Pseudopotential Method Research Articles

DFT study of structural, mechanical, thermodynamic, electronic, and thermoelectric properties of new PdTiZ (Z = Ge and Pb) half Heusler compounds

AbstractA first principle investigation of structural, mechanical, thermodynamical, electronic, and thermoelectric properties of PdTiZ (Z = Ge and Pb) were carried out using the plane‐wave pseudopotential (PP‐PW) method. The exchange‐correlation between the electrons was considered with the generalized gradient approximation of Perdew‐Burke‐Ernzerhof (PBE‐GGA). The energy band structure of the sample materials shows that it is a semiconductor with indirect energy band gaps of 0.66 eV and 0.387 eV in PdTiGe and PdTiPb, respectively, with the density of states being mainly dominated by the p states of Z and d states of Ti atom in the vicinity of the Fermi energy level. The sample compounds were found to be mechanically and dynamically stable in their non‐magnetic cubic phase. PdTiGe was found to be harder and stiffer than PdTiPb. The thermodynamic study revealed that pressure has a negative effect on heat capacity, whereas Debye temperature increases with enhanced pressure in both the sample compounds. The thermoelectric properties of the sample compounds predicted that p‐type nature of the sample compounds possess better thermoelectric performance. The room temperature Seebeck coefficient (S) values were found to be 98.73 μV/K and 94.82 μV/K for PdTiGe and PdTiPb, respectively, whereas S values of 245.73 μV/K and 218.77 μV/K at 1200 K at n = 1021 cm−3. The lowest values of lattice thermal conductivity () of 2.28 W/m‐K and .98 W/m‐K were observed for PdTiGe and PdTiPb, respectively. The optimum dimensionless figure of merits of .66 (1200 K) and .64 (1000 K) were found for p‐type PdTiGe and PdTiPb, respectively, at optimum carrier concentration.

The Response of Phonon Frequencies, Sound Velocity, Electronic, Optical, and Mechanical Properties of Indium (Phosphide, Arsenide, and Antimonide) to Hydrostatic Pressure

The pressure dependence of phonon vibrations, the velocity of sound, and some other related physical quantities of and semiconductors have been determined. This investigation is done using the empirical pseudopotential method. We proposed a pressure dependence of the pseudopotential form factors; hence we were able to calculate the pressure dependence of the fundamental energy gaps. Using the values of the fundamental band gaps, we were able to calculate the refractive index, static dielectric constant, high-frequency dielectric constant, longitudinal and transverse phonon frequencies, elastic constants, mechanical moduli, and velocity of sound. Our findings may serve as a reference for experimental work at high pressures.

Ab initio investigations of Fe(110)/graphene interfaces

Interfacial bonding of three different semi-coherent bcc-Fe(110)/graphene interfaces is investigated using the plane-wave pseudopotential method within density functional theory. The analysis of bond lengths, charge densities, charge transfer, magnetic moments and densities of states shows that interfacial adhesion can be understood from the electronic structure. Graphene is considered on Fe(110) surfaces as well as embedded in (110) planes of bulk Fe. Moreover, the influence of single vacancies in graphene is studied in case of graphene on the Fe(110) surfaces. It is found that a single vacancy in graphene leads to a strong increase of interfacial adhesion. The most important contribution to the adhesion is covalent bonding with hybridization of Fe states and C states which is most pronounced for neighboring atoms of the vacancy.

Effects of rare earth element Ce on structural, electronic, magnetic and optical properties of CexY1-xFeO3 (x ​= ​0.0625, 0.125): ab initio study

First-principles calculations based on density functional theory are used to study the effect of doping rare earth elements on certain characteristics of perovskite systems CexY1-xFeO3 (CYFO). Structural, electronic, magnetic and optical properties are studied using the pseudo-potential plan-wave method as implemented in the VASP code. The calculated density of states proposed in this work predicts a semiconductor behavior of YFeO3 and a semi-metallic behavior of CexY1-xFeO3. In addition, in order to study the magnetic properties of several compounds, four magnetic configurations (FM, A-AFM, C-AFM, and G-AFM) were considered. By doping rare earth elements, the system exhibits different changes in magnetic order and electronic structure. Further, analysis of optical properties discloses that incorporation of Ce into YFeO3 lattice (where x ​= ​0.125) remarkably improves absorbance and reflectivity.

Structural, electronic and magnetic properties of inverse spinel NiFe2O4: DFT + U investigation

We have used ab initio density functional theory (DFT) to explore the structural, mechanical, electronic and magnetic properties of inverse spinel NiFe2O4. The ultra-soft pseudopotential method parametrized under generalized gradient approximation (GGA) for the exchange-correlation functional with Hubbard correction (GGA + U) was applied. The equilibrium lattice parameter was estimated to be 8.4620 Å and the 3 independent elastic constants C11, C12 and C44 were found to be 304.12 GPa, 144.45 GPa and 57.46 GPa, respectively. GGA + U formalism could successfully explain the semiconducting and ferrimagnetic behaviour of NiFe2O4. The spin-polarized electronic density of states reveals that the p−d hybridization between O and Fe atoms is responsible for the semiconducting nature of NiFe2O4. The ferrimagnetic behaviour of inverse spinel NiFe2O4 can be explained through the anti-parallel spin configuration of tetrahedrally and octahedrally occupied Fe atoms. The resultant magnetic moment is thus solely contributed by the octahedrally occupied Ni atoms. Spin-polarized band structure calculations reveal indirect band gap of 1.6 eV for spin-up channel and direct band gap of 2.2 eV for spin-down channel.

First-principles study on the electronic structures and optical properties of Cs2XInCl6 (X= Ag, Na)

The excellent properties and diversity of structural composition of environmentally benign and stable double perovskite materials have piqued the curiosity of a wide range of researchers. In the field of optoelectronic materials, all-inorganic halide double perovskite has emerged as a viable alternative to lead-based perovskite in recent years. The energy band structure, density of states, and optical properties of the all-inorganic halide double perovskite Cs2XInCl6 (X = Ag, Na) were calculated and compared in this paper using the Castep code of Materials Studio software based on the density generalized function theory first principle plane wave pseudopotential method (DFT). The analysis of the results shows that Cs2XInCl6 (X = Ag, Na) is a material with a wide direct band gap and exhibits good light absorption in the visible and ultraviolet regions. Cs2AgInCl6 has better optical properties than Cs2NaInCl6 due to differences in reflection spectra, absorption spectra, and energy loss, although the differences are not considerable, and the compound Cs2XInCl6 (X = Ag, Na) has prospective applications in the field of optoelectronics. The computational results of this work contribute to a better understanding of the electronic and optical properties of Cs2XInCl6 (X = Ag, Na), as well as their similarities and differences, and provide some ideas for the design of corresponding innovative experiments to seek more valuable discoveries in the field of optoelectronic and optical applications.

Pore-Scale Study on Convective Drying of Porous Media.

In this work, a numerical model for isothermal liquid-vapor phase change (evaporation) of the two-component air-water system is proposed based on the pseudopotential lattice Boltzmann method. Through the Chapman-Enskog multiscale analysis, we show that the model can correctly recover the macroscopic governing equations of the multicomponent multiphase system with a built-in binary diffusion mechanism. The model is verified based on the two-component Stefan problem where the measured binary diffusivity is consistent with theoretical analysis. The model is then applied to convective drying of a dual-porosity porous medium at the pore scale. The simulation captures a classical transition in the drying process of porous media, from the constant rate period (CRP, first phase) showing significant capillary pumping from large to small pores, to the falling rate period (FRP, second phase) with the liquid front receding in small pores. It is found that, in the CRP, the evaporation rate increases with the inflow Reynolds number (Re), while in the FRP, the evaporation curves almost collapse at different Res. The underlying mechanism is elucidated by introducing an effective Péclet number (Pe). It is shown that convection is dominant in the CRP and diffusion in the FRP, as evidenced by Pe > 1 and Pe < 1, respectively. We also find a log-law dependence of the average evaporation rate on the inflow Re in the CRP regime. The present work provides new insights into the drying physics of porous media and its direct modeling at the pore scale.

Application of the Molecular Pseudopotential Method for Modeling the Toxicity of Chemical compounds

Statistical modeling of the relationship between the toxicity of a number of substituted benzo-2,1,3-thia- and selenadiazoles depending on the number of various substituents and their position in the benzene ring was performed. It has been statistically reliably established that the toxicity of the analyzed series of chemical compounds is closely related to the value of the molecular pseudopotential. It has been shown that the relationship between the toxicity of drugs correlates linearly with the molecular electronic factor, which characterizes the magnitude of the pseudopotential of the molecule.

First-principles study on the optical spectrum of N/P doped TiO2-anatase

In this paper, the geometry structure, electronic structure and optical properties of TiO2-anatase, TiO2-anatase doped with N and TiO2-anatase doped with N/P atoms were investigated by the plane-wave ultrasoft pseudopotential method based on the first-principles density functional theory. It demonstrates that there exist additional states in the band gap as TiO2-anatase was doped with N and TiO2-anatase was doped with N/P atoms, and the band gap narrows. The conduction band and valence band position of TiO2-anatase doped with N/P move to lower energy levels, that enhance the ability of oxidation and reduction, which is conducive to improve the photocatalytic performance. Red shift phenomenon, oxygen vacancies increase and enhanced photocatalytic properties of TiO2-anatase doped with N/P has been found by XRD, IR, RRS optical spectrum of the N/P doped TiO2- anatase, which verify our theory analysis.

Electronic, optical, and mechanical properties of AlxIn1−xP alloys under temperature and pressure

In this work, the optoelectronic and mechanical properties of AlxIn1−xP combinations in the zinc-blende structure were studied for different Al concentrations. The energy band gaps left( {{text{E}}_{{text{g}}}^{{text{L}}} ,,{text{E}}_{{text{g}}}^{Gamma } ,,{text{E}}_{{text{g}}}^{{text{X}}} } right), refractive index (n), high frequency and static dielectric constants (ɛ∞, ɛo), elastic parameters (C11, C12, C44) were investigated. Other mechanical properties such as bulk (Bu), shear (Cs), Young’s (Y0) moduli, Poisson ratio (σ), linear compressibility (C_{o} ), Cauchy (C_{a} ) ratio, isotropy factor (A), bond stretching parameter (alpha ,), bond-bending force parameter (beta ,), internal-strain parameter (xi ), and the transverse effective charge (e_{T}^{*} ) were calculated. Also, the temperature and pressure dependences of these properties were studied. Our estimations were made with the empirical pseudo-potential method combined with the virtual crystal approximation incorporated the compositional disorder impact. There was a reasonable agreement between our determined outcomes and the accessible experimental values for the binary materials AlP and InP which give help for the consequences of the ternary combinations.

Kinetic theory of sech2 x electron holes and applications to Kappa-distributed plasmas

The kinetic theory of sech2 x-type electron holes (EHs) is studied. The potential of the EHs is solved in the weak amplitude limit by the pseudo-potential method. We investigate the existence condition of the sech2 x EHs. It indicates that the derivatives of trapped and untrapped distributions at the separatrix play significant roles in determining the potential profile. The theory is then applied to the Kappa-distributed plasmas. The amplitude and width of the sech2 x EHs are analyzed. Finally, the theoretical results are verified by numerical calculations.

First-principle study of new insights on the magnetic mechanism of ZnO through rare-earth Nd doping and Zn vacancies

Theoretical and experimental studies on the magnetic mechanism of Nd doping or Nd doping and Zn vacancies on ZnO have been reported. However, the magnetic source and mechanism of the doping system are controversial. Thus far, no reasonable theoretical explanation exists. First principles can solve such problems. In this study, the first principles are used to investigate the influence of Nd doping or Nd doping and Zn vacancies on the magnetic source and mechanism of ZnO based on the generalized gradient approximation plane wave supersoft pseudopotential method under the framework of spin density functional theory. The calculation results show that the formation energy of the Zn15NdO16 system or the Zn14NdO16 system is smaller and the doping is easier under the O-rich condition than those under the Zn-rich condition. Both doping systems have high binding energy and high stability. The study found that the magnetism of the Zn15NdO16 system is mainly produced by the spin polarization of the s, p, d, and f states of a single Nd3+ ion. This phenomenon is consistent with the mechanism by which a single ion in a compound produces magnetism. The magnetism of the Zn14NdO16 system is produced not only by the double exchange of the p, d, and f state itinerant electrons of a single Nd3+ ion but also by the d and f state exchange mediated of the O1−-2p state itinerant electrons. This finding is consistent with the explanation of the carrier-mediated double exchange mechanism. The new insights on the magnetic origin and mechanism of doped Nd or doped Nd and Zn vacancy coexisting ZnO mediated by itinerant electron have a leading role in studying similar dilute magnetic semiconductors.

Mesoscale Simulations on the Ultrahigh Heat Flux Evaporation of R143a within Ultrathin Nanoporous Membrane Using a Modified Dimensionless Lattice Boltzmann Method

A modified dimensionless pseudo-potential lattice Boltzmann method capable of restoring all related thermophysical properties of any working fluid and spatial/temporal scales of the computational domain is developed in this work. Direct numerical investigations of ultrathin liquid film evaporation of R143a within nanoporous membranes to its ambient pure vapor are carried out. The characteristics of evaporation-driven replenishing flow and the self-regulation of liquid-vapor interface profile under different heat fluxes are captured. In the case of intensive ultrathin liquid film evaporation within nanopore, the apparent contact angle could not reach as small as Young's contact angle θ0 for isothermal conditions due to the self-regulation of liquid-vapor interface. As a result, the maximum capability of nanoporous membranes to replenish the liquid is limited. Under the ambient temperature T0 = 318.19 K, before the three-phase contact line (TCL) recedes into the nanoporous membrane, a heat flux as high as qin,max = 1.52 kW/cm2 can be sustained with a superheat about Ts = 7.79 K for a membrane with pore radius rp = 38.71 nm, pore depth Lp = 294.96 nm, Young's contact angle θ0 = 23° and porosity η = 0.66. The maximum heat flux qin,max increases with the increase of rp and the decrease of Lp and θ0. The present work presents a promising numerical method to optimize thermal management devices using complex micro-nano composite structures.

Nonperturbative ab initio approach for calculating the electrical conductivity of a liquid metal

We propose a nonperturbative ab initio approach to calculate the electrical conductivity of a liquid metal. Our approach is based on the Kubo formula and the theory of electron-phonon coupling (EPC) and, unlike the conventional empirical approach based on the Kubo-Greenwood formula, fully takes into account the effect of coupling between electrons and moving ions. We show that the electrical conductivity at high temperature is determined by an EPC parameter ${\ensuremath{\lambda}}_{\mathrm{tr}}$, which can be inferred, nonperturbatively, from the correlation of electron scattering matrices induced by ions. The latter can be evaluated in a molecular dynamics simulation. Based on the density-functional theory and pseudopotential methods, we implement the approach in an ab initio manner. We apply it to liquid sodium and obtain results in good agreement with experiments. This approach is efficient and based on a rigorous theory suitable for applying to general metallic liquid systems.

On the force scheme influence on pseudopotential method coexistence curve

Pseudopotential method has been drawing attention in simulating multiphase flows by adding an interparticle force to lattice Boltzmann method (LBM). However, implementing a known force field effect into evolution of particle distribution function is still an active field of interest, specially when higher order terms play significant role in resulting macroscopic equations. A considerable number of forcing schemes has already been proposed, and although many have shown their differences up to second order analysis, there is still some debate about high order terms. In this work, a study taking into account third order forcing terms is carried out, in order to better identify the reasons for existing distinct coexistence curves when the same interparticle force is employed through distinct forcing schemes. Third order terms related to spatial interparticle force gradients are taken into account, and a theoretical analysis is carried out in order to study gas–liquid coexistence curves. It is shown that this curve can be characterized by a single parameter, Γ, dependent on employed relaxation time and forcing scheme, which allows the current analysis to be used for other schemes as well. Flat interface simulations are carried out in order to study the behavior of gas–liquid coexistence curve. A good agreement between the numerical and theoretical results validates the presented approach. Static droplet simulations were carried out, where surface tension behavior also respect the expected trend of higher values for lower Γ. Maximum spurious velocities of renowned forcing schemes from literature were also studied, without a clear correlation with Γ parameter.

Modified AKNS model, Riccati-type pseudo-potential approach and infinite towers of quasi-conservation laws

In this paper, a dual Riccati-type pseudo-potential formulation is introduced for a modified AKNS system (MAKNS) and infinite towers of novel anomalous conservation laws are uncovered. In addition, infinite towers of exact nonlocal conservation laws are uncovered in a linear formulation of the system. It is shown that certain modifications of the nonlinear Schrödinger model (MNLS) can be obtained through a reduction process starting from the MAKNS model. So, the novel infinite sets of quasi-conservation laws and related anomalous charges are constructed by an unified and rigorous approach based on the Riccati-type pseudo-potential method, for the standard NLS and modified MNLS cases, respectively. The nonlocal properties, the complete list of towers of infinite number of anomalous charges and the (nonlocal) exact conservation laws of the quasi-integrable systems, such as the deformed Bullough–Dodd, Toda, KdV and SUSY sine-Gordon systems, can be studied in the framework presented in this paper. Our results may find many applications since the AKNS-type system arises in several branches of nonlinear physics such as Bose–Einstein condensation, superconductivity and soliton turbulence.

Simulation of the Band Structure of InAs/GaSb Type II Superlattices Utilizing Multiple Energy Band Theories

Antimonide type II superlattices is expected to overtake HgCdTe as the preferred materials for infrared detection due to their excellent photoelectric properties and flexible and adjustable band structures. Among these compounds, InAs/GaSb type II superlattices represent the most commonly studied materials. However, the sophisticated physics associated with the antimonide-based bandgap engineering concept started at the beginning of the 1990s gave a new impact and interest in the development of infrared detector structures within academic and national laboratories. InAs/GaSb superlattices are a type II disconnected band structure with electrons and holes confined in the InAs and GaSb layers, respectively. The electron miniband and hole miniband can be regulated separately by adjusting the thickness of InAs and GaSb layers, which facilitates the design of superlattice structures and optimizes the value of band offset. In recent years, both domestic and foreign researchers have made many attempts to quickly and accurately predict the bandgaps of superlattice materials before superlattice materials grow. These works constituted a theoretical basis for the effective utilization of the InAs/GaSb system in material optimization and designing new SL structures; they also provided an opportunity for the preparation and rapid development of InAs/GaSb T2SLs. In this paper, we systematically review several widely used methods for simulating superlattice band structures, including the k·p perturbation method, envelope function approximation, empirical pseudopotential method, empirical tight-binding method, and first-principles calculations. With the limitations of different theoretical methods proposed, the simulation methods have been modified and developed to obtain reliable InAs/GaSb SL energy band calculation results. The objective of this work is to provide a reference for designing InAs/GaSb type II superlattice band structures.

Effect of Sc and Zn doping on structure and electro-optical behavior in c-BiAlO3: A DFT trial

Density Functional Theory (DFT) is an attractive route to search for new substitute lead free perovskite materials for linear and non-linear optics, with superior properties similar to PZT materials. We consider environment friendly facile c-BiAlO3 piezoelectric material, where Bi is substituted by transition metals (Sc, Zn). The effect of these dopant on the structural, electronic and optical properties of c-BiAlO3 are investigated by using generalized gradient approximation and ultrasoft pseudopotential method as implemented in CASTEP. Structural properties experience reciprocal effect on lattice constant as a function of ionic size of Impurities. In addition, a transition of indirect band gap of c-BiAlO3 to the direct one is obtained after TM impurity substitution. The optical properties such as absorption, energy loss, refractive index, dielectric function, function are computed for the energy range of 30eV. The static refractive index for pure and TM-doped obtained are 2.87, 2.99 and 3.5 respectively. Upsurges in static dielectric constants are also sighted, predicting a shift from semiconducting to metallic behavior after TM doping in c-BiAlO3. TM-doping has also boosted the absorption spectra response and reflectivity in the UV-region. Sc and Zn-doped c-BiAlO3 can be considered as potential materials in photovoltaic application.

Effect of Fe2+/3+ doping on the new magnetic mechanism and itinerant electronic characteristics of ZnO

Fe2+/3+ doping has been experimentally proven to have ferromagnetic effects on ZnO. However, the source and mechanism of magnetic properties are still explained by using the traditional double exchange theory with oxygen atoms as the intermediary. Therefore, the theoretical explanation of these phenomena needs to be expanded urgently. In this study, first principles calculations are used to investigate the effect of Fe2+/3+ doping on the magnetic mechanism of ZnO on the basis of the generalized gradient approximation plane wave ultrasoft pseudopotential method within the framework of spin density functional theory. This study shows that the Zn34Fe2O36(Fe3+) system exhibits the characteristics of ferromagnetism above room temperature and that the total magnetic moment is not an integer when the distance between two Fe ions is 4.790 Å. The Zn34Fe2O36(Fe3+) system has a part of O1− ions in addition to O2− ions, that is, it contains the characteristics of itinerant electrons (donors) and local electrons (acceptors). This study is the first to discover that Fe3d states also produce local and itinerant electrons. It also reveals a new magnetic mechanism for sp–d double exchange with itinerant electrons as the media in the Zn34Fe2O36(Fe3+) system.

Impact of Substrates on the Electronic and Mechanical Properties of AlxIn1\u2212xPySb1\u2212y Alloys

The compositional dependence of electronic and mechanical properties of a AlxIn1−xPySb1−y quaternary alloy in zinc-blende structure lattice-matched to GaSb, InAs, and InP substrates is studied. The calculations are done based on the empirical pseudopotential method modified with virtual crystal approximation. Our calculations are obtained for the energy band gaps, elastic constants, elastic moduli, bond stretching, bond bending forces, and internal strain parameter. The material system of interest is found to be a direct semiconductor within a small range of Al concentrations about 0–0.07 and an indirect one outside this region. The Debye temperature and the Fröhlich coupling constant have been determined at different values of composition lattice-matched to different substrates. The phonon frequencies and the sound velocity for different substrates at various compositions have been studied. There is a good agreement between our results and the experimental data for its constituent binary compounds, AlP, AlSb, InP, InSb, and ternary alloys, InPSb, AlPSb, which supports the calculated results of the studied AlxIn1−xPySb1−y quaternary alloy. The studied properties for the considered alloy may be helpful for the fabrication of optoelectronic devices.