Virtual Crystal Approximation Research Articles

First principles calculation and corrosion resistance study of Fe60CrxNi40-x medium entropy alloy

In this paper, Fe60CrxNi40-x medium entropy alloys (MEAs) were synthesized through vacuum arc melting. Based on the plane wave pseudo-potential and density functional theory, the solid solution structure is modeled by virtual crystal approximation (VCA). The first principle method was employed to examine how Cr content affects the phase structure, elastic constant, and elastic properties of the Fe60CrxNi40-x. Physical phase analysis of the MEAs by X-ray diffraction reveals that the Fe60CrxNi40-x maintain a single FCC solid solution structure. Meanwhile, the impact of Cr content on MEAs' resistance to corrosion in a Cl- setting was examined and contrasted with 316 L stainless steel. The result indicated that as the Cr content rises, the MEAs exhibit an extended passivation period, a reduced dimensional passivation current, and a more compact and stable passivation film. When considering the aggregate outcomes of pitting potential (Epit), self-corrosion potential (Ecorr), and corrosion current density (icorr) for 316LSS and MEAs, it's evident that Fe60CrxNi40-x in 3.5 wt% NaCl solution exhibits superior corrosion resistance compared to 316 L stainless steel, with Fe3 alloy leading in corrosion resistance among medium entropy alloys.

Computational study of lattice dynamics and mechanical properties of AlxIn1-xPySbzAs1-y-z/InP under the effect of composition

For the first time, the composition dependence of pentanary AlInPSbAs alloy is described. The effect of composition on the polarity (∝p), mechanical parameters have been calculated for the AlxIn1-xPySbzAs1-y-z alloy. The impact of composition on the internal strain parameter (ζ), bond-bending force constant (α), bond-stretching force constant (β), and sound velocity (v) has been reported. Results from experiments and the investigated properties of the new alloy show a high degree of agreement. In this work, the density function theory (DFT) and the empirical pseudo-potential approach (EPM) with the virtual crystal approximation (VCA) have been used. The outcomes are expected to be used in optoelectronic applications and in understanding the mechanical properties of AlxIn1-xPySbzAs1-y-z alloy.

DFT studies on electronic and structure properties of PbSe1-xSx alloys for thermoelectric materials using VCA and RS

In this work, Density Functional Theory (DFT) was employed to investigate the crystal structure and electronic properties of lead selenide (PbSe) and lead sulfide (PbS) semiconductors and their pseudo binary alloys PbSe1-xSx. The virtual crystal approximation (VCA) and randomly substituted solid models were employed to study these materials. In addition, the thermoelectric properties as function of the alloy composition were studied. Lattice parameters obtained using the generalized gradient approximation (GGA) were compared with theoretical and experimental results. The electronic bands were calculated for different sulfur compositions (0≤x ≤ 1, Δx = 0.1) using the Generalized Gradient Approximation of TB09LDA (MGGA). It was observed that the transition from the valence band to the conduction band takes place at the L point, which agrees with previous theoretical investigations. Slight deviations from Vegard's law were observed in the lattice parameters and bandgap of the alloys, especially for the sulfur-rich alloys. Effective band diagrams obtained from the unfolding of supercell band diagrams, reported for the first time for this system, show that the impacts of alloy disorder are low in the vicinity of the L point. The influence of the alloy composition on the transport properties was studied within the framework of Boltzmann's semiclassical theory. The electrical conductivity shows a thermally activated increase which influences the total thermal conductivity above T > 600–700 K, and the figure of merit ZT. Seebeck coefficient dependence with the chemical potential suggests an ambipolar behavior of the alloys, which present both n and p character. The alloys have the ZT maxima above 900 K influenced by the PbSe1-xSx alloy composition. Remarkably, the alloy with x = 0.5 present a maximum ZT value comparable to that of PbSe, which indicates that is promising to substituting selenium by sulfur atoms to tailor the thermoelectrical properties. This work shows the suitability of the VCA approximation and the band unfolding method, to describe the composition-dependent properties of the PbSe1-xSx pseudo binary alloys.

Prediction of NbTaTiZr-based high-entropy alloys with high strength or ductility: First-principles calculations

Refractory high-entropy alloys (RHEAs) have a great potential in high-temperature electronics, extensive aerospace and biomedical applications due to their unique strength and ductility. Herein, the effects of alloying elements on the mechanical and electronic properties of NbTaTiZrX (X = V, Mo, Hf, W and Re) RHEAs are investigated by using the density functional theory (DFT) in combination with the special quasi-random structure (SQS) and virtual crystal approximation (VCA) methods. The calculated results show that alloying of Mo, W and Re with high elastic modulus can enhance the strength and the hardness of RHEAs. Among them, NbTaTiZrRe RHEA with the best strengthening effect has a yield strength of exceeds 2 GPa, which is notably higher than other considered high-entropy alloys. For alloying of V and Hf, the ductility of RHEAs is improved, and among which, NbTaTiZrHf RHEA has a relatively best ductility. Compared with other NbTaTiZrX RHEAs, spiral dislocations in NbTaTiZrHf RHEA are more prone to nucleate, and which confirms its excellent ductility. Moreover, the reduction of pseudo energy gaps and the formation of Hf–Hf strong metallic bonds further confirm its metallic ductility from the electronic density of states and charge density. The theoretical predictions in this study are congruent with the existing experimental data, and have certain theoretical guidance relevance for enhancing the mechanical characteristics of NbTaTiZrX (X = V, Mo, Hf, W and Re) RHEAs.

Precipitation kinetics of Cu in FeCu and FeCuX (X=B, N, and BN) alloys using kinetic Monte Carlo simulations

We employed kinetic Monte Carlo simulations to investigate the precipitation of Cu in FeCu and FeCuX (X=B, N, and BN) alloys. The parameters of kinetic Monte Carlo simulations were obtained through first-principles calculations. In this study, we utilized a vacancy-mediated model that considers the migration barriers dependent on local impurity atomic environments. The dynamical stability of both FeCu and FeCuX (X=B, N, and BN) alloys was examined by the virtual crystal approximation method. Through the assessment of advancement factor, cluster number density, and mean cluster radius, the simulation results indicated that both B and N promote precipitation of Cu. While B doping has a minor effect, N doping significantly promotes Cu precipitation.

Composition-Dependent Phonon and Thermodynamic Characteristics of C-Based XxY1−xC (X, Y ≡ Si, Ge, Sn) Alloys

Novel zinc-blende (zb) group-IV binary XC and ternary XxY1−xC alloys (X, Y ≡ Si, Ge, and Sn) have recently gained scientific and technological interest as promising alternatives to silicon for high-temperature, high-power optoelectronics, gas sensing and photovoltaic applications. Despite numerous efforts made to simulate the structural, electronic, and dynamical properties of binary materials, no vibrational and/or thermodynamic studies exist for the ternary alloys. By adopting a realistic rigid-ion-model (RIM), we have reported methodical calculations to comprehend the lattice dynamics and thermodynamic traits of both binary and ternary compounds. With appropriate interatomic force constants (IFCs) of XC at ambient pressure, the study of phonon dispersions ωjq→ offered positive values of acoustic modes in the entire Brillouin zone (BZ)—implying their structural stability. For XxY1−xC, we have used Green’s function (GF) theory in the virtual crystal approximation to calculate composition x, dependent ωjq→ and one phonon density of states gω. With no additional IFCs, the RIM GF approach has provided complete ωjq→ in the crystallographic directions for both optical and acoustical phonon branches. In quasi-harmonic approximation, the theory predicted thermodynamic characteristics (e.g., Debye temperature ΘD(T) and specific heat Cv(T)) for XxY1−xC alloys. Unlike SiC, the GeC, SnC and GexSn1−xC materials have exhibited weak IFCs with low [high] values of ΘD(T) [Cv(T)]. We feel that the latter materials may not be suitable as fuel-cladding layers in nuclear reactors and high-temperature applications. However, the XC and XxY1−xC can still be used to design multi-quantum well or superlattice-based micro-/nano devices for different strategic and civilian application needs.

First-principles study on structural, electronic, and optical properties of mixed alloys of Cs2Ag(SbxBi1−x)Br6 double perovskites for optoelectronic applications

Halide perovskites have attracted promising capabilities of solar cells and light-emitting diode applications due to their high stability and optoelectronics performance. In this work, the effect of chemical substitution on the structural, electronics, and optical properties of mixed alloy Cs2Ag(SbxBi1-x)Br6 perovskites (when x is a mixing ratio varying from 0.0 to 1.0) was explored by using the virtual crystal approximation within the first-principle framework based on DFT-GGA, GGA-TB09, RPA, and BSE. Calculation results reveal that the lattice constant increases linearly as a function of the mixing ratio of a(x) = 11.246–0.0012x Å. These results satisfied the Vegard’s law as well as the reliability of virtual crystal approximation. The band structure calculated with and without the spin–orbit coupling was compared. Band gap of the mixed alloys are lower than that of pure Sb or pure Bi compounds. The band gap increases as a quadratic function of Eg = 2.07 – 0.279x – 0.121x2 eV. The smaller band gap of the mixed alloy results from the energy mismatch between Sb and Bb s and p orbitals. Calculations from DFT-TB09 are in good agreement with experimental results. Blue-shifted absorption spectra to the ultraviolet region were found when the Sb content increased. A linear increase of the static dielectric constant was found when the Sb content increased as a function of εs(x) = 2.849 + 0.266x. BSE@PBEsol calculations are consistent with experimental profiles. Calculation results indicate that the mixed alloy Cs2Ag(SbxBi1-x)Br6 can be tuned regarding its electronic and optical properties.

Magnetic structures and magnetic anisotropy of Mn3−x Fe x Sn studied by first-principles calculations

Aiming at clarifying the structures and magnetic anisotropies of kagome Mn3−x Fe x Sn crystal over the wide range of x, we theoretically investigate the x-dependence of lattice parameters and bond lengths of their six magnetic phases by the first-principles calculations, and discuss their stabilities. Under the virtual crystal approximation at the Mn/Fe sites with a composition increment of Δx = 0.25, we find the following points. First, the lattice parameters and intra-plane first-neighbor bond lengths highly depend on the magnetic structures. Second, at x ≤ 0.25, the in-plane noncollinear antiferromagnetic (AFM) phase is most stable. Third, at x = 0.50, magnetic structures such as the out-of-plane spin canting structure, which are like superpositions of the AFM and ferromagnetic phases, are stable, and the magnetization easy axis is the c-axis. Finally, at x ≥ 1.00, the in-plane ferromagnetic phase is most stable, indicating that the easy axis is in the ab-plane, and the magnetocrystalline anisotropy energy reaches −1.60 MJ m−3 at x = 3.00.

First-principles study of the magnetic anisotropy of ultrathin B-, C-, and N-doped FeCo films

Iron-based layered systems are of great interest because of their ability to tune effective material parameters such as magnetic anisotropy energy (MAE). The influence of the crystallographic structure of Fe, its thickness, and the presence of other layers above and below the Fe layer on magnetic parameters, such as the MAE of the studied system, is an intriguing and important topic from an application point of view. Here, we present a density functional theory (DFT) study of the magnetic anisotropy of nine-monolayer Fe, FeCo, and FeCo films with B, C, and N dopants placed in octahedral interstitial positions. The theoretical study is based on calculations using the full-potential local-orbital code FPLO and the generalized gradient approximation. The chemical disorder in the FeCo layers was modeled using the virtual crystal approximation. The structures of the layers were subjected to optimization of the geometry of the interlayer spacings and the neighborhood of the dopant sites. We determined the local magnetic moments and the excess charge at each layer position. We also identified the influence of dopant atoms on the magnetic properties of FeCo layers, such as magnetization and magnetic anisotropy.

Quantum Transport in Large-Scale Patterned Nitrogen-Doped Graphene.

It has recently been demonstrated how the nitrogen dopant concentration in graphene can be controlled spatially on the nano-meter scale using a molecular mask. This technique may be used to create ballistic electron optics-like structures of high/low doping regions; for example, to focus electron beams, harnessing the quantum wave nature of the electronic propagation. Here, we employ large-scale Greens function transport calculations based on a tight-binding approach. We first benchmark different tight-binding models of nitrogen in graphene with parameters based on density functional theory (DFT) and the virtual crystal approximation (VCA). Then, we study theoretically how the random distribution within the masked regions and the discreteness of the nitrogen scattering centers impact the transport behavior of sharp n-p and n-n' interfaces formed by different, realistic nitrogen concentrations. We investigate how constrictions for the current can be realized by patterned high/low doping regions with experimentally feasible nitrogen concentrations. The constrictions can guide the electronic current, while the quantized conductance is significantly washed out due to the nitrogen scattering. The implications for device design is that a p-n junction with nitrogen corrugation should still be viable for current focusing. Furthermore, a guiding channel with less nitrogen in the conducting canal preserves more features of quantized conductance and, therefore, its low-noise regime.

First-principles calculation of phase stability and elastic properties of CrxMoNbTiV refractory high-entropy alloys

Compared with traditional alloys, high-entropy alloys (HEAs) have been widely studied because of their unique phase formation rules and excellent physical properties. This work used the first-principle calculation method to study the effect of Cr content on the phase formation, stability, and mechanical properties of MoNbTiV refractory HEAs (RHEAs). The structural model of CrxMoNbTiV (x = 0.00, 0.25, 0.50, …, 2.00) RHEAs was constructed by the virtual crystal approximation method. The structural model was geometrically optimized using the Cambridge Sequential Total Energy Package code, and the structures’ binding energy, enthalpy of formation, and elastic constants were calculated. The results show that the CrxMoNbTiV RHEAs can form a stable body-centered cubic structure, and the addition of Cr significantly impacts the lattice constant, elastic constant, plastic toughness, and elastic anisotropy of the alloy. At the same time, the three-dimensional surface map of Young’s modulus anisotropy is also drawn.

First-Principles Study on the Mechanical Properties of Gd-Doped BCZT Ceramics

Due to their remarkable piezoelectric characteristics, (BaCa)(ZrTi)O3 (BCZT) ceramics exhibit vast potential for being employed in cutting-edge electromechanical apparatus. Extensive experimental studies have been conducted to better meet the practical needs of BCZT-based materials, focusing on their mechanical performance. However, there is a serious lack of research on the theoretical computational aspects. Here, first-principles calculations were utilized to evaluate the mechanical properties of BCZT-xGd ceramics. The structural models were established using the virtual crystal approximation (VCA) method. The investigated compounds demonstrate structural and mechanical strength, as evidenced by their negative formation energies and adherence to the Born stability criteria. Compared to pure BCZT, the substitution of Gd leads to a significant enhancement in the system’s elasticity and stiffness. The BCZT-0.05Gd with B-site doping demonstrates the highest level of Vicker’s hardness (HV), with the noteworthy observation that the inclusion of Gd concomitantly augments its machinability performance. Upon the incorporation of the Gd element, the anisotropic elasticity in the systems gradually transitions into isotropic elasticity, which favors a more uniform stress distribution and consequently reduces sensitivity to the formation and propagation of microcracks. These results indicate that BCZT-xGd exhibits potential for application in electromechanical systems.

The improvement of room temperature plasticity of refractory high entropy alloy based on different first principles calculation models and experiment verification

The limited ductility of Refractory High Entropy Alloy (RHEA) at room temperature hinders its widespread application. However, optimizing their properties through traditional “trial and error” methods is a challenging due to the intricate composition of High Entropy Alloys (HEAs). First principles calculation is an efficient and cost-effective method for predicting material properties. Three modeling methods for high entropy alloys are the most commonly used, namely Supercell (SC), Special quasi-random structure (SQS) and Virtual crystal approximation (VCA). To improve the plasticity of NbMoTaTiV at room temperature, the effects of Ti and Ta contents on the mechanical properties were studied. First principles calculations have been employed to predict the phase structure, elastic properties and electronic structure by three common modeling methods and validated against experimental data. The result revealed that MoNbTaTiV and MoNbTa0.5Ti1.5V RHEAs have BCC structure and increasing Ti and decreasing Ta contributed to enhanced toughness of the alloy. Notably, the three methods demonstrated good accuracy in predicting the effects of Ti and Ta content on the performance of NbMoTaTiV. The VCA method was particularly well-suited for predicting elastic properties, offering a balance between computational accuracy and efficiency. SQS and SC considered the complex atomic occupation in HEAs, making them more suitable for studying the electronic structure.

Rectifying behavior of inhomogeneous BCN alloy nanotubes

Based on virtual crystal approximation, the electronic properties and rectifying behavior of inhomogeneous BCN alloy nanotubes are investigated using a first-principles study combined with the non-equilibrium Green's function method. Consistent with previous experimental reports, carbon atoms in the BCN alloy nanotubes can randomly replace boron and nitrogen atoms. Hence, the asymmetrical distribution of B, C, and N atoms could result in different charge transmissions under the applied forward and reverse bias voltages, and the I–V characteristics of these inhomogeneous alloy nanotubes exhibit a rectifying behavior. A BCN alloy nanotube exhibits a larger current when more B and N atoms are replaced by C atoms. Consequently, the current depends on the concentration of the carbon atoms. Based on these results, tuning the electronic properties and rectification ratio of inhomogeneous BCN alloy nanotubes with an asymmetrical random distribution of carbon atoms can be applied in the design of nanoelectronic devices.

First-principle calculations and experimental investigations of thermal barrier coating on (AlxY1-x)TaO4 ceramics

In this paper, the crystal structure models of (AlxY1-x)TaO4 were constructed using the virtual crystal approximation method, and the crystal structure, electronic structure, and mechanical properties were calculated with the first-principle calculations. The bulk specimens were synthesized by hot-press sintering method, and their physical phase structures as well as mechanical properties were tested. Moreover, the calculated results were validated using the experimental data. Calculated results indicate that as Al content increases, the energy band gap of (AlxY1-x)TaO4 crystal decreases, the total density of states for Fermi energy level increases, the chemical stability of the compound decreases, and the chemical bonding between atoms diminishes. In accordance with the variation law of electronic structure, the Young's modulus, shear modulus, bulk modulus, and Vickers hardness of the compounds decrease as the Al content rises, whereas Poisson's ratio rises, indicating that the materials are more susceptible to deformation. In addition, the Young's modulus of (AlxY1-x)TaO4 (127–150 GPa) is lower than that of YSZ, and the hardness values (8.8–11.3 GPa) are comparable to those of YSZ, indicating that the material has high stress-strain tolerance.

A Hybrid‐Density Functional Theory Study of Intrinsic Point Defects in MX2 (M = Mo, W; X = S, Se) Monolayers

Defects can strongly influence the electronic, optical, and mechanical properties of 2D materials, making defect stability under different thermodynamic conditions crucial for material–property engineering. Herein, an account of the structural and electronic characteristics of point defects in monolayer transition metal dichalcogenides MX2 with M = Mo/W and X = S/Se is investigated through density functional theory using the hybrid HSE06 exchange–correlation functional including many‐body dispersion corrections. For the simulation of charged defects, a charge compensation scheme based on the virtual crystal approximation (VCA) is employed. The study relates the stability and the electronic structure of charged vacancy defects in monolayer MoS2 to an explicit calculation of the S monovacancy in MoS2 supported on Au(111), and finds convincing indication that the defect is negatively charged. Moreover, it is shown that the finite‐temperature vibrational contributions to the free energy of defect formation can change the stability transition between adatoms and monovacancies by 300–400 K. Finally, defect vibrational properties are probed by calculating a tip‐enhanced Raman scattering image of a vibrational mode of a MoS2 cluster with and without an S monovacancy.

Hole and Electron Doping Outcomes in Bi2YO4Cl.

Recognizing the deficiency in the hole and electron doping outcomes in layered bismuth-based oxyhalides intergrowths, the current study was addressed to the doping of Ca2+ and Zr4+ for Y3+ in Bi2YO4Cl. The samples were rapidly synthesized by a sol-gel auto combustion method and characterized extensively. Up to 30 mol % Y could be substituted with Ca in tetragonal symmetry and without the appearance of any additional phase. The unit cell parameters varied nonlinearly with the elongation of the Y-O bond. The Raman spectra supported the local site distortion. The calcium-substituted samples displayed selected area electron diffraction characteristics similar to those of Bi2YO4Cl. A blueshift of the absorption edge was noticed with increasing calcium content yielding optical band gap values in the 2.40-2.57 eV range. The creation of 10% Bi5+ in Bi2Y0.70Ca0.30O4Cl was established with the help of XPS measurements and redox titrations. The higher reactivity of Bi5+ in an aqueous solution has been demonstrated for the oxidation of As(III) to As(V). Electron doping through Zr4+ incorporation was possible up to 30 mol % in Bi2YO4Cl. The Y-O bonds are contracted, and the Bi-O bonds are elongated with increasing Zr4+ content. Zr4+'s incorporation induced a local distortion. The color of the sample changed from bright yellow to deep yellow with Zr inclusion, resulting in a progressive decrease in optical band gap values. The introduction of electrons caused the reduction of 13.6% of Bi(III) to Bi(0). These results have established the vulnerability of Bi2O2 chains to charge carriers in Bi2YO4Cl. Density functional theory (DFT) calculations were implemented to understand the electronic and optical properties of the pristine and doped compounds. From the band structure calculations, the chosen compounds were found to be indirect band gap semiconductors. The results of the DFT calculations were in good agreement with the experiment; however, for the doped cases, virtual crystal approximation has been used considering uniform doping at the Y-site.

Development of an Al rich Ti–Al alloy with better ductility

In this work, structural and mechanical properties of Ti–56Al–4Zr–2Nb (wt.%) alloy have been investigated by first-principles calculations based on density functional theory and experimentally. Theoretical studies have been performed with the crystal structures created with the Virtual crystal approximation approach. Ti–56Al–4Zr–2Nb (wt.%) alloy have been synthesized first time by vacuum induction melting. The phases in the microstructure were examined by scanning electron microscopy and X-Ray diffraction. Simulated alloy is found as mechanically stable by first-principles calculations. Elastic modulus and hardness results of theoretical and experimental studies are in good agreement with each other, and acceptable difference shows the reliability and success of the theoretical results in this study. Ti–56Al–4Zr–2Nb (wt.%) alloy is suggested as a promising new Ti–Al alloy for industrial applications.

Doping induced phase transition, elastic, electronic, vibrational and thermal properties of LiBeA1-x Sb x (A= P and As)

ABSTRACT Phase transition, elastic, electronic, vibrational and thermal properties of (A = P and As) are investigated via performing ab-initio calculations combined with the virtual crystal approximation (VCA) and the quasi-harmonic Debye model. Our total energy results show that and undergo a phase transition from the (P4/nmm) phase to the polar LiGaGe (P) one beyond antimony concentration of x = 0.21 and 0.36, respectively. The calculated elastic constants show that the studied alloys are mechanically stable and they also exhibit abrupt discontinuities at the transition concentrations. From Poisson's ratio and the B/G ratio, both alloys are brittle. Our results indicate that the studied alloys are semiconductors and the gap value decreases as Sb concentration increases. The calculated phonon dispersion curves for different antimony concentrations have positive frequencies. The phonon frequencies at the zone centre for the Raman-active and infrared-active modes are predicted for the and the LiGaGe structures for different antimony concentrations. The value of the polarisation is almost constant for different values of concentration. The variations of thermal expansion coefficient (α), bulk modulus (B), Debye temperature (), heat capacities at constant volume (C) and the parameter (γ) as a function of pressure and temperature were all obtained and analysed in detail.

A Theoretical Estimation of Optical, Vibrational and Structural Properties of II–VI Quaternary Alloy Zn0.5Cd0.5SySe1–y

We present a theoretical estimation of optical, vibrational, and structural properties of II–VI semiconducting quaternary alloy Zn0.5Cd0.5SySe1−y for 0 < y < 1 giving total 10 compositions. The estimation of refractive index, elastic constants, bulk modulus, and vibrational frequencies are performed using the important input parameters provided by the empirical pseudopotential method. In this method, the bandgaps are computed, and the alloying effects are modeled through the modified virtual crystal approximation. We have computed the static refractive index, static and high-frequency dielectric constants, longitudinal and transverse optical phonon frequencies, elastic constants, bulk modulus, and cohesive energy for 10 compositions of the alloy. The results are compared to other experimental and theoretical values wherever available.