Becke's Exchange Research Articles

New insights into the bonding, electronic and optical properties of fergusonite and scheelite ReNbO4 (Re = Y, La and Lu) compounds

A systematic study of the structural, bonding, electronic, and optical properties of the ReNbO4 (Re = Y, La, and Lu) compounds with the fergusonite (space group I/2a) and scheelite (space group I41/a) structure was performed using density functional theory calculations. The aim of these calculations was to resolve some doubt about these properties for the fergusonite phase and to fill the existing knowledge gap for the scheelite phase. To carry out geometry optimization, the generalized gradient approximation for solids was used. Based on these optimized crystal structures, the bonding, electronic and optical properties were determined using the modified Becke and Johnson exchange potential. An analysis of the bonds between Nb and O atoms showed that Nb atoms in the fergusonite structure of the rare-earth niobates were octahedrally coordinated to oxygen atoms, unlike in the scheelite phase, which had tetrahedral coordination. The calculated values of the direct energy band gap for the fergusonite YNbO4, LaNbO4 and LuNbO4 compounds were 4.435, 4.427 and 4.431 eV, respectively. The optical spectrum for these systems is characterized by a small exciton binding energy, which affects the experimental prediction of the energy band gap when the Tauc plot method is used. The values for the energy band gap in the scheelite phase were 4.625 eV for YNbO4, 4.844 eV for LaNbO4 and 4.611 eV for LuNbO4 and all were indirect. The real and imaginary parts of the dielectric tensor were computed up to 25 eV for both phases, and were interpreted based on their electronic structure. The lower energy regions of the optical spectra of these systems are mainly due to electronic transitions within the NbO6 polyhedron (for the fergusonite phase) and the NbO4 polyhedron (for the scheelite phase), i.e., electronic transitions from the occupied O-2p states at the top of the valence band to the unoccupied Nb-4d states at the bottom of the conduction band.

Theoretical prediction of low-energy photoelectron spectra of AlnNi- clusters (n = 1-13).

Mixed-metal clusters have long been studied because of their peculiar properties and how they change with cluster size, composition, and charge state and their potential roles in catalysis. The characterization of these clusters is therefore a very important issue. One of the main experimental tools for characterizing their electronic structure is photoelectron spectroscopy. Theoretical computation completes the task by fully determining the structural properties and matching the theoretical predictions to the measured spectra. We present density functional theory computations of the structural, magnetic, and electronic properties of negatively charged mixed AlnNi- clusters with up to 13 Al atoms. The lowest energy structures of the anionic clusters with up to 7 atoms are also found to be low-energy isomers of the neutral counterparts found in the literature. The 13-atom cluster is found to be a quartet and a perfect icosahedron. The predicted photoelectron spectra are also presented and can be used to interpret future experimental data. We also presented indicators that can be used to determine the potential of these systems for single-atom catalysis. These indicators point to smaller clusters to be more reactive as the gap between the Fermi energy and the center of the d-band increases with cluster size and that Ni occupies an internal site for n = 11-13. We speculate that reactivity can be enhanced if one adds an additional Ni atom. The DFT calculations were performed using the Becke exchange and Perdew-Wang/91 correlation functionals (BPW91), a DFT-optimized all-electron basis set for the aluminum atom, and the Stuttgart small core pseudopotential for the Ni atom. All of the computations used the Gaussian 03 software.

First-principles investigation of structural electronic magnetic and thermodynamic properties of CdS alloyed with Co transition metal

This research work investigates the structural, electronic, and magnetic properties of Cd1–xCoxS compounds. The supercells with 16 atoms have been studied by substituting a Cd atom with a Co atom in the cubic unit cell CdS from x = 0 to x = 1 in a Zinc-Blende structure based on the first principles of spin-polarized density functional theory as implemented in the WIEN2k code. The full potential augmented plane wave method FP-LAPW (Full potential linear augmented plane wave) was used in this study to evaluate the structural properties of the compounds, the exchange–correlation term is based on the PBE-GGA (Perdew–Burke–Ernzerhof generalized gradient approximation). For other properties, such as electronic structures, densities of states, and magnetic properties, the TB-mbj GGA (Tight Binding modified Becke–Johnson) exchange potential approximation is utilized. In addition to these properties, the thermodynamic properties were added advantage to clarify their comportment as temperature variation. The analysis of the density of spin-polarized states showed the ferromagnetic character with a from x = 0.125 to x = 0.875. The calculated total magnetic moments of Cd1–xCoxS are addressed concerning the change in lattice parameters. To track the shift brought on by adding Co, we ran our calculationsfrom x = 0.125 to x = 0.875 for Cd1–xCoxS compounds. Hence, the negative energyformation indicates that our compound can be made experimentally. The material may be used for solar cell applications. In summary, our findings represent a guide for future research. The findings of this work might provide useful direction for the usage of this alloy system in a variety of device applications, For example in high-efficiency tandem solar cells, magnetic sensors, and photoconductive detector applications.

Study of new lead-free double perovskites halides Tl2TiX6 (X = Cl, Br, I) for solar cells and renewable energy devices

In recent years, lead-free double perovskites have been found tremendously advantageous due to their unique optoelectronic and environmentally benign photovoltaic characteristics. Herein, we have explored optical, thermoelectric, and thermodynamical properties of Tl2TiX6 (where X ​= ​Cl, Br, I) using modified Becke and Johnson exchange potential. The calculated value of the Goldschmidt tolerance factor in the range 0.90–1.04 demonstrates the structurally stable cubic phase of our materials. Moreover, the negative value of formation energy together with positive phonon dispersion further reinforces the thermodynamically stable nature of the crystal structures. Further, a decrease in calculated bandgaps of Tl2TiCl6 (2.99 ​eV) was observed in Tl2TiBr6 (2.26 ​eV) and Tl2TiI6 (1.42 ​eV), obtained by changing Cl with Br and I anions, respectively. The first absorption bands 413 ​nm–310 ​nm for Tl2TiCl6, 496 ​nm–400 ​nm for Tl2TiBr6, and 775 ​nm–496 ​nm for Tl2TiI6 increase the potential of studied compounds for solar cells, and optoelectronic applications. In the end, the transport properties were examined based on Boltzmann transport theory via BoltzTrap code. Moreover, the reasonably high values of figure of merits (0.76 and 0.75) measured at room temperature further authenticate their utilization as thermoelectric generators.

Theoretical investigations of optoelectronic and thermoelectric properties of the XIn2O4 (X = Mg, Zn, Cd) spinel oxides

The spinel oxides have received remarkable attention in recent years for being promising materials for optoelectronic and thermoelectric applications. In this paper, the thermodynamic stability of the XIn2O4 (X = Mg, Zn, Cd) spinels is examined by calculating the formation energy whereas the mechanical stability is evaluated by Born mechanical stability criteria. Our results indicate adequate thermodynamic and mechanical stability of the studied spinels. Ductile behavior of these materials is established through Pugh's and Poisson's ratios. The bandgaps of magnitudes 4.0 eV, 2.79 eV, and 2.21 eV have been calculated respectively for MgIn2O4, ZnIn2O4 and CdIn2O4 by modified Becke and Johnson (mBJ) exchange potential. The optical spectra of dielectric constants, refraction, absorption and other related parameters are determined to explore their potential for optoelectronic applications. Furthermore, thermoelectric properties are investigated in terms of thermal to electrical conductivities, Seebeck coefficients, and figures of merit (ZT). The high ZT of magnitude 0.84, 0.74, 0.79 are observed for MgIn2O4, ZnIn2O4 and CdIn2O4 that highlight important of these materials' potential applications in thermoelectric generators and other thermal energy conversion devices.

Piezoelectric properties of Ga2O3: a first-principle study

The compounds exhibit piezoelectricity, which demands to break inversion symmetry, and then to be a semiconductor. For $\mathrm{Ga_2O_3}$, the orthorhombic case ($\epsilon$-$\mathrm{Ga_2O_3}$) of common five phases breaks inversion symmetry. Here, the piezoelectric tensor of $\epsilon$-$\mathrm{Ga_2O_3}$ is reported by using density functional perturbation theory (DFPT). To confirm semiconducting properties of $\epsilon$-$\mathrm{Ga_2O_3}$, its electronic structures are studied by using generalized gradient approximation (GGA) and Tran and Blaha's modified Becke and Johnson (mBJ) exchange potential. The gap value of 4.66 eV is predicted with mBJ method, along with the the effective mass tensor for electrons at the conduction band minimum (CBM) [about 0.24 $m_0$]. The mBJ gap is very close to the available experimental value. The elastic tensor $C_{ij}$ and piezoelectric stress tensor $e_{ij}$ are attained by DFPT, and then piezoelectric strain tensor $d_{ij}$ are calculated from $C_{ij}$ and $e_{ij}$. In this process, average mechanical properties of $\epsilon$-$\mathrm{Ga_2O_3}$ are estimated, such as bulk modulus, Shear modulus, Young's modulus and so on. The calculated $d_{ij}$ are comparable and even higher than commonly used piezoelectric materials such as $\alpha$-quartz, ZnO, AlN and GaN.

Theoretical investigation of structural, electronic and thermoelectric properties of $$p{-}n$$ type $$\hbox {Mg}_{2}\hbox {Si}_{1-x}\hbox {Sn}_{x}$$ system

Based on the density functional theory and the Boltzmann transport theory, the thermoelectric properties of $$\hbox {Mg}_{2}\hbox {Si}_{1-x}\hbox {Sn}_{x}$$ solid solution with $$x= 0.25, 0.5 \hbox { and } 075$$ were investigated. The calculated structural parameters were in good agreement with the previous work and the mechanical and dynamical stabilities were confirmed. The electronic band structure computed using the Tran-Blaha-modified Becke and Johnson (TB-mBJ) exchange potential indicated that the band gap can be tuned by the alloy effect. We combined first-principles calculations and the semiclassical Boltzmann transport theory by considering the electronic transport in the $$\hbox {Mg}_{2}\hbox {Si}_{1-x}\hbox {Sn}_{x}$$ solid solution to determine the effect of varying the Sn composition on the thermoelectric performance. Our results have shown exceptionally high electrical conductivity for $${\hbox {Mg}}_{2}\hbox {Sn}$$ and higher Seebeck coefficient for $$\hbox {Mg}_{2}\hbox {Si}$$ . The highest figure of merit (ZT) was predicted for $$\hbox {Mg}_{2}\hbox {Si}_{1-x}\hbox {Sn}_{x}$$ solid solution with $$x = 0.5$$ where ZT has reached 0.55 with carrier concentration charge $$n = 10^{20}\hbox { cm}^{-3}$$ (p-type doping) at intermediate temperatures. Consequently, the alloying system with p-type doping may improve the thermoelectric properties compared to the $$\hbox {Mg}_{2}\hbox {Si}$$ and $$\hbox {Mg}_{2}\hbox {Sn}$$ pristine compounds.

Ab-initio investigations of structural, optoelectronic and thermoelectric properties of AIn2Se4 (A = Zn, Cd) spinels

Current investigations are based on ab-initio total energy calculations to study structural, optoelectronic and thermo-electric aspects of AIn2Se4 (A = Zn, Cd) spinels. Perdew–Burke–Ernzerhof-sol technique along with the modified Becke and Johnson exchange potential are exploited to determine the structural, optoelectronic and thermoelectric characteristic of AIn2Se4 (A = Zn, Cd). Enthalpy of formation is employed to investigate the structural stability of understudy compounds. Electronic band structures of the compounds reveal that they possess direct bandgap as top and bottom of valence and conduction bands are located at the Γ-point. Optical characteristics are deduced from complex dielectric constant and refractive index. The deduced optical parameters reveal a shift towards lower energies because of atomic radii increase from Zn to Cd and yields potential of these study compounds in the optoelectronic industry. Finally, thermoelectric aspects of the studied compounds are expressed via Wiedemann-Franz constant, See-beck coefficient and power factor and these thermoelectric parameters deliver necessary information for thermoelectric device fabrication.

Ab initio study of F-centers in alkali halides

The structural and electronic properties of an electron trapped at vacant anion site in alkali halides are investigated using first principles electronic structure calculations with the supercell method. In order to determine the spatial electronic charge density and band structure of the studied systems we used the Augmented Plane Waves plus local orbital (APW+lo) method in the framework of the Density Functional Theory (DFT), considering the Wu and Cohen parametrization of the generalized gradient approximation (WCGGA) for the exchange and correlation energy, and the modification of Tran and Blaha to the Becke and Johnson exchange potential (mBJ method). We discuss the improvements in the description of the defect levels induced by the vacancies using mBJ compared to WCGGA. Additionally, we revisit the experiment to perform a new determination of the UV/Vis absorption energies in F-centers. In the theoretical framework used, we demonstrate that the bound electron at the F-center is localized within a sphere with diameter twice the lattice parameter. From the comparison of our theoretical predictions with the Mollwo-Ivey relation that comes from this new experiment, we show that the mBJ method predicts accurately the energy band gaps and gives better energy values for the s-p transitions that give rise to the optical absorption energies.

Electronic structure, optical and thermoelectric properties of half-Heusler ZrIrX(X $$=$$ = As, Sb, Bi): a first principles study

The electronic structure, optical and thermoelectric properties of half-Heusler ZrIrX (X $$=$$ As, Sb, Bi) compounds were investigated under pressure by using the modified Becke and Johnson exchange potential. The band gaps of ZrIrAs and ZrIrBi increase from tensile strain ( $$\varepsilon $$ $$=$$ 5%) to compressive strain ( $$\varepsilon $$ $$=$$ $$-5$$ %), and at about $$\varepsilon $$ $$=$$ $$-4$$ % they changed from indirect to direct band gaps. Their absorption efficiencies increase from compressive to tensile strain in the low energy ( $$ \leqslant $$ 2.8 eV). For p-type doped compounds, the Seebeck coefficients and the power factors increase from tensile to compressive strain. While the the Seebeck coefficients and the power factors of n-type compounds, first increase and then decrease from tensile to compressive strain, reaching the largest values at the critical strain (direct-indirect transition). ZrIrSb is an indirect band gap insulator, but it translates to direct band gap insulator when the tensile strain $$\varepsilon \geqslant $$ 2%.

Structural stabilities and band structure characteristics of platinum nitride (PtN) via first-principles calculations

The structural phase transformations of the PtN compound with a 1:1 stoichiometric ratio of Pt:N were investigated using the framework of density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method within the generalized gradient (PBE-GGA) and the Engel–Vosko generalized gradient (EV-GGA) approximations were used. A comparative study of the experimental and theoretical results is provided on the structural properties of zinc-blende (ZB), rock-salt (RS), cesium chloride (CsCl), wurtzite (WZ), nickel arsenide (NiAs), lead monoxide (PbO), and tungsten carbide (WC) phases. The calculated band structure using the modified version of the Becke and Johnson (mBJ) exchange potential reveals the metallic character of the PtN compound. The present study also shows that the PtN compound crystallizes in the WZ phase under ambient conditions. The theoretical transition pressures from WZ to RS, NiAs, PbO, and CsCl transformations are found to be 9.441GPa, 7.705GPa, 18.345GPa and 31.9GPa, respectively, using the PBE-GGA method.

Thermoelectric properties of topological insulator BaSn2

Recently, has been predicted to be a strong topological insulator by the first-principle calculations. It is well known that topological insulators have a close connection to thermoelectric materials, such as the family. In this work, we investigate thermoelectric properties of by the first-principles calculations combined with the Boltzmann transport theory. The electronic part is carried out by a modified Becke and Johnson (mBJ) exchange potential, including spin–orbit coupling (SOC), while the phonon part is performed using a generalized gradient approximation (GGA). It was found that the electronic transport coefficients between the in-plane and cross-plane directions showed strong anisotropy, while lattice–lattice thermal conductivities demonstrated almost complete isotropy. Calculated results revealed a very low lattice thermal conductivity for , and the corresponding average lattice thermal conductivity at room temperature is 1.69 , which is comparable or lower than those of lead chalcogenides and bismuth-tellurium systems as classic thermoelectric materials. Due to the complicated scattering mechanism, calculating the scattering time τ is challenging. By using an empirical s, the n-type figure of merit ZT is greater than 0.40 in wide temperature ranges. Experimentally, it is possible to attain better thermoelectric performance by strain or tuning size parameters. This work indicates that may be a potential thermoelectric material, which can stimulate further theoretical and experimental work.

Topological phase, structural, electronic, thermodynamic and optical properties of XPtSb (X=Lu, Sc) compounds

The electronic, thermodynamic and optical properties of XPtSb (X=Lu, Sc) half Heusler compounds are studied based on density functional theory. The calculations are carried out in the presence of spin orbit interaction. The exchange correlation part of total energy is calculated within local density approximation, generalized gradient approximation, Engel-Vosco generalized gradient approximation and modified Becke and Johnson exchange potential with the correlation potential of the generalized gradient approximation. The effect of pressure on the electron density of states and linear coefficient of the electronic specific heat is studied. Using the band structure calculations at different pressures, the band inversion strength and topological phase transition of these compounds are investigated. Some thermodynamic properties of XPtSb compounds by different thermal models using the non-equilibrium Gibbs function are studied and compared with experiment. Furthermore the effect of pressure on dielectric function of XPtSb (X=Lu, Sc) compounds is investigated.

Interesting pressure dependence of power factor in BiTeI

We investigate pressure dependence of electronic structures and thermoelectric properties in BiTeI by using a modified Becke and Johnson exchange potential. Spin–orbit coupling (SOC) effects are also included due to giant Rashba splitting. Thermoelectric properties are illuminated through solving Boltzmann transport equations within the constant scattering time approximation. The calculated energy band gap of 0.36 eV agrees well with the experimental value of 0.38 eV. As the pressure increases, the energy band gap first decreases, and then increases. The Rashba energy has the opposite trend with the energy band gap. SOC has obvious detrimental influence on the power factor in both n-type and p-type doping. For low doping concentration, the power factor has the same trend with the energy band gap with increasing pressure, but shows a monotonic changing trend in high doping. It is found that the pressure can induce a significantly enhanced power factor in high n-type doping, which can be understood as pressure leading to two-dimensional-like density of states in the conduction bands. These results suggest that BiTeI may be a potential candidate for efficient thermoelectricity in n-type doping by pressure, turning an ordinary insulator into a topological insulator.

Electronic structure and thermoelectric properties of (Mg2X)2 / (Mg2Y)2 (X, Y = Si, Ge, Sn) superlattices from first-principle calculations

To identify thermoelectric materials containing abundant, low-cost and non-toxic elements, we have studied the electronic structures and thermoelectric properties of (Mg2X)2/ (Mg2Y)2 (X, Y = Si, Ge, Sn) superlattices with state-of-the-art first-principles calculations using a modified Becke and Johnson (mBJ) exchange potential. Our results show that (Mg2Ge)2/ (Mg2Sn)2 and (Mg2Si)2/ (Mg2Sn)2 are semi-metals using mBJ plus spin-orbit coupling (mBJ + SOC), while (Mg2Si)2/ (Mg2Ge)2 is predicted to be a direct-gap semiconductor with a mBJ gap value of 0.46 eV and mBJ + SOC gap value of 0.44 eV. Thermoelectric properties are predicted by through solving the Boltzmann transport equations within the constant scattering time approximation. It is found that (Mg2Si)2/ (Mg2Ge)2 has a larger Seebeck coefficient and power factor than (Mg2Ge)2/ (Mg2Sn)2 and (Mg2Si)2/ (Mg2Sn)2 for both p-type and n-type doping. The detrimental influence of SOC on the power factor of p-type (Mg2X)2/ (Mg2Y)2 (X, Y = Si, Ge, Sn) is analyzed as a function of the carrier concentration, but there is a negligible SOC effect for n-type. These results can be explained by the influence of SOC on their valence and conduction bands near the Fermi level.

Pressure induced magnetic and semiconductor–metal phase transitions in Cr2MoO6**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. 2015XKMS073).

We investigate magnetic ordering and electronic structures of Cr2MoO6 under hydrostatic pressure. To overcome the band gap problem, the modified Becke and Johnson exchange potential is used to investigate the electronic structures of Cr2MoO6. The insulating nature at the experimental crystal structure is produced, with a band gap of 1.04 eV, and the magnetic moment of the Cr atom is 2.50 μB, compared to an experimental value of about 2.47 μB. The calculated results show that an antiferromagnetic inter-bilayer coupling–ferromagnetic intra-bilayer coupling to a ferromagnetic inter-bilayer coupling–antiferromagnetic intra-bilayer coupling phase transition is produced with the pressure increasing. The magnetic phase transition is simultaneously accompanied by a semiconductor–metal phase transition. The magnetic phase transition can be explained by the Mo–O hybridization strength, and ferromagnetic coupling between two Cr atoms can be understood by empty Mo-d bands perturbing the nearest O-p orbital.

Improved half-metallic gap of zincblende half-metal superlattices with the Tran–Blaha modified Becke–Johnson density functional

Binary transition-metal pnictides and chalcogenides half-metallic ferromagnetic materials with zincblende structure, being compatible with current semiconductor technology, can be used to make high-performance spintronic devices. Here, we investigate electronic structures and magnetic properties of composite structure ((CrX)2/(YX)2 (X=As, Sb; Se, Te and Y=Ga; Zn) superlattices) of zincblende half-metallic ferromagnetism and semiconductor by using Tran and Blaha's modified Becke and Johnson (mBJ) exchange potential. Calculated results show that they all are half-metallic ferromagnets with both generalized gradient approximation (GGA) and mBJ, and the total magnetic moment per formula unit follows a Slater–Pauling-like “rule of 8”. The key half-metallic gaps by using mBJ are enhanced with respect to GGA results, which is because mBJ makes the occupied minority-spin p-bands move toward lower energy, but toward higher energy for empty minority-spin Cr-d bands. When the spin–orbit coupling (SOC) is included, the spin polarization deviates from 100%, and a most reduced polarization of 98.3% for (CrSb)2/(GaSb)2, which indicates that SOC has small effects, of the order of 1%, in the considered four kinds of superlattice.

Thermoelectric studies of IV–VI semiconductors for renewable energy resources

In the present study we explore the structural, electronic and thermoelectric properties of IV-VI semiconductors using first principle calculations based on the density functional theory. Generalized gradient approximation (GGA-PBEsol) is used as an exchange-correlation functional for the structural properties of the different crystal phases of these semiconductors. The compounds SnS, SnSe, GeS and GeSe favor orthorhombic structure, lead chalcogenides (PbS, PbSe and PbTe) are stable in the cubic rocksalt structure, while SnS2 and SnSe2 exist in hexagonal structure. For band structures additional to GGA, modified Becke and Johnson (mBJ) exchange potential is used. These compounds are narrow band gap semiconductors in the energy range 0.2–2.8eV. These binary IV-VI chalcogenides have direct band gap nature in cubic crystals while indirect in the orthorhombic phases. The post-DFT (BoltzTraP) calculations are performed to estimate Seebeck coefficient, electric and thermal conductivities, power factor and figure of merit of these compounds to check their applicability in thermoelectric devices. Spin orbit coupling effect influences the electronic and thermoelectric properties of these compounds due to the large size of the constituent elements. Further, the total thermal conductivity is computed using ad-hoc calculation with the experimental results. The calculated values of figure of merit for PbS, PbSe, PbTe and SnTe are in the range 0.68–0.77 at room temperature.

Electronic, structural and magnetic properties of TbO under pressure: FP-LAPW study

ABSTRACTUsing the framework of the density functional theory, we calculated electronic, magnetic and structural properties of terbium oxide (TbO) in rocksalt (RS), cesium chloride (CsCl) and zincblende (ZB). Full potential linearized augmented plane wave (FP-LAPW) method within the local spin density approximation (LSDA) and generalized gradient (PBE-GGA) approximations are used. Magnetic and non-magnetic calculations are performed and a modified version of Becke and Johnson (mBJ) exchange potential has been used to calculate the band gaps. We found that, although TbO is stable in a ferromagnetic state, it is stable in RS phase at ambient condition. Both LSDA and PBE-GGA calculations revealed that the three structures are metallic. However, using the mBJ calculation, it is clear that RS and CsCl phases of TbO compound are metallic, while ZB phase is found to be an insulator in the spin-up case and a semiconductor in the spin-down case at ambient pressure.

Pressure enhanced thermoelectric properties in Mg2Sn

The pressure dependence of the electronic structure and thermoelectric properties of Mg2Sn are investigated by using a modified Becke and Johnson exchange potential, including spin–orbit coupling.