Virtual Crystal Approximation Approach Research Articles

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.

Impact of random alloy fluctuations on the electronic and optical properties of (Al,Ga)N quantum wells: Insights from tight-binding calculations.

Light emitters based on the semiconductor alloy aluminum gallium nitride [(Al,Ga)N] have gained significant attention in recent years due to their potential for a wide range of applications in the ultraviolet (UV) spectral window. However, current state-of-the-art (Al,Ga)N light emitters exhibit very low internal quantum efficiencies (IQEs). Therefore, understanding the fundamental electronic and optical properties of (Al,Ga)N-based quantum wells is key to improving the IQE. Here, we target the electronic and optical properties of c-plane AlxGa1-xN/AlN quantum wells by means of an empirical atomistic tight-binding model. Special attention is paid to the impact of random alloy fluctuations on the results as well as the Al content x in the well. We find that across the studied Al content range (from 10% to 75% Al), strong hole wave function localization effects are observed. Additionally, with increasing Al content, electron wave functions may also start to exhibit carrier localization features. Overall, our investigations on the electronic structure of c-plane AlxGa1-xN/AlN quantum wells reveal that already random alloy fluctuations are sufficient to lead to (strong) carrier localization effects. Furthermore, our results indicate that random alloy fluctuations impact the degree of optical polarization in c-plane AlxGa1-xN quantum wells. We find that the switching from transverse electric to transverse magnetic light polarization occurs at higher Al contents in the atomistic calculation, which accounts for random alloy fluctuations, compared to the widely used virtual crystal approximation approach. This observation is important for light extraction efficiencies in (Al,Ga)N-based light emitting diodes operating in the deep UV.

First-principles investigation of W[sbnd]V and W[sbnd]Mo alloys as potential plasma facing materials (PFMs) for nuclear application

Density-functional theory (DFT) based first-principles calculations were used to investigate the crystal structure, binding energy, phase stability and elastic properties of body-centered cubic (BCC) W-based binary solid solutions. The effect of alloying up to 50 atomic percent (at.%) concentration range is determined from the virtual-crystal approximation (VCA) approach. Resulting BCC solid solutions are assessed in comparison to the ideal Vegard's law. Solubility of the alloying elements is characterized by the negative enthalpy of mixing. The values of elastic constants computed for the ground state structures are used to assess the effect of alloying on the ductility and hardness. Based on current results, it seems key to strike a tricky balance between moderate Pugh's modulus ratio (B/G) between bulk and shear moduli (B,G) and elastic anisotropy (A) such that high hardness is not completely compromised at the expense of ductility. The predicted property trends are in agreement with existing theoretical and experimental data, which is indicative that the VCA approach is reliable in development of such alloys.

First-principles study on material properties and stability of inorganic halide perovskite solid solutions CsPb(I1−xBrx)3

All-inorganic halide perovskites have attracted a great interest as a promising light harvester of perovskite solar cells due to their enhanced chemical stability. In this work we investigate the material properties of solid solutions $\mathrm{CsPb}{({\mathrm{I}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{3}$ in cubic phase by applying the virtual crystal approximation approach within a density functional theory framework. First we check the validity of constructed pseudopotentials of the virtual atoms ($\text{X}={\text{I}}_{1\ensuremath{-}x}{\text{Br}}_{x}$) by verifying that the lattice constants follow the linear function of mixing ratio. We then suggest an idea of using the hybrid HSE functional with linear increasing value of exact exchange term as increasing the Br content $x$, which produces the band gaps of $\mathrm{C}\mathrm{s}{\mathrm{PbX}}_{3}$ in good agreement with the available experimental data. The calculated light absorption coefficients and reflectivity show the systematic varying tendency to the Br content. We calculate the phonon dispersions of $\mathrm{C}\mathrm{s}{\mathrm{PbX}}_{3}$, CsX, and ${\mathrm{PbX}}_{2}$ as slightly changing their volumes, revealing the phase instability of $\mathrm{C}\mathrm{s}{\mathrm{PbX}}_{3}$ and calculating the thermodynamic potential function differences. By projecting Gibbs free energy differences onto the plane of $\mathrm{\ensuremath{\Delta}}G=0$, we determine the $P\ensuremath{-}T$ diagram for $\mathrm{C}\mathrm{s}{\mathrm{PbX}}_{3}$ to be stable against the chemical decomposition, highlighting that the area of being stable extends gradually as the Br content increases.

Gradual reduction of the superconducting transition temperature of H3S by partial replacing sulfur with phosphorus

Recently, using the first-principles calculations based on the virtual crystal approximation (VCA), it has been reported that hydrogen sulfide with low phosphorus substitution possesses higher critical temperature (280 K), compared to experimentally studied H3S (203 K). Herein, using the supercell method for doping, we discuss the correctness of this result through study of superconducting properties of H3S1-xPx mixture. We proved that electron-phonon coupling constants and superconducting transition temperatures (Tc) calculated using supercell approach deviate from these one obtained earlier within the framework of the virtual crystal approximation. In particular, supercell method indicates that Tc is shifted towards lower values in comparison to experimental measurements of H3S which is the evidence for failure in increasing Tc of hydrogen sulfide through partial phosphorus substitution. Moreover, the obtained trend of critical temperature as a function of pressure, in contrast to VCA approach, is consistent with experimental results.

Electronic structures of doped BaFe2As2 materials: virtual crystal approximation versus super-cell approach

Employing virtual crystal approximation and super-cell methods for doping, we have performed a comparative study of the electronic structures of various doped BaFe$_2$As$_2$ materials by first principles simulations. Both of these methods give rise to a similar density of states and band structures in case of hole doping (K doping in Ba site) and iso-electronic P doping in As site. But in case of electron doped systems with higher doping concentration, electronic structures, calculated using virtual crystal approximation approach deviates from that of the super-cell method. On the other hand in case of iso-electronic Ru doping implemented by virtual crystal approximation, an extra shift of the chemical potential in electronic structure in comparison to super-cell method is observed and that shift can be used to predict the correct electronic structure within virtual crystal approximation as reflected in our calculated Fermi surfaces. But for higher Ru doping concentration, simple shifting of chemical potential does not work as the electronic structure calculated by virtual crystal approximation approach is entirely different from that of the calculated by super-cell formalism.

Electronic structure and optical properties of dilute InAs1−xNx: pseudopotential calculations

The energy band gaps, the electron and heavy hole effective masses and dielectric constants of ternary dilute alloys InAs1−xNx are calculated using the pseudopotential method. The study was undertaken for small amounts of N. The disorder in these alloys is modeled by an improved virtual crystal approximation approach. The numerical results thus obtained are compared where possible with experimental and previous theoretical data. It is found that the incorporation of a few per cent of N in InAs reduces substantially the fundamental band-gap energy, which is a manifestation of the huge band-gap bowing commonly reported in the III–V–N-type alloys. The behavior of all studied quantities with respect to the nitrogen concentration x is shown to be nonlinear. The information gathered by the present study may be useful for mid-infrared applications.

Electronic structure and optical properties of CdSe xTe 1− x mixed crystals

The band structure and optical properties of the CdSe x Te 1− x ternary mixed crystals have been studied using the pseudopotential formalism under an improved virtual crystal approximation approach. Quantities such as, energy gaps, band-gap bowing parameters, electron effective mass and dielectric constants are calculated. Our results agree well with the available data in the literature. The composition dependence of all studied quantities has been expressed by quadratic polynomial forms.

On the bowing parameter in Cd1−xZnxTe

Cd 1−x Zn x Te thin films were prepared on 7059 Corning glass substrates using an rf magnetron sputtering system and CdTe(1−y)+ZnTe(y) targets. The concentration (x) of Zn in the films did not coincide with the relative weight (y) of the ZnTe powder in the compressed targets. Values of x were in the range 0–0.30 as determined from x-ray diffraction patterns. The band gap energy (Eg) of the Cd1−xZnxTe samples was calculated from the photoreflectance spectra measured on the films. The position of the experimental points in the Eg versus x plot show a deviation from the phenomenological quadratic relation Eg=Eg0+ax+bx2 calculated within the virtual crystal approximation (VCA). The depart of the Eg values for higher Zn concentrations from the expected VCA model is probably due to a larger clustering of Zn atoms and/or a percolation phenomena. We obtain fitted values for the parameters a and b within the VCA approach. Comparison with data reported by other authors is made.

Band structure nonlocal pseudopotential calculation of the III-nitride wurtzite phase materials system. Part II. Ternary alloys AlxGa1−xN, InxGa1−xN, and InxAl1−xN

This work presents detailed information on the band structures of the III-nitride wurtzite ternary alloys, computed through the virtual crystal approximation approach. The key ingredient of this study is the set of realistic atomic effective potentials described in Part I of the present work, dedicated to the constituent binary compounds. The model relies on the linear interpolation of the structural parameters and of the local and nonlocal effective potentials: no further empirical corrections are included. The dependence on the mole fraction is computed for the energy gaps at all the high-symmetry points, the valence-band width, and the electron effective masses in the valleys relevant for carrier-transport simulation.

Virtual crystal approximation revisited: Application to dielectric and piezoelectric properties of perovskites

We present an approach to the implementation of the virtual crystal approximation (VCA) for the study of properties of solid solutions in the context of density-functional methods. Our approach can easily be applied to any type of pseudopotential, and also has the advantage that it can be used to obtain estimates of the atomic forces that would arise if the real atoms were present, thus giving insight into the expected displacements in the real alloy. We have applied this VCA technique within the Vanderbilt ultrasoft-pseudopotential scheme to predict dielectric and piezoelectric properties of the Pb(Zr[0.5],Ti[0.5])O[3] solid solution in its paraelectric and ferroelectric phases, respectively. Comparison with calculations performed on ordered alloy supercells and with data on parents compounds demonstrates the adequacy of using the VCA for perovskite solid solutions. In particular, the VCA approach reproduces the anomalous Born effective charges and the large value of the piezoelectric coefficients.

Quantum tunneling effects in dipolar ferroelectrics: A nuclear-quadrupole-resonance study in HCl-DCl mixed crystals

Static and dynamical effects from a quantum tunneling contribution to the dipole reorientations in the ferroelectric phases of the mixed crystals HCl-DCl have been studied through $^{35}\mathrm{Cl}$ nuclearquadrupole-resonance pulse Fourier-transform measurements. Experimental data for the transition temperatures ${T}_{c}$ and the local correlation times $\ensuremath{\tau}$, of both the HCl and DCl dipoles, for several percentages of deuteration are analyzed in the light of a dynamical Ising model in a transverse field which includes damping in a heuristic way. The virtual-crystal approximation (VCA) is used in deriving both the static properties (local order parameters and transition temperatures) and the reorientational dynamics (local correlation functions). Estimates of the interaction energy and the tunneling integrals are obtained from the dependence of ${T}_{c}$ on deuteration. The behavior of the correlation times for the DCl dipoles as a function of deuteration, predicted within the framework of the VCA approach, is consistent with the experimental results. However, for the HCl dipoles this theoretical picture does not account for the correlation times in the dilute limit of almost-complete deuteration. Instead, the persistence of quantum tunneling (namely, short correlation times) is observed in spite of the interaction with the "classical" DC1 dipoles (long correlation times). Possible sources for this phenomenon are discussed. The only hypothesis that appears suitable involves the effect of local phonon modes on the tunneling of the HCl dipoles.