4 under high pressures were studied using the full potential linearized augmented plane wave (FP‐LAPW) method within the GGA and mBJ exchange correlation potentials from 0.0 to 16.92 GPa. The obtained results show that the lattice constants, bandgap values, and optoelectronic properties are sensitive to applied external pressures. The mBJ results indicate that the bandgap increases and the static dielectric constants decrease with increasing the pressure. The two none zero dielectric tensor components show considerable anisotropy between the perpendicular and parallel components. The maximum absorption for x direction in all pressures takes place in vacuum UV region. Also, the plasma frequency shifts to the higher energies with increasing the pressure for application in optical devices. The calculated results by mBJ are in close agreement with the experimental values.