Coherent Potential Approximation Research Articles

Origin of large magnetostriction in palladium cobalt and palladium nickel alloys: Strong pseudo-dipole interactions between palladium–cobalt and palladium–nickel atomic pairs

The origin of the large magnetostriction in palladium cobalt and palladium nickel alloys was investigated. Density functional theory calculations based on the Korringa–Kohn–Rostoker Green function method with the coherent potential approximation revealed that alloying with palladium results in increased magnetization of cobalt and nickel atoms. Also, anomalous magnetization of palladium atoms occurs simultaneously. Employing calculated spin and orbital angular momenta of the atoms, magnetostriction was discussed based on the two-spin model for disordered alloys. Under the assumption that the pseudo-dipole interaction is proportional to the orbital and total angular momenta, the experimental magnetostriction curves can be reproduced. The estimated contributions of each atomic pair to magnetostriction revealed that the large magnetostriction at the palladium-rich side originates from the strong pseudo-dipole interactions between 4d and 3d transition metal atoms, namely, palladium–cobalt and palladium–nickel atomic pairs.

Electronic properties of semiconducting Zn(Si,Ge,Sn)N2 alloys

We report on results of first-principles electronic structure calculations on disordered $\mathrm{Zn}(\mathrm{Si},\mathrm{Ge},\mathrm{Sn}){\mathrm{N}}_{2}$ alloys. These calculations on substitutional disordered alloys are carried out using the Korringa-Kohn-Rostoker Green function (KKR-GF) method in combination with the coherent potential approximation (CPA) alloy theory. The band gaps and effective masses as well as the disorder-induced finite lifetime of electronic states at the conduction band minimum and valence band maximum are evaluated by analyzing the Bloch spectral functions. Relativistic effects are found to have a small impact and in particular the influence of the spin-orbit coupling is negligible. The alloys with low Si content show band gaps and effective masses which change almost linearly with the composition. Their relatively small effective mass and long lifetime indicate a high charge carrier mobility.

Impacts of atomic and magnetic configurations on the phase stability of Fe–Pd shape memory alloys: A first-principles study

The effects of local atomic and magnetic configurations on the phase stability and elastic property of the face-centered cubic (fcc) and two body-centered tetragonal [face-centered tetragonal (fctI) and fctII, with 0.9<c/a<1 and 0.71<c/a<0.9, respectively, in the fct unit cell] phases of Fe1−xPdx (0.28≤x≤0.34) shape memory alloys are systematically investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. It is shown that, considering four types of atomic configurations in a fcc unit cell, the two with one random sublattice are both preferable in each x below 300 K. When T=300 K, the one with three random sublattices also changes to be stabilized for x≤0.30, whereas that with four random sublattices becomes stable in most of these alloys until T≥600K. Upon tetragonal distortions, in these fully disordered alloys, both the fctI and fctII phases are unstable. The fctI phase is found for 0.29≤x≤0.33, having only the configuration with one random sublattice on the same layer with the Pd site in the unit cell, whereas the fctII phase is obtained for x≤0.30, possessing all the configurations with one, two, and three random sublattices. These results representing the phase diagram of these alloys, their determined equilibrium lattice parameters, and elastic constants of the three phases at 0 K are in line with the experimental and theoretical data, and their estimated structural (TM) and magnetic (TC) transition temperatures are also close to the experimental data. Adding 4% magnetic disorder in Fe0.70Pd0.30, the fctII structure is effectively prevented, whereas the thermoelastic martensitic transformation of fcc–fctI can still be retained at 0 K.

Electron localization function implementation in the exact muffin-tin orbitals method

We report implementation of the electron localization function (ELF) within the exact muffin-tin orbitals (EMTO) formalism. The ELF is often used to study the nature of electronic bonding in different types of materials, and it is also an important ingredient in meta-generalized gradient approximations, which are one of the classes of exchange-correlation functionals. The correctness of the ELF implementation is verified with test calculations and comparison with previous literature results. The implementation supports not only regular ordered systems but also disordered systems that have been calculated using the coherent potential approximation method.

Theoretical investigation of electronic and superconducting properties of Beryllium-doped iron-based superconductor: A first-principles study

We have performed a comprehensive set of calculations to study the influence of Be-alloying on the electronic and superconducting properties of Iron-Selenium (FeSe). We have considered FeSe1-xBex (where x = 0.0–1.0) and performed density functional theory (DFT) calculations implemented in the Vienna ab initio simulation package (VASP) and SPRKKR electronic structure code. In our study, both spin-unpolarized and spin-polarized calculations using LDA functional have been executed to assess the role of substitutional alloying in this material. We found that these systems are non-magnetic under Be alloying into FeSe. In addition, we calculated Bloch spectral function and Fermi surface of FeSe1-xBex alloys and found that alloying through Be, makes the system disordered and shows its robust nature against alloying. The effect of disorder on the total density of states (DOS) has been investigated and as a result, it shifts the DOS towards higher values at Fermi level. Coherent potential approximation implemented in SPRKKR and supercell method using VASP have been used to handle the substitutional disorder for our studies. Based on the DFT calculations at zero temperature, we found that electrical conductivity decreases by increasing the concentration of Be into FeSe at Se sites. Furthermore, we have also estimated superconducting transition temperature (Tc) for FeSe1-xBex alloys. However, we have experimental evidence for FeSe and FeSe0.94Be0.06 alloys, we have reported the Tc for other concentrations hypothetically and that might be helpful for the synthesis of new superconducting materials, experimentally.

Outstanding Thermoelectric Performance Of N-Type Half-Heusler V(Fe <sub>1-X</sub> Co <sub>x</sub>)Sb Compounds at Room-Temperature

The effects of cobalt substitution at the Fe-site on the crystal structure, electronic structure and thermoelectric (TE) properties of half-Heusler (HH) V(Fe1-X Cox)Sb compounds are investigated. Crystal structural analysis using powder X-ray diffraction clarified that all compounds have deficient HH structures (Huang et al., Chem. Mater., 32, 5173-5181, 2020). With Co-substitution at the Fe 4c sites, defects are observed in the V 4a and Fe 4c sites, and a small amount of Fe atoms enter into 4d sites, which is normally vacant in an ideal HH structure. The deficient HH structures of the V(Fe1-X Cox)Sb compounds are supported by electronic structural analysis using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA). All compounds exhibit n-type behavior in both calculations and experiments with deficient HH structures. The absolute value of the Seebeck coefficient |S|, decreases with an increase in the cobalt content x . The tendency of the experimental |S| value to decrease corresponds well with the |S| determined by KKR-CPA calculations. Cobalt substitution acts as an electron dopant that effectively tunes the carrier concentration. The optimized carrier concentration of 2.1×1020 cm-3 leads to the highest power factor of 5.7×10-3 W/K2 m at room temperature for the V(Fe 0.99Co0.01)Sb compound. The V(Fe1-X Cox)Sb compounds are thus potential candidates as n-type TE materials for power generation near room temperature.

Magnetic Phase Transition, Elastic and Thermodynamic Properties of L12-(Ni,Cu)3(Al,Fe,Cr) in 3d High-Entropy Alloys

The phase stability and elastic properties of paramagnetic (PM), ferromagnetic (FM) and antiferromagnetic (AFM) phases in L12-(Ni,Cu)3(Al,Fe,Cr) alloy are first investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA). The result shows the AFM structure phase of the three is the most stable in the ground state. Calculated elastic constants show that all the phases are mechanically stable, and have uncovered that L12-(Ni,Cu)3(Al,Fe,Cr) can achieve good strength and ductility simultaneously. Then, crucial thermal properties are described satisfactorily using the Debye–Grüneisen model, showing heat capacity, Gibbs free energy G, the competitive contribution of entropy −TS and enthalpy H exhibiting significant temperature dependences. Moreover, the magnetic phase transition thermodynamics was studied, which suggests that −TS has a primary contribution to Gibbs free energy and may play a key role in the phase transition. The present results can benefit the understanding of the mechanical, thermodynamic and magnetic properties of the L12 structure phase in 3d high-entropy alloys.

Effect of Ta and Ti content on high temperature elasticity of HfNbZrTa1−xTix refractory high-entropy alloys

The finite temperature elastic properties of non-equiatomic refractory high-entropy alloys (RHEAs) HfNbZrTa1−xTix are investigated using the exact muffin‑tin orbitals method in combination with coherent potential approximation and Debye-Grüneisen model, especially the effect of Ta and Ti addition is emphasized. Taking HfNbZr as a reference, the lattice constants decrease when alloying with Ta, and continue to decline slightly with replacement of Ta by Ti. Estimating from calculated elastic parameters, mere Ta addition leads to the high strength and stiffness accompanying evident lowering of ductility, while sole Ti addition brings about the high ductility at the expense of reducing strength and hardness. With substitution of Ti for Ta, HfNbZrTa1−xTix alloys achieve a relatively high ductility accompanied a gentle decline in high strength and hardness. As temperature increases, although the obtained single-elastic and polycrystalline elastic parameters exhibit decreasing trend, the anti-softening ability is enhanced with substitution of Ti for Ta, and HfNbZrTa1−xTix alloys remain an overall good combination of strength and ductility at high temperature. The proper strength-ductility trade-off can be gained by tuning Ti/Ta ratio, which is beneficial for designing the high temperature mechanical properties of alloy.

Full counting statistics of phonon transport in disordered systems

The coherent potential approximation (CPA) within full counting statistics (FCS) formalism is shown to be a suitable method to investigate average electric conductance, shot noise as well as higher order cumulants in disordered systems. We develop a similar FCS-CPA formalism for phonon transport through disordered systems. As a byproduct, we derive relations among coefficients of different phonon current cumulants. We apply the FCS-CPA method to investigate phonon transport properties of graphene systems in the presence of disorders. For binary disorders as well as Anderson disorders, we calculate up to the $8$-th phonon transmission moments and demonstrate that the numerical results of the FCS-CPA method agree very well with that of the brute force method. The benchmark shows that the FCS-CPA method achieves $20$ times more speedup ratio. Collective features of phonon current cumulants are also revealed.

Short-range order in face-centered cubic VCoNi alloys

Concentrated solid solutions including the class of high entropy alloys (HEAs) have attracted enormous attention recently. Among these alloys a recently developed face-centered cubic (fcc) equiatomic VCoNi alloy revealed extraordinary high yield strength, exceeding previous high-strength fcc CrCoNi and FeCoNiCrMn alloys. Significant lattice distortions had been reported in the VCoNi solid solution. There is, however, a lack of knowledge about potential short-range order (SRO) and its implications for most of these alloys. We performed first-principles calculations and Monte Carlo simulations to compute the degree of SRO for fcc VCoNi, namely, by utilizing the coherent-potential approximation in combination with the generalized perturbation method as well as the supercell method in combination with recently developed machine-learned potentials. We analyze the chemical SRO parameters as well as the impact on other properties such as relaxation energies and lattice distortions.

Magnesium vacancies and hydrogen doping in [formula omitted] for improving gravimetric capacity and desorption temperature

Performing an ab initio analysis, we inspect the effect of magnesium vacancies and hydrogen doping on the magnesium hydride (MgH2). The Korringa – Kohn – Rostoker method integrated with the coherent potential approximation is used to perform our calculations. In particular, we find that the gravimetric capacity of MgH2 increases from 7.658 to 9.816 wt% when the concentrations of magnesium vacancies and hydrogen dopant atoms increase from 0 to 10%. Concretely, the results reveal that the magnesium vacancies and the hydrogen doping have a beneficial effect on the hydrogen storage properties of the hydride by decreasing its desorption temperature and stability. This decrease can be explained on the one hand by the diminution of the number of Mg atoms that establish strong bonds with H atoms, and on the other hand by using the density of states, which indicates that when the concentrations increase, the Mg and H states shift to the conduction band. We also obtain that the value of the desorption temperature can be controlled by varying the concentrations of magnesium vacancies and hydrogen dopant atoms from 4.2 to 5.8% in order to reach the optimum range 289–393 K for the practical use of fuel cell vehicles.

Polaritonic cylinders as multifunctional metamaterials: Single scattering and effective medium description

Polaritonic materials, owing to a strong phonon-polariton resonance in the THz and far-infrared parts of the electromagnetic spectrum, offer both high-index dielectric and metallic response in this regime. This complex response makes them suitable candidates for the design of metamaterial-related phenomena and applications. Here we show that one type of polaritonic-material-based structures that are particularly suitable for the achievement of a wide range of metamaterial properties are systems of polaritonic rods. To study the interplay between the material and the structural resonances in such systems we employ as model systems rods of LiF and SiC and we calculate first the scattering properties of a single rod, identifying and discussing the behavior of the different resonances for different rod diameters. To analyze the response of ensembles of polaritonic rods we employ an effective medium approach based on the Coherent Potential Approximation (CPA), which is shown to be superior to the simple Maxwell-Garnett approximation for polaritonic and high-index dielectric metamaterials. Calculating and analyzing the CPA effective parameters, we found that our systems exhibit a large variety of interesting metamaterial properties, including hyperbolic dispersion, epsilon-near-zero and negative refractive index response. This rich response, achievable in almost any system of polaritonic rods, is highly engineerable by properly selecting the radius and the filling ratio of the rods, making polaritonic rod systems an ideal platform for demonstration of multifunctional metamaterials.

On segregation in multicomponent alloys: Surface segregation in austenite and FeCrCoNiMn alloys

Segregation modelling in multicomponent random alloys within the single-site mean-field approximation is considered. As an example, the surface segregation in austenite Fe70Cr20Ni10 and equimolar Fe20Mn20Co20Cr20Ni20 random fcc alloy towards the (111) facet is calculated using ab initiomultiple scattering technique in the coherent potential approximation (CPA). The results show a very similar trend in both alloys: relatively strong surface segregation of Ni and strong anti-segregation of Cr. However, in the case of Fe70Cr20Ni10, the reversal of the surface enrichment from Ni to Fe is observed at 1750 K, while the surface of FeMnCoCrNi is Ni-rich up to 2500 K.

Ab initio calculations of conduction band effective mass parameters of thermoelectric {hbox {Mg}}_{2} {hbox {X}}_{1{-}x} {hbox {Y}}_x (X, Y\xa0=\xa0Si, Ge, Sn) alloys

Since there are still research interests in the physical properties of quasi-binary thermoelectric {hbox {Mg}}_{2} {hbox {X}}_{1-x}{hbox {Y}}_{x} alloys, with X, Y = Si, Ge, Sn, we present an ab initio analysis that yields the relative formation energy and effective masses of the conduction bands, in the whole compositional range x. We base our calculations on the full-relativistic Korringa, Kohn and Rostocker (KKR) Green’s functions formalism within the coherent potential approximation (CPA). Formation energies, measured relative to the end {hbox {Mg}}_{2} hbox {X} compounds, show no excess energy for the {hbox {Mg}}_{2} hbox {Si} {-} {hbox {Mg}}_{2} hbox {Ge} substitution thus indicating a complete solubility. In contrast, concave and asymmetric formation energies for intermediate compositions in the {hbox {Mg}}_{2} hbox {X} {-} {hbox {Mg}}_{2} hbox {Sn} alloys manifest a miscibility gap. With this basis, we compute and discuss the crossing of the conduction bands observed in n-type {hbox {Mg}}_{2} {hbox {X}}_{1-x} {hbox {Sn}}_x materials. We present direction- and band-dependent effective masses using a generalized single parabolic band effective mass approximation to discuss anisotropic effects, to interpret available experimental and theoretical data, and to predict intermediate and not yet published transport parameters on these alloys.

Seismic attenuation in partially molten rocks

We compare viscoelastic models to obtain the seismic properties of a partially molten rock as a function of temperature, pressure and tectonic stress. Invoking the correspondence principle, the material of the inclusions is represented by a Maxwell mechanical model, where the Arrhenius equation and the octahedral stress criterion define the Maxwell viscosity. One of the most advanced models is the self-consistent or coherent-potential approximation (CPA), which considers oblate spheroidal inclusions of arbitrary aspect ratio and high concentration. The physical mechanism behind the Arrhenius equation is grain-boundary relaxation, and melt occurs beyond a critical temperature. The seismic properties (stiffness, wave velocity and dissipation factor) are obtained with the CPA, Hill, Hashin-Shtrikman, Walsh and Krief-Gassmann equations. The latter model and the Hashin-Shtrikman average make no assumption on the shape of the inclusions. All the models show similar trends, predicting relaxation peaks at seismic frequencies and at the brittle-ductile transition.

Half-metallicity in [formula omitted] and V doped tin-carbide SnC

Electronic and magnetic properties of V and Mn doping SnC are investigated in this work. The ab-initio structure calculation based on the Korring-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA) have been used. We have found that the substitution of Sn atoms in Tin-Carbide by Mn and V Transition Metals brings a half-metallic ferromagnetic character with a double exchange mechanism. The partial moments of impurities in the compound varies slightly from 1.11 to 1.09 μB in V case and 3.56 to 3.5 μB for Mn case. While the polarization is found to be equal 100%, in both cases where the up-spins channel is conductive, while the down channel is insulating. Using the Vegard's law, we have determined the laticce parameter of the doping compounds as function of the impurities concentrations. Such features has a significant implications for the performance of spintronic instruments and boosts their magnetoresistance ability at an enormous rate. Hence, the critical temperature can be controlled by varying the concentration of impurities.

Computational study of the magnetic and electronic properties of the LiMgN(Vc1-xZx) and LiMg(N1-xZx) alloys where Z is B, C or S

In this investigation, we performed our calculations within the Korringa-Kohn-Rostoker (KKR) scheme and Coherent Potential Approximation (CPA) as implemented in the MACHIKANEYAMA2002v08 package based on density-functional-theory to compute the electrical and magnetic properties of LiMg(N1-xZx) and LiMgN(Vc1-xZx). Z ≡ “B, C and S” where “Vc” stands for vacancy sites. The goal is to examine the ferromagnetic characteristic at low concentration amount: x = 0.02, 0.03, 0.04, 0.05. The obtained band gap is of 2.76eV. The density of states (DOS) have been analyzed, the LiMg(N1-xBx) shown the ferromagnetic (FM) stability, where there is no evidence of this behavior in LiMg(N1-xSx), since the TDOS is symmetric and the total spin moment is null. For the LiMgN(Vc1-xSx), the ferromagnetic behavior is induced by the S element incorporated in vacancy sites, which exhibits the p-type conductivity. the main contribution in the total magnetic moment is from the sulfur, it is of MS = 0.40802 μB. The outcomes of this work made these systems as promising candidates in magnetoelectronic application thanks to their ferromagnetism.

Band engineering of Dirac cones in iron chalcogenides

By band engineering the iron chalcogenide Fe(Se,Te) via ab-initio calculations, we search for topological surface states and realizations of Majorana bound states. Proposed topological states are expected to occur for non-stoichiometric compositions on a surface Dirac cone where issues like disorder scattering and charge transfer between relevant electronic states have to be addressed. However, this surface Dirac cone is well above the Fermi-level. Our goal is to theoretically design a substituted crystal in which the surface Dirac cone is shifted towards the Fermi-level by modifying the bulk material without disturbing the surface. Going beyond conventional density functional theory (DFT), we apply the coherent potential approximation (BEB-CPA) in a mixed basis pseudo-potential framework to scan the substitutional phase-space of co-substitutions on the Se-sites. We have identified iodine as a promising candidate for intrinsic doping. Our specific proposal is that FeSe$_{0.325}$I$_{0.175}$Te$_{0.5}$ is a very likely candidate to exhibit a Dirac cone right at the Fermi energy without inducing strong disorder scattering.

Averaged cluster approach to including chemical short-range order in KKR-CPA

The single-site Korringa-Kohn-Rostoker Coherent Potential Approximation (KKR-CPA) ignores short range ordering present in disordered metallic systems. In this paper, we establish a new technique to fix this shortcoming by embedding an averaged cluster that displays chemical short range order (SRO). The degree of SRO can be tuned by externally defined order parameters. This averaged cluster can be embedded in the single site CPA medium, or a self-consistently obtained effective medium that contains SRO information. The validity of this method is demonstrated by applying it to two alloy systems - the CuZn body centered cubic (BCC) solid solution, and AlCrTiV, a four-element BCC high entropy alloy. A comparison between the non-self-consistent and self-consistent modes is also provided for the two above mentioned systems. We make the code available on the internet. Planned extensions to this work are discussed.

Generalized stacking fault energies and critical resolved shear stresses of random α-Ti-Al alloys from first-principles calculations

The critical resolved shear stress (CRSS) and its related plastic deformation of titanium with hexagonal close packed structure are highly anisotropic, leading to low ductility of the material. Understanding the alloying effect on the CRSS is crucial for the improvement of the mechanical properties through rational composition design. Accurate prediction of the CRSS is not straightforward due to the atomic randomness of the alloy. The generalized stacking fault energies (GSFEs) for the basal and prismatic plane 〈a〉 slips of random α-Ti1−xAlx alloys (0≤x≤0.1875) are calculated by using first-principles methods including exact muffin-tin orbitals (EMTO) and plane-wave psuedopotential (VASP) methods in this work. The random distribution of Al in the alloy is treated by using both coherent potential approximation (CPA) for EMTO and special quasirandom structure (SQS) techniques for VASP. The CRSSs are then evaluated within the frame work of semi-discrete variational Peierls-Nabarro model. The VASP-SQS calculations with atomic relaxation generate reasonably good GSFE and CRSS compared with the EMTO-CPA and VASP-SQS calculations without atomic relaxation. For pure Ti, the unstable stacking fault energy (γusf) and CRSS (τa) for the basal 〈a〉 slip are higher than those for the prismatic 〈a〉 slip. With increasing Al concentration x, γusfb for the basal 〈a〉 slip decreases whereas γusfp for the prismatic 〈a〉 increases. The CRSSs for the basal 〈a〉 slip (τab) and the prismatic 〈a〉 slip (τap) both increase with x while τap increases faster than τab such that τab approaches to τap. The calculated CRSSs explain successfully our recently measured mechanical properties (yield strength, fracture toughness, ultimate strength, and elongation) of the α-Ti1−xAlx alloy against x.