Korringa Kohn Rostoker Coherent Potential Approximation Research Articles

Understanding thermal transport in magnesium solid solutions through first-principles approaches and machine learning feature screening

The solid solution phases have a large influence on the thermal conductivity of alloys, but the physics behind this effect is complicated. Existing works mainly utilize experimental methods to study the thermal conductivity of alloys. However, due to the coexistence of multi-phase and insufficient understanding of the microscopic property variations in different alloy systems, it remains unclear how different solute atoms can affect the thermal transport of solid solutions. In this work, we employ first-principles simulations based on the Korringa–Kohn–Rostoker coherent potential approximation method and the Boltzmann transport equation to calculate the thermal conductivity of 38 binary magnesium solid solutions with dilute solute atom concentration. Solute atoms can greatly reduce the thermal conductivity of magnesium. Phonon contribution is found to be less than 10 %. The machine learning feature screening methods are further applied to identify the key properties that influence their thermal conductivity the most. Different from the commonly used Linde's law, which states that the resistivity is proportional to the square of valence difference, the machine learning screening indicates that thermal conductivity is predominantly influenced by six features in total. Among them, the variance of the density of states of s + p, d + f orbitals at Fermi level is highly correlated to the electronic thermal conductivity of magnesium alloys. These correlations underscore the role of virtual bond states formed in magnesium alloys due to the presence of solute atoms. The developed method and uncovered physical mechanism can potentially be applied to other solid solution systems, which is an important step to achieve predictive design of high thermal conductivity metallic alloy.

An investigation of high entropy alloy conductivity using first-principles calculations

The Kubo–Greenwood equation, in combination with the first-principles Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) can be used to calculate the DC residual resistivity of random alloys at T = 0 K. We implemented this method in a multiple scattering theory based ab initio package, MuST, and applied it to the ab initio study of the residual resistivity of the high entropy alloy AlxCoCrFeNi as a function of x. The calculated resistivities are compared with experimental data. We also predict the residual resistivity of refractory high entropy alloy MoNbTaVxW. The calculated resistivity trends are also explained using theoretical arguments.

Hydrogen Adsorption on Ordered and Disordered Pt-Ni Alloys

The bulk properties and chemical reactivity of disordered Pt-Ni alloys in the A1 (fcc) structure are investigated using different methods: Virtual Crystal Approximation (VCA), Korringa–Kohn–Rostoker Coherent Potential Approximation (KKR-CPA), and large explicit supercells generated using Super-Cell Random Approximates (SCRAPs). While VCA predicts lattice constants that closely follow Vegard’s law, the large supercells and KKR-CPA predict lattice constants that are consistently larger than Vegard’s law. KKR-CPA results closely agree with those from the large supercells for the disordered alloys, producing similar projected density of states and magnetic moment across the composition range. For instance, while VCA predicts the disordered alloys to be non-magnetic at a Pt concentration (xPt) ≥ 0.5, KKR-CPA and SCRAPs predict the disordered alloys to remain ferromagnetic to higher Pt concentrations. As xPt decreases, the adsorption of H becomes more exothermic on bulk-terminated (111) surfaces but less exothermic on Pt monolayer-terminated (111) surfaces due largely to strain effects. (111) surfaces cut from the large supercells predict average H adsorption energies on the disordered alloys similar to those on the ordered phases of the same compositions, while VCA predicts H adsorption to be more exothermic.

Effect of Dopant Concentrations on Magnetic States of Ti, Mn, And Ni Doped Iii-V Zincblende Compounds

The effect of Ti, Mn, and Ni dopants concentration on various magnetic states of III-V zincblende compounds (Al1-xTx)M is presented by calculation of multiple scattering theory based on Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA), where T denotes Ti, Mn, and Ni atoms and M represents As and P atoms. The calculations reveal the phase changes in various magnetic states, which are solely controlled by dopant concentrations. The turning points of these behaviors are reported, which are responsible for the change of stable ferromagnetic (FM) state to unstable disordered local moment (DLM) state. The Curie temperature, magnetic moments, and density of states at different concentrations of the dopants are calculated and explain the concurrency with magnetic phase stability.
 Journal of Bangladesh Academy of Sciences, Vol. 42, No. 2, 177-182, 2018

Effect of Grain Size on Magnetic Properties of ZnO Doped with Nickel Single Impurities

The purpose of our study is to find a suitable material to be used in spintronic applications and to find the relation between the parameters of deposition by spray pyrolysis technic (temperature, concentration of Ni doping) and the ferromagnetic properties (Curie temperature, magnetic moment). The nickel-doped zinc oxide, Zn.1−x.NixO (x = 0.01, 0.03, 0.05), and diluted magnetic semiconductors (DMSs) are synthesized by the spray pyrolysis technic. The results of The X-ray diffraction (XRD) of prepared substrates confirm the incorporation of the dopants into the ZnO lattice structure. The spin-polarized electronic properties has been found and was investigated in detail by using the density-functional theory (DFT), the local density approximation (LDA), and The Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). As result, nickel doping brings up a half-metallic appearance due to the hybridization between the 3d state of Nickel impurities and the oxygen 2p state. The mechanism of the interatomic exchange has been explained as being a p-d double exchange.

Beyond Atomic Sizes and Hume-Rothery Rules: Understanding and Predicting High-Entropy Alloys

High-entropy alloys constitute a new class of materials that provide an excellent combination of strength, ductility, thermal stability, and oxidation resistance. Although they have attracted extensive attention due to their potential applications, little is known about why these compounds are stable or how to predict which combination of elements will form a single phase. In this article, we present a review of the latest research done on these alloys focusing on the theoretical models devised during the last decade. We discuss semiempirical methods based on the Hume-Rothery rules and stability criteria based on enthalpies of mixing and size mismatch. To provide insights into the electronic and magnetic properties of high-entropy alloys, we show the results of first-principles calculations of the electronic structure of the disordered solid-solution phase based on both Korringa–Kohn–Rostoker coherent potential approximation and large supercell models of example face-centered cubic and body-centered cubic systems. We also discuss in detail a model based on enthalpy considerations that can predict which elemental combinations are most likely to form a single-phase high-entropy alloy. The enthalpies are evaluated via first-principles “high-throughput” density functional theory calculations of the energies of formation of binary compounds, and therefore it requires no experimental or empirically derived input. The model correctly accounts for the specific combinations of metallic elements that are known to form single-phase alloys while rejecting similar combinations that have been tried and shown not to be single phase.

Computational materials design of LiZnAs-, LiZnP-, and LiZnN-based n-type magnetic semiconductors

The computational materials design of n-type filled tetrahedral compound magnetic semiconductors is proposed on the basis of first-principles calculations within the density functional theory. Using the Korringa–Kohn–Rostoker coherent potential approximation, the electronic structures of electron-doped Li(Zn,Mn)As, Li(Zn,Mn)P, and Li(Zn,Mn)N are calculated. By estimating free energy, the phase diagrams of these systems are predicted. It is shown that these systems are phase-separated and favor spinodal decomposition when the electrons are not doped. By introducing Li interstitials as donor impurities, spinodal decomposition is strongly suppressed and Mn can be doped at a high concentration. Moreover, ferromagnetic interactions between Mn atoms are induced by electron doping. Thus, we can expect electron-mediated ferromagnetism in these systems with a reasonable Curie temperature.

Magnetic Properties of Transition Metal-Doped CdSe

In this paper, the electronic, structural, and magnetic properties of transition metal (TM) (V, Cr, Mn, Fe, Co, Ni, Cu)-doped CdSe are investigated using first-principles calculations within the density functional theory (DFT) and the local density approximation (LDA) in connection with the Korringa–Kohn–Rostoker coherent potential approximation (KKR–CPA) implemented in the AKAI MACHIKANEAMA code. The 3d level locations of majority spin of those magnetic ions are studied. The spin–spin interaction at low concentration between magnetic impurities leads to ferromagnetic behavior. While the increase of TM concentration beyond 10 % leads to antiferromagnetic interactions between the magnetic ions, which suppress the ferromagnetism. Magnetic stability and Curie temperature are also calculated for different TM doping. And finally, two TMs are chosen to make a codoping in CdSe semiconductor material.

Materials design of interstitial Be-doped ferromagnetic (In,Fe)As

Using the first-principles calculation method, we study the ferromagnetic states of electron-doped (In,Fe)As by the Korringa–Kohn–Rostoker coherent potential approximation method and calculate Curie temperature (TC) by Monte Carlo simulation. We show that the chemical pair interactions and magnetic exchange interactions between Fe atoms depend on the concentration of the doped interstitial Be donor. We obtain the so-called ordered phase of Fe-rich regions in this material. With this type of structure, Be-doped (In,Fe)As can exhibit ferromagnetism and its TC could be up to approximately 85 K. We propose that the ordered phase is necessary for enhancing ferromagnetism and increasing TC.

Computational materials design of ferromagnetic Fe–Cu alloy by phase separation

Using ab initio electronic structure calculations by the Korringa–Kohn–Rostoker coherent potential approximation and Monte Carlo simulation of the two-dimensional spinodal nano-decomposition, we propose a new materials design for ferromagnetic Fe–Cu alloys. We demonstrate that Fe nano-wires of the size of 3–5 nm are surrounded by Cu matrix under a modified layer by layer crystal growth. Ferromagnetism is enhanced by the inhomogeneous distribution of Fe atoms in the alloy.

Correlation effects in fcc-FexNi1−x alloys investigated by means of the KKR-CPA ∗∗This work is dedicated to the memory of Balazs Gyorffy.

The electronic structure and magnetic properties of the disordered alloy system fcc-FexNi1−x (fcc: face centered cubic) have been investigated by means of the KKR-CPA (Korringa–Kohn–Rostoker coherent potential approximation) band structure method. To investigate the impact of correlation effects, the calculations have been performed on the basis of the LSDA (local spin density approximation), the LSDA + U as well as the LSDA + DMFT (dynamical mean field theory). It turned out that the inclusion of correlation effects hardly changed the spin magnetic moments and the related hyperfine fields. The spin–orbit induced orbital magnetic moments and hyperfine fields, on the other hand, show a pronounced and element-specific enhancement. These findings are in full accordance with the results of a recent experimental study.

Computational materials design of defect-induced ferrimagnetic MnO

We present a computational materials design for defect-induced ferrimagnetic MnO. The magnetic properties of MnO containing Mn vacancies were investigated using first-principle calculations. For these electronic structure calculations, we employed a pseudo-self-interaction-corrected local density approximation (PSIC-LDA). We used the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) to create a random distribution of atoms at the assigned sites. Having described the magnetic properties with a classic Heisenberg model, we calculated the effective exchange coupling constants by applying the magnetic force theorem to two magnetic sites embedded in the CPA medium. We estimated the Curie temperatures from the calculated exchange interactions. This study found that the Mn vacancies induced ferrimagnetic ground states in MnO, and that the Curie temperature could reach room temperature at Mn vacancy concentrations above 20%. These findings suggest a new route for designing ferrimagnetic materials from anti-ferromagnetic host materials.

Theoretical Study of TiO2 Doped with Single and Double Impurities

Using first principles calculations within the density-functional theory (DFT) and the local density approximation (LDA), the electronic structure and magnetic properties of rutile TiO2 doped with single and double-impurities Ti(Cr, Mn) O2 has been investigated. The Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) has been used. Chromium doping brings up a half-metallic appearance; it is due to the hybridization between the 3d state of chromium and the oxygen 2p state. The mechanism of the interatomic exchange in this case has been deducted as a double exchange. We also studied Mn doped TiO2, which gave rise to ferromagnetism, due to the p–d exchange. We also study the advantage of codoping TiO2 diluted magnetic oxide to find a suitable material to be used in spintronics.

Search for Resonant-Like Impurity in Ag-Doped CoSb3 Skutterudite: Theoretical and Experimental Study

Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) calculations of Ag-doped CoSb3 point to the presence of either an extra sharp peak of s-symmetry Ag density of states near the valence-band edge when filling the void (2a) or to conventional p-type doping when substituting Sb site (24g). These results suggest a resonant-like impurity level in the former or nearly rigid-band behavior in the latter. To confirm the theoretical predictions, a series of samples with nominal composition Co8Sb24:Agx (x = 0, 0.1, 0.3, 0.4, 0.5) were prepared. Structural and phase composition analyses were carried out by x-ray diffraction, scanning electron microscopy, and scanning thermoelectric microprobe. Investigations of the influence of Ag impurity on the electrical conductivity and Seebeck coefficient were performed over the temperature range from 300 K to 560 K. It was found that doping CoSb3 with Ag leads to an increase of the thermoelectric power factor α2σ in the temperature range from 300 K to 475 K of about an order of magnitude for all doped samples. However, electron probe microanalysis revealed accumulation of Ag mainly in grain boundaries while the presence of Ag in CoSb3 crystallites was not confirmed. This observation corroborates the results of KKR-CPA calculations concerning the formation energy of the AgxCo4Sb12 system, which is much lower than values calculated for AxCo4Sb12 (A = Ca, Ba).

Evaluation of terminal composition of palladium–silver hydrides in plateau region by electronic structure calculations

Abstract The electronic structures of palladium, palladium–silver alloys (Pd–Ag) and their hydrides were calculated to evaluate the terminal composition in the plateau region in the pressure–composition isotherm. The electronic structures were calculated by the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA), MACHIKANEYAMA 2002 developed by Akai. The Fermi level of palladium at various hydrogen contents moved to the edge of the d-band with increasing the hydrogen content and the Fermi level of PdH 0.6 reached the edge of the d-band. This hydrogen composition, [H]/[Pd] = 0.6, agreed well with the terminal composition in the plateau region. The Fermi level of Pd–Ag was similarly lifted up to the edge of the d-band with increasing the hydrogen content. The terminal compositions in isotherms of Pd–Ag alloys were estimated from the Fermi level position and the density of states at the Fermi level. The estimated compositions agreed with those experimentally obtained. Therefore, the terminal composition in the plateau region in the pressure–composition isotherms of Pd–Ag alloys could be evaluated by using with the KKR-CPA method.

Ferromagnetism from Acceptors of Native Point Defects in (Zn, Mn)O Doped Systems

In this paper, we represent a theoretical study of the (Zn, Mn)O doped system with native point defects of ZnO as oxygen interstitials (Oi) and zinc vacancies (VZn). Under these defects, it has been shown that the ground state can be converted from spin glass to ferromagnetic phase, by performing ab initio density functional theory calculations relying on the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method employing the local density approximation (LDA) with the parameterization by Morruzi, Janak, and Williams. The stability of magnetism in the (Zn, Mn)O doped system with different native point defects has been discussed. We find that acceptor-like defects (that is, Oi, OZn, and VZn) may cause the enhancement of the ferromagnetic characteristics in ZnMnO with increasing Tc. Based on the theoretical results, we suggest that the native point defects have a key role with respects to the FM properties. Using the mean field approximation, the Curie temperature in our studied model is estimated.

Computational Nano-Materials Design of Low Cost and High Efficiency Cu2ZnSn[Se1-xSx]4 Photovoltaic Solar Cells by Self-Organized Two-Dimensional Spinodal Nanodecomposition

We propose the possibility of spinodal nanodecomposition in Cu2ZnSn[Se1-xSx]4 (CZTSeS) for high efficiency photovoltaic solar cells, based on the first-principles calculations within the self-interaction-corrected local density approximation. By using the Korringa–Kohn–Rostoker coherent potential approximation method, electronic structures of CZTSeS are calculated. Due to the calculated positive mixing energy and type II band alignment between Cu2ZnSnSe4 and Cu2ZnSnS4, we can expect that the efficiency of CZTSeS becomes higher by spinodal nano-decomposition. Then we simulate the self-organized two-dimensional spinodal nanodecomposition by Monte Carlo method using the Ising model with chemical pair interactions calculated from the first principles.

Materials design of CuIn1−xZn0.5xSn0.5xSe2 for low cost photovoltaic solar cells

Computational material design of a strategy for resource security of chalcopyrite-based photovoltaic solar cells (PVSC) is investigated. The electronic structure of CuIn1−xZn0.5xSn0.5xSe2 (CIZTS) is calculated by using the self-interaction-corrected local density approximation (SIC-LDA) and the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA). It is shown that for any degree of substitution for In, CIZTS has a direct band gap and no impurity state is induced in the band gap. We propose a possibility of substitutional co-doping of Zn and Sn for In in CuInSe2 photovoltaic material to reduce the cost of materials for PVSC production.

Electronic structure and magnetism of IV–VI compound based magnetic semiconductors

The electronic structure and the magnetic properties of Mn-doped GeTe are calculated from first-principles. The calculations are based on the local density approximation, and we use the Korringa–Kohn–Rostoker Coherent Potential Approximation (KKR-CPA) method for simulating substitutional and magnetic disorder. It is shown that Mn-3d states appear deep in the valence bands with d5 electron configuration and the system is insulating. By introducing Ge vacancies to dope hole carriers, the ferromagnetism is induced via the p–d exchange mechanism. On the other hand, the electron doping is not efficient to stabilize the ferromagnetic state.

Origin of Magnetism from Native Point Defects in ZnO

Based on the first-principle calculations by using the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of different point defects in ZnO, which are Zinc interstitials (Zni), Zinc antisites (ZnO), Oxygen interstitials (Oi) and Oxygen antisites (OZn) defects in ZnO. The supercell calculations were also performed using the full potential local-orbital (FPLO) band structure scheme. This work presents detailed information about total and local density of states at some concentrations of these defects; the stability of the ferromagnetic state compared with the spin-glass state is investigated by comparing calculating their total energy. The results show on one hand that Zni and ZnO produce a shallow donor bellow the bottom of the conduction band (CB), while Oi and OZn produces the shallow acceptors above the top of the valence band (VB), and moment magnetic; on other hand that the ferromagnetic state is more stable than the spin-glass in Oxygen interstitials (Oi) and vice versa for oxygen antisites (OZn) of native point defects in ZnO. The other native point defects (Zni, ZnO, VO, and VZn) have a zero magnetic moment. The results show that the Curie temperature increases with the concentration of interstitial oxygen.