Generalized Gradient Approximation Approaches Research Articles

Enhancement of the magneto-electronic properties by GGA and TB-mBJ approaches for KMgO3 perovskite oxide

We investigate the structural, elastic, electronic and magnetic properties of KMgO3. First-principal calculations, based on the formalism of the density functional theory (DFT) and the method of full potential augmented and linearized plane waves (FP-LAPW) implemented in the Wien2k code. The exchange and correlation effects were treated by the following two approximations: generalized gradient approximation (GGA) and Tran-Blaha modified beck Johnson (TB-mBJ) potentials. After analyzing the obtained structural parameters, the results revealed that KMgO3compound is most stable in its ferromagnetic configuration. The formation energy value showed that this compound can be experimentally synthesized. Furthermore, the calculated band structures, and density of states (DOSs) indicate the half-metallic behavior of KMgO3. We found also that the total magnetic moment is an integer value of 3μBwhich confirms the half-metallic character. The magnetic moment specially issues from the spin-polarization of p electrons of O atoms.

Calculations of Ba(1-x)SrxTio3 structure and band gap properties by using density functional theory

TThe aim of this study is to simulate features using molecular modeling methods. The point is to show that it will accelerate research in material development studies by directing us, researchers, in terms of gaining time, material and workforce. In this study, the structural and electronic properties of undoped BaTiO3 and Sr-doped BaTiO3 were calculated by molecular modeling. In the study, energy calculations were made with the PBE and GGA (Generalized Gradient Approximation approach) developed by Perdew, Burke and Ernzerhof (PBE) using the density functional theory (DFT) calculation method, the CASTEP module of the Materials Studio program. First, the structural and electronic properties of the BaTiO3 crystal phase were calculated. Then, the lattice constants, band gap values and electron state densities of the Sr doped structure to BaTiO3 structure were calculated. The values in the literature were compared with the calculations made using the( DFT) density functional theory and it was determined that the calculations were in agreement with the values in the literature. It has been revealed that it will accelerate research in material development studies by giving direction to us researchers in terms of gaining from materials and workforce. As a result of geometric optimization of the non-stoichiometric Ba(1-x)SrXTiO3 structure and DFT calculations, it was determined that the electronic band gap shifted after %1 and %3Sr addition towards the conduction band and the band gap respectively decreased to 1,911 eV and 1.989eV.

Machine learning accelerated study for predicting the lattice constant and substitution energy of metal doped titanium dioxide

Currently, titanium dioxide (TiO2) has been extensively studied for its wide applications in many fields, and metal doping is regarded as one of the important methods for modifying TiO2 to improve its performance. Previous explorations on metal doped TiO2 mainly relied on density functional theory (DFT) calculations or chemical experiments, which required lots of time, computation resource, and costs. In this study, we validated the potential of combining machine learning (ML) methods with DFT calculations to improve the efficiency of developing metal doped TiO2 materials. The doped oxide systems comprising 22 different metal elements were investigated, and the lattice constant and substitution energy were calculated via the generalized gradient approximation approach to form the dataset for ML. It is found that through reasonable feature selection and ML modeling validation, the gradient boosting decision tree model performs exceptionally well in predicting the lattice constant and substitution energy. This study provides an effective strategy for developing doped oxide systems based on ML method.

Role of Native Defects in Fe-Doped β-Ga2O3.

Iron impurities are believed to act as deep acceptors that can compensate for the n-type conductivity in as-grown Ga2O3, but several scientific issues, such as the site occupation of the Fe heteroatom and the complexes of Fe-doped β-Ga2O3 with native defects, are still lacking. In this paper, based on first-principle density functional theory calculations with the generalized gradient approximation approach, the controversy regarding the preferential Fe incorporation on the Ga site in the β-Ga2O3 crystal has been addressed, and our result demonstrates that Fe dopant is energetically favored on the octahedrally coordinated Ga site. The structural stabilities are confirmed by the formation energy calculations, the phonon dispersion relationships, and the strain-dependent analyses. The thermodynamic transition level Fe3+/Fe2+ is located at 0.52 eV below the conduction band minimum, which is consistent with Ingebrigtsen's theoretical conclusion, but slightly smaller than some experimental values between 0.78 eV and 1.2 eV. In order to provide direct guidance for material synthesis and property design in Fe-doped β-Ga2O3, the defect formation energies, charge transitional levels, and optical properties of the defective complexes with different kinds of native defects are investigated. Our results show that VGa and Oi can be easily formed for the Fe-doped β-Ga2O3 crystals under O-rich conditions, where the +3 charge state FeGaGai and -2 charge state FeGaOi are energetically favorable when the Fermi level approaches the valence and conduction band edges, respectively. Optical absorption shows that the complexes of FeGaGai and FeGaVGa can significantly enhance the optical absorption in the visible-infrared region, while the energy-loss function in the β-Ga2O3 material is almost negligible after the extra introduction of various intrinsic defects.

First-principles calculations to investigate structural, electronic, magnetic, mechanical and thermodynamic properties of Half-Heusler alloy CoMnTe: Using GGA and GGA+U methods

In this paper, we study the structural, electronic, magnetic, mechanical and thermal properties of Half-Heusler CoMnTe, using the Density Functional Theory (DFT) with the generalized gradient approximation (GGA) and GGA plus the Hubbard correction (GGA+U). The obtained results show that the compound is more stable in γ-phase with the ferromagnetic (FM) configuration than the nonmagnetic (NM) among the three considered structural phases (α, β, γ) and there is no possibility of martensitic transformation under the tetragonal distortion effect. The computed total magnetic moments of CoMnTe by GGA and GGA+U are 4 μB and 4.13 μB, respectively, arising mainly from the Mn atom. The obtained density of states and band structures using GGA approach show that the compound exhibits half-metallic behavior with 100% spin polarization. As the Hubbard is added, the electronic properties show an electronic phase transition from half-metallic to metallic behavior with 68% spin polarization. While the thermal and mechanical properties show very small changes, confirming that the studied compound is mechanically and dynamically stable.

Study the change of some physical and chemical properties of TiN under high pressure

The pressure equation of state of Titanium nitrate TiN was investigated in the current work using two equations of state (EOS) from the literature, including the Barden EOS and the Birch-Murnaghan EOS, whereas the Bardeen EOS, which is based on interstellar atomic potentials, and the Birch-Murnaghan EOS, which is based on the solid mechanics notion of finite strain. The EOSs were processed to identify the impacts of high pressure on the bulk modulus B, Debye temperature θD and lattice constant a, which are characterizations of TiN. Ultimately, a fair comparison of the current findings with the first principle approximation and the generalized gradient approximation approach was conducted, and a perfect agreement was found. It was demonstrated that TiN EOS can be used to calibrate high pressure for chemical compound TiN .

Effect of hydrostatic strain on the mechanical properties and topological phase transition of bi-alkali pnictogen NaLi2Bi

The bi-alkali pnictogens have attracted significant attention for optoelectronic and photocathodic device applications. However, in most of the compounds belonging to this family, there has been less effort put into investigating the mechanical properties and topological phase transitions (TPT) of the compounds. Here, in the framework of density functional theory, the mechanical properties and topological phase transition of NaLi2Bi under hydrostatic pressures are investigated. Elastic constants and phonon calculations have shown the mechanical and dynamical stability of this compound under hydrostatic tension and compression. The analysis of the elastic constants show that the NaLi2Bi in the equilibrium state is an auxetic material with a negative Poisson’s ratio of -0.285, which changes to a material with a positive Poisson’s ratio under hydrostatic tension. Meanwhile, Poisson’s ratio and Pugh ratio indicate that this compound has brittle behavior and maintains it under hydrostatic pressures. The calculated results of the band structure within the generalized gradient approximation (GGA) (Tran-Blaha modified Becke-Johnson exchange potential approximation (TB-mBJ)) show that NaLi2Bi is a nontrivial topological material (trivial topological material). It was found that hydrostatic compression (tension) in the GGA (TB-mBJ) approach leads to a transition from a nontrivial (trivial) to a trivial (nontrivial) topological phase for this compound. Moreover, the calculated Wannier charge centers confirm the TPT. Identifying the mechanisms controlling the auxetic behavior and TPT of this compound offers a valuable feature for designing and developing high-performance nanoscale electromechanical and spintronic devices.

Search for semiconducting materials among 18-electron half-Heusler alloys

A high throughput search for semiconductors among 153 half-Heusler phases with 18 valence electrons was performed. The alloys considered were as follows: III–X–XV, IV–X–XIV, IV–IX–XV, V–IX–XIV, and V–VIII–XV, wherein the particular groups of the elements were confined as shown: III (Sc, Y, La, Ga, In), IV (Ti, Zr, Hf), V (V, Nb, Ta), VIII (Fe, Ru, Os), X (Ni, Pd, Pt), XIV (Ge, Sn, Pb), and XV (As, Sb, Bi). The generalized gradient approximation (GGA) and modified Becke–Johnson GGA (MBJGGA) approaches were employed in order to calculate the band structures as well as determine the types and widths of band gaps in the materials. Furthermore, the strength of spin–orbit coupling in some systems was discussed based on the heavy- and light-hole energy split-offs. Some compounds are expected to be topological insulators, e.g., the novel Pd-based arsenide (LuPdAs). The set of 24 novel semiconductors following the ‘10 kBT rule’, which suggests their potential for application in thermoelectrics, was revealed. The results of this work should encourage further experimental efforts in synthesis of novel half-Heusler phases.

Influence of hydrostatic pressure and concentration of Ge on the topological band order of SnSi1-xGex alloys

(Si, Ge, Sn)-based alloys are generally compatible with silicon technology and offer many options to engineer the optical properties. Furthermore, topological phases in some of these alloys have been investigated. In this work, through the calculations of the first-principles, the possibility of realization of topological phases in new SnSi1-xGex alloys (x = 0.0, 0.25, 0.5, 0.75, and 1.0) by hydrostatic pressure has been investigated. Furthermore, the effects of Ge concentration and the hydrostatic pressure on the topological phase of these alloys are studied and the band inversion strength (BIS) as a function of concentration (x) and pressure is plotted in a matrix image. The calculations of electronic band structure and band inversion of these alloys within two generalized gradient approximation (GGA) and Heyd-Scuseria-Ernzerhof screened hybrid (HSE06) functional are compared. It was found that the HSE06 approach was more effective than the GGA approach in improving bandgap and BIS. The results show that the nontrivial topological phase of these alloys in both approaches is due to an s-p band inversion at the Gamma (G) point. Moreover, the calculated topological surface states and ℤ2-index confirm the topological phase transition in these alloys. According to the HSE06 approach, at x ≤ 0.5, pressure changes play an essential role in the topological phase transition, while at x > 0.5, pressure changes affect only the BIS. The SnSi1-xGex alloys are dynamically stable, and it is expected that these alloys can be experimentally synthesized.

DFT Investigation of the Structual and Optoelectronic Properties of Alkali Metal Hydrides MH (M=Li, Na)

This paper presents ab initio calculations within the Density Functional Theory (DFT) for the structural and optoelectronic properties of the alkali metal hydrides LiH and NaH in rocksalt structure (B1). This study used the Generalized Gradient Approximation (GGA) of Wu-Cohen to consider the electronic exchange and correlation interactions. In addition, the Tran-Blaha modified Becke-Johnson exchange potential was used with the GGA approach (GGA-TBmBJ) to calculate the band structure with high accuracy. The structural properties, namely the lattice parameter, the bulk modulus, and the pressure derivative of the bulk modulus were determined and found to be generally in good agreement with other research findings. Furthermore, the energy band gaps, the Density Of States (DOS), the static and high-frequency dielectric constant, along the refractive index were addressed and analyzed. These results could be useful for hydrogen storage purposes.

Structural, elastic, mechanical, electronic, magnetic and optical properties of half-Heusler compounds CoFeZ (Z = P, As, Sb): A GGA+U approximation

The first-principles study technique is implemented to explore Co-based H–H compounds, namely CoFeP, CoFeAs, and CoFeSb. In this study we have investigated the structural, elastic, electronic, magnetic, and optical properties using DFT as a fundamental approach within simulation package WIEN2k, utilizing linearized augmented plane wave method with full potential. Calculations for electronic exchange–correlation energy are performed using GGA (Generalized Gradient Approximation) approach together with the on-site Coulomb interactions effect as (GGA + U). Using Phono3py code, we have calculated the phonon spectrum of CoFeZ compounds and there is no negative frequency in the phonon spectrum which shows the dynamic stability of CoFeZ (Z = P, As, Sb) compounds. Large lattice constants are seen for all three compounds CoFeP, CoFeAs, and CoFeSb. Elastically, the former two compounds CoFeP and CoFeAs are ductile while the latter one, CoFeSb is brittle. All the compounds show small band gaps for spin-down channels hence confirming half-metallic character. It is found that magnetic moments follow the Slater–Pauling 18Mt=Zt-18 electron rule with the high total magnetic moments. Also, optical properties such as refractive index, absorption coefficient, dielectric function, conductivity, and reflectivity are calculated and discussed, which show that all the compounds are highly active at intermediate energy spectra.

Computational and experimental approach for investigation of electronic and optical response of MgxCu2-xO

The electronic and optical characteristics of Cu2O based semiconductor materials are recently focused on due to having applications in modern electronic devices and solar cells. In this study, Mg incorporated Cu2O structures were simulated using density functional theory calculations for the prediction of electronic and optical response. The simulations were carried out using the generalized gradient approximation approach by considering the full-potential linearized augmented plane wave method. Pure and Mg incorporated Cu2O thin films with 3.7, 5.5, and 12.5% Mg contents were deposited in uniform morphology using spin coating technique. Ab-initio and experimental investigations present a strong correlation of electronic and optical properties. The bandgap was observed to decrease with the incorporation of Mg in the Cu2O structure. The absorption coefficient and real epsilon depict a sharp increment with Mg doping in the visible regime. The specific outcomes make these materials excellent candidates for dielectric and optoelectronic applications.

Molecular electronics behaviour of L-aspartic acid using symmetrical metal electrodes.

Protein-based electronics is one of the growing areas of bio-nanoelectronics, where novel electronic devices possessing distinctive properties are being fabricated using specific proteins. Furthermore, if the bio-molecule is analysed amidst different electrodes, intriguing properties are elucidated. This research article investigates the electron transport properties of L-aspartic acid (i.e. L-amino acid) bound to symmetrical electrodes of gold, silver, copper, platinum and palladium employing NEGF-DFT approach using self-consistent function. The theoretical work function of different electrodes is calculated using local density approximation and generalized gradient approximation approach. The calculated work function correlates well with the hole tunneling barrier and conductance of the molecular device, which further authenticate the coupling strength between molecule and electrode. Molecule under consideration also exhibits negative differential resistance region and rectification ratio with all the different electrodes, due to its asymmetrical structure. The molecular device using platinum electrodes exhibits the highest peak to valley ratio of 1.38 and rectification ratio of 3.20, at finite bias. The switching characteristics of different molecular device are justified with detailed transmission spectra and MPSH. These results indicate that L-aspartic acid and similar biomolecule can be vital to the growth of Proteotronics.

The spin-polarized electronic and magnetic properties of zinc selenide heavy doped with chromium

Atthe first stage, the structureof theZnSe crystal doped with chromium atoms (ZnCrSe)has been found by optimization procedure. At the second stage, the electronic properties of this material have been evaluated within the two approaches. The exchange-correlation functionals used here are based on the generalized gradient approximation (GGA) and the hybrid functional PBE0. The GGA approach provides the metallic state for electrons with the spin up, and for opposite spin orientation the material ZnCrSe bahaves as semiconductor, with the band gap of 2.48 eV. The hybrid functional approach also gives a gapless state for a spin up electron states, and for a spin down it provides the forbidden gap value of 2.39 eV. The magnetic moment of the unit cell, found with the two functionals, is the same and equals to 4 Bμ(Bohr magnetons). So, the calculations with the two exchange-correlation functionals provide the prediction of half-metallic properties of the ZnCrSe material, which is an interesting candidate for spintronic applications.

First‐principles calculations to investigate structural stabilities, mechanical and optoelectronic properties of NbCoSn and NbFeSb half‐Heusler compounds

AbstractThe aim of this work was to study by means of the full potential linear muffin‐tin orbital method within generalized gradient approximation (GGA) and GGA + U approach the various physical properties of the NbCoSn and NbFeSb half‐Heusler compounds. The equilibrium ground states properties were calculated and compared with available experimental and theoretical data. The elastic constants have been calculated, and revealed that our compounds are mechanically stable. The obtained elastic modulus divulged that our compounds are elastically anisotropic and categorizing them as brittle compounds. The GGA approach showed a semi‐conductor nature. However, the GGA + U approach showed a significant improvement over other theoretical work. We remarked from the band structures that the two materials showed a p‐type semiconductor, with relatively high power factors. Furthermore, the optical quantities are calculated and discussed in detail. Hence, by our findings, the studied compounds could be used for thermoelectric and optoelectronic applications.

Structural, electronic and magnetic properties of double layered perovskite Sr2BiMn2O7: DFT+U study

The structural, electronic and magnetic properties of the Ruddlesden-Popper type Sr2BiMn2O7 (SBMO) has been investigated using ab-initio calculations based on the density functional theory (DFT). Calculations have been accomplished by adopting the full-potential linearized augmented plane wave (FP-LAPW) method. The exchange correlation term was treated using both the Generalized Gradient approximation (GGA) of Polarized Spin and the GGA approach including the Hubbard correction term (GGA+U). The ferromagnetic (FM), antiferromagnetic (AFM) and non-magnetic (NM) phases of SBMO compound were examined. The results predict that the most stable moment arrangement is that of ferromagnetic ordering. We also calculated the density of state, structural band and magnetic moment of SBMO in its ground state (FM). The calculations indicate that SBMO compound has a 100% spin-polarized at Fermi level (EF) with a total magnetic moment of 6.99 μβ/f.u, which is mainly localized at Mn atoms. The present study shows that SBMO exhibits magnetic half-metallic (HM) behavior with an indirect half gap of 3 eV, which is a major key for spintronic applications.

A combined density functional (PBE, WC and Tb-mBJ) study about the optoelectronic properties of copper-rich ternary chalcogenide materials

Here, we thoroughly studied the ground state structural, electronic and optical properties of high crystalline type cubic ACu3Se4 (A = Ta, Nb and V) ternary chalcogenide materials. The the FP-LAPW method within the framework of density functional theory is employed. The electronic structure calculations are treated with highly accurate modified-Becke-Johnson potential to yield the exact band gap values along with the use of Perdew-Burke-Ernzerhof and Wu-Cohen generalized gradient approximation approach for the corresponding optical properties. The band structures and density of states for the three materials are compared and discussed in detail. The complex optical dielectric function’s and the related optical parameters like the absorption coefficient, reflectivity, energy loss function refractive indices and extension coefficient were calculated and discussed in detail for radiations up to energy value of 14 eV. The calculated optical properties display a significant anisotropy between the two components. We further studied the inter-band contributions for the optical characteristics in these ternary chalcogenides. Our described results could pave the way towards further fundamental investigations of the optoelectronic properties of these direct band gap semiconductors in nanoscale regime for solar energy harvesting.

Comparing GGA, GGA+U, and meta-GGA functionals for redox-dependent binding at open metal sites in metal-organic frameworks.

Metal-organic frameworks (MOFs) with open metal sites have been widely investigated for the selective adsorption of small molecules via redox mechanisms where charge transfer can take place between the binding site and the adsorbate of interest. Quantum-chemical screening methods based on density functional theory have emerged as a promising route to accelerate the discovery of MOFs with enhanced binding affinities toward various adsorbates. However, the success of this approach is linked to the accuracy of the underlying density functional approximations (DFAs). In this work, we compare commonly used generalized gradient approximation (GGA), GGA+U, and meta-GGA exchange-correlation functionals in modeling redox-dependent binding at open metal sites in MOFs using O2 and N2 as representative small molecules. We find that the self-interaction error inherent to the widely used Perdew, Burke, and Ernzerhof (PBE) GGA predicts metal sites that are artificially redox-active, as evidenced by their strong binding affinities, short metal-adsorbate bond distances, and large degree of charge transfer. The incorporation of metal-specific, empirical Hubbard U corrections based on the transition metal oxide literature systematically reduces the redox activity of the open metal sites, often improving agreement with experiment. Additionally, the binding behavior shifts from strong chemisorption to weaker physisorption as a function of U. The M06-L meta-GGA typically predicts binding energies between those of PBE-D3(BJ) and PBE-D3(BJ)+U when using empirically derived U values from the transition metal oxide literature. Despite the strong sensitivity of the binding affinities toward a given DFA, the GGA, GGA+U, and meta-GGA approaches often yield the same qualitative trends and structure-property relationships.

High pressure-induced phase transitions in AgI semiconducting compound up to 1 Mbar

In this work, we have resorted to a recent version of the full potential linear muffin-tin orbitals (FP-LMTO) method based on the density functional theory (DFT), within both the local density approximation (LDA) and the generalized gradient approximation (GGA) for the calculation of the equilibrium structural parameters, and phase transition under high pressure of AgI semiconducting compound. Results are given for lattice parameters, bulk modulus and its first pressure derivative in different structures. In general, the use of the GGA approach in this work appears more appropriate than the LDA, and it is most correctly predicts the majority of the ground state properties for AgI material. The energy differences of NiAs (B81)/NaCl (B1) phases are around 1.76 and 1.15 meV, using the LDA and the GGA, respectively. Importantly, the phase transitions for this material from sixfolds coordinated NaCl-type structure (B1) to the eightfold coordinated CsCl-type structure (B2), and then to the hexagonal close packed HCP-type structure (A3) are possible under high pressure. Our calculations show that AgI semiconducting compound transforms firstly from B1 to B2 at pressure of around 55.58 GPa, and then from B2 to A3 at pressure of around 95.03 GPa, respectively.

Structural, magnetic, electronic and elastic properties of half-metallic ferromagnetism full-Heusler alloys: Normal-Co2TiSn and inverse- Zr2RhGa using FP-LAPW method

The equilibrium structural parameters, electronic, magnetic and elastic properties of the inverse Zr2RhGa and normal Co2TiSn Full-Heusler compounds have been studied using density-functional theory (DFT) and full-potential linearized augmented plane-wave (FP-LAPW) method as implemented in the WIEN2k package. The Generalized Gradient Approximation (GGA) has been used for the exchange-correlation potential (Vxc) to compute the equilibrium structural parameters; lattice constant (a), bulk modulus (B) and bulk modulus pressure derivative (B′). In addition to the GGA approach, the modified Becke Johnson (mBJ) scheme has been used to calculate the band gap energies. The normal Heusler Co2TiSn compound and the inverse Heusler Zr2RhGa compound within GGA and mBJ approaches are found to have a half-metallic behavior, with an indirect energy gap in the spin down configuration. The calculated total magnetic moments for both compounds are to some extent compatible with the available experimental and theoretical results. Elastic constants and their related properties were obtained by using the IRelast package. These compounds are mechanically stable. The elastic constants satisfy the Born mechanical stability criteria. B/S ratio shows that the normal Co2TiSn has a brittle nature, while the inverse Zr2RhGa has a ductile nature; Poisson's ratio (ν) values show that both compounds have an ionic bond nature.