Coherent Potential Approximation Research Articles

Superconductivity in high-entropy alloy system containing Th

Th-containing superconducting high entropy system with the nominal composition (NbTa)_{0.67}(MoWTh)_{0.33} was synthesized. Its structural and physical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, specific heat, resistivity and magnetic measurements. Two main phases of alloy were observed: major bcc structure and minor fcc. The experimental results were supported by numerical simulation by the DFT Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA).

Temperature-dependent models for wave velocity of oil sand

Knowledge of how temperature affects the oil–sand acoustic response is useful to exploit these reservoir rocks with seismic methods. We propose three models: double-porosity coherent potential approximation (CPA), lower-bound Hashin–Shtrikmann (HS-), and contact cement (CC), based on different spatial distributions of heavy oil and temperature and frequency-dependent empirical equations. The shear modulus and S-wave velocity are affected by temperature in all the cases. Moreover, the properties of oil sands with heavy oil as a continuous matrix and higher viscosity are more sensitive to temperature. The models can provide theoretical support for the establishment of rock physical models in heavy oil reservoirs so as to quantitatively characterize the seismic response changes caused by thermal mining.

First-principles study of the phase stability and elastic properties of TiVNbMoM (M = Al, Sc, Ni and Cu) high entropy alloys

The ab initio exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) were used to study the influence of alloying elements M = Al, Sc, Ni and Cu on the phase stability, lattice constants, elastic constants, polycrystalline elastic moduli and electronic structure of equiatomic and non-equiatomic TiVNbMoM refractory high entropy alloys. The agreement between our results and the available experimental and theoretical data is quite good. It was found that the equiatomic systems are stable in the body-centered cubic (bcc) structure. Alloying elements decrease the stability of the bcc against the face-centered cubic (fcc) and the hexagonal close-packed (hcp) structures. Scandium enlarges the lattice constants of equiatomic and non-equiatomic systems significantly. According to the calculated bulk modulus to shear modulus, Poisson’s ratio and Vickers hardness, all studied equiatomic and non-equiaomic systems are found to be ductile. However, alloying elements Al, Ni and Cu reduce the ductility and improve the hardness of equiatomic and non-equiatomic TiVNbMoM systems, while the ductility (hardness) of non-equiatomic systems enhances (reduces) by substitution with Sc element. The present theoretical results provide insight for the design and improvement of high entropy alloys and complete information on the alloying effects.

Enhanced Superconducting Critical Parameters in a New High-Entropy Alloy Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08.

The structural and physical properties of the new titanium- and niobium-rich type-A high-entropy alloy (HEA) superconductor Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08 (in at.%) were studied by X-ray powder diffraction, energy dispersive X-ray spectroscopy, magnetization, electrical resistivity, and specific heat measurements. In addition, electronic structure calculations were performed using two complementary methods: the Korringa-Kohn-Rostoker Coherent Potential Approximation (KKR-CPA) and the Projector Augmented Wave (PAW) within Density Functional Theory (DFT). The results obtained indicate that the alloy exhibits type II superconductivity with a critical temperature close to 7.5 K, an intermediate electron-phonon coupling, and an upper critical field of 12.2(1) T. This finding indicates that Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08 has one of the highest upper critical fields among all known HEA superconductors.

First-principles calculation of magnetocrystalline anisotropy of Y(Co,Fe,Ni,Cu)[formula omitted] based on full-potential KKR Green’s function method

The performance of permanent magnets YCo5 can be improved by replacing cobalt with other elements, such as iron, copper, and nickel. In order to determine its optimum composition, it is necessary to perform systematic theoretical calculations in a consistent framework. In this study, we calculated the magnetocrystalline anisotropy constant Ku of Y(Co1−x−yFexCuy)3(Co1−zNiz)2 on the basis of the full-potential Korringa–Kohn–Rostoker Green’s function method in conjunction with the coherent potential approximation. The calculated Ku of YCo5 was smaller than the experimental value because of a missing enhancement due to orbital polarization. Although the value of Ku of Y(Co1−x−yFexCuy)3(Co1−zNiz)2 was systematically underestimated compared to their experimental counterparts, the doping effect can be analyzed within a consistent framework. The results have shown that YFe3Co2 has much higher Ku=5.00 MJ/m3 than pristine YCo5 (Ku=1.82 MJ/m3), and that nickel as a stabilization element decreases Ku and magnetization in YFe3(Co1−zNiz)2. However, the anisotropy field of z∼0.5 can compete with the value of YCo5.

Effect of 3d transition metal impurities doping on electronic and magnetic properties of CeO2

The effect of transition metal (TM) impurities, namely Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, on the electronic and magnetic properties of Ceria dioxide is studied using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation (KKR-CPA). Calculations indicate that pure CeO2 is a nonmagnetic indirect semiconductor. Doping this compound with transition metals results in three main groups based on their electromagnetic behavior. The first group involves CeO2 doped with Sc, Ti and Zn which show a lack of magnetism at low concentrations. The second group of TMs, including V, Mn, Fe and Ni, induces half-metallic character and ferromagnetic behavior. Finally, when Cr, Co and Cu are substituted in CeO2, this doping creates magnetic semiconductors with a direct band gap. The origin of the magnetic states is further discussed and the Curie temperatures are determined. The present work shows that controlling impurity type and concentration renders Cerium dioxide a potential candidate for spintronic applications.

Effect of atomic anti-site disorder on the anisotropic magnetoresistance in Fe50Co50 alloys

In order to understand the anti-site disorder effect on the anisotropic magnetoresistance (AMR) of alloys, alloys were studied in this work using the fully relativistic spin-polarized Korringa–Kohn–Rostoker method. The anti-site disorder was modeled by interchanging Fe and Co atoms and treated by the coherent potential approximation. It is found that the anti-site disorder broadens the spectral function and decreases the conductivity. Our work emphasizes that the absolute variations of resistivity under magnetic moment rotation are less affected by atomic disorder. The annealing procedure improves the AMR by reduction of the total resistivity. At the same time, we also find that the fourth-order term in the angular dependent resistivity becomes weaker when the disorder increases, resulting from increased scattering of the states around the band-crossing.

First-principles study on the effects of atomic configuration on the magnetic anisotropy energy of (Fe,Co)16(N,C)2 alloys

Herein, the first-principles technique combined with a coherent potential approximation is utilized to investigate the magnetic anisotropy energy (MAE) of (Fe1-xCox)16(N,C)2 alloys. The alloys contain an ordered α″-phase and partially ordered α′-phase, where α-Fe is distorted to form a bct structure with c/a = 1.1 owing to the random intrusion of (N, C) atoms. For the α″-phase, the MAE is sensitive to Co substitution sites and reaches approximately 3 MJ/m3 at an x of 0.25 when Co atoms occupy the 4d sites. For the α′-phase, the lattice distortion c/a = 1.1 alone cannot provide a positive MAE when x = 0 (FeN0.125 and FeC0.125). Evidently, both Co substitution and further increasing the c/a over 1.1 must be performed to realize a positive MAE in the α'-phase. Finally, the importance of local distortion of the (Fe, Co) lattice in further increasing the c/a beyond 1.1 is discussed as it increases the effective c/a even when the global c/a is fixed at 1.1.

Predicting elastic properties of refractory high-entropy alloys via machine-learning approach

Refractory high entropy alloy (RHEA) is an important family of alloy with excellent mechanical properties and broad industrial applications. Different chemical and stoichiometric compositions make RHEA a highly tunable material with a huge space for optimization. However, the virtual screening of RHEA is extremely nontrivial, considering the high cost of conventional first-principles calculations. In this work, we compute the ground state and the elastic properties of 2487 RHEAs using exact muffin-tin orbital method combined with coherent potential approximation (EMTO-CPA). Using these data, we train an accurate and robust random forest model that can predict the elastic properties of RHEA systems much more efficiently. Furthermore, we also examine the correlation between seven independent input features and the elastic properties of the alloy. Through this analysis, we identify the most important features that control the elastic properties of RHEA, paving the road to a rational design of the RHEA materials.

Properties of electronic and magnetic states of ternary (MgA)O diluted magnetic insulators (A = Cr, Mn and Co)

Diluted magnetic states of the ternary compounds (Mg1-xAx)O are calculated by the method of Korringa-Kohn-Rostoker (KKR) Green's function, where A and x denote the Cr, Mn, and Co atoms with fractional concentration. The fractional impurities are controlled with the coherent potential approximation (CPA). The total energy of the ferromagnetic (FM) and local moment disordered (LMD) states are calculated for the compounds and found them stable in FM structure. The difference of energy between the FM and the LMD states for each unit cell is utilized to estimate the Curie temperature (TC) within framework of the mean-field approximation (MFA). The values of TC for (Mg1-xMnx)O is higher than that of the room temperature (RT) for x = 3%–15% of Mn dopant and follow a decreasing trend with dopant concentrations. Moreover, the low TC is reported for Cr, and Co doped compounds. Furthermore, the induced local moments and the half-metallic conditions of the compounds are obtained, where the magnetic atoms act as the sole contributor for the carrier of magnetic moments.

Experimental and simulation study of microstructure and magnetic properties of (AlFeMnNi)1−x Nd x

ABSTRACT The high-entropy alloy (Al25Fe30Mn25Ni20)1−x Nd x (all in at.%) (AlFeMnNi)1−x Nd x ) was prepared by vacuum arc melting (AM) and casting method (CM). X-Ray Diffraction (XRD) and Scanning Electron Microscopy, transmission electron microscope and Integrated Energy-Dispersive X-ray spectroscopy were used to study the structure of our system. The (Al25Fe30Mn25Ni20)1−x Nd x crystallises in a Face-centred cube and centred cubic (BCC), depending on the x (x = 0.00, x = 0.01 at.% Nd). The increase of Neodymium (0.01 at.% Nd) content results in the formation of BCC structure. The chemical composition analysis of FCC and BCC crystal structures, their lattice constants demonstrate that the formation of a single BCC solid solution between 760°C and 1480°C. The Korringa–Kohn–Rostoker Method and Green's function combined with coherent potential approximation were used to study the magnetoelectronic properties of (Al25Fe30Mn25Ni20)1−x Nd x . We have obtained the value optimised of lattice parameter 2.9 Å and it is near to that obtained by experimental value using High Score programme and results XRD experimental.

Improving the hydrogen storage properties of lithium hydride (LiH) by lithium vacancy defects: Ab initio calculations

Using the coherent potential approximation (CPA) integrated into the AkaiKKR package, the effect of lithium vacancies on the temperature of desorption and the energy of formation in the lithium hydride (LiH) is investigated. Firstly, the calculations show that the LiH has high stability and consequently a high decomposition temperature, and that is due to the strong interaction between Li and H atoms. Secondly, the results reveal that the Li vacancy defects have a beneficial effect on the hydrogen storage properties of the hydride by increasing its formation energy and gravimetric density, and decreasing its desorption temperature and stability. Two reasons can explain this decrease in stability as a function of the concentration. First, there is a decrease in Li atoms that form strong bonds with hydrogen atoms. Second, the lithium and hydrogen states shift towards the conduction band, which is shown using the density of states (DOS).

Theoretical Calculations and Experimental Measurements on the Two-Component Au - Pt Alloys with Ultralow Magnetic Susceptibility

A solid-state material with ultralow magnetic susceptibility $\ensuremath{\chi}$ is the key material candidate in aeronautical devices and special targeted drugs. In this work, we design a series of two-component alloy samples ${\mathrm{Au}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}$, and study their magnetic structures and susceptibilities by using the first-principles calculations within a spin-polarized relativistic Korringa-Kohn-Rostoker coherent potential approximation. Our theoretical results demonstrate that by controlling the ratio parameter $x$, ultralow magnetic susceptibilities ($\ensuremath{\chi}<{10}^{\ensuremath{-}6}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{3}/\mathrm{mol}$) occur in $\mathrm{Au}\mathrm{Pt}$ alloys around the sample ${\mathrm{Au}}_{0.75}{\mathrm{Pt}}_{0.25}$. Following the theoretical calculations, several realistic $\mathrm{Au}\mathrm{Pt}$ alloy samples with ultralow magnetic susceptibilities are fabricated successfully in experiments, and the experimental measurements on magnetic susceptibilities are well consistent with our theoretical predications. This work not only provides us changes to study physical properties of the materials with ultralow magnetic susceptibility, but also constructs unique materials towards special device applications in TianQin project and others.

Ab-initio study of electronic, mechanical and thermodynamic properties of β-Ti–15Nb–xSi alloys for biomaterials applications

In this paper, we used the first-principles method to investigate the structural, electronic, mechanical and thermodynamic parameters of the ternary [Formula: see text]-Ti–15Nb–xSi alloys with [Formula: see text][Formula: see text]wt.%. We have carried out theoretical computations inside the density functional theory (DFT) utilizing the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) model. The random distribution of Nb atoms in the alloy was described by using both virtual crystal approximation (VCA), special quasirandom structure (SQS) and the coherent potential approximation (CPA) techniques, in combination with first-principles plane-wave pseudopotential (PW-PP) and exact muffin-tin orbital (EMTO) methods. We determined the elastic constants as well as the bulk, shear, Young’s modulus and Poisson’s ratio. Our structural results are in good agreement with the available experimental and theoretical results for the pure structure of the titanium. In addition, we have estimated the band structure and the density of state (DOS) for the electronic computations. Our findings demonstrate that all of the compounds are metallic, stable and meet the requirements for stability. Young’s modulus of Ti–15Nb–0.6Si and Ti–15Nb–1.6Si is 86.5[Formula: see text]GPa and 15.11[Formula: see text]GPa, respectively, which are similar to Young’s moduli of human bone (10–30[Formula: see text]GPa). All calculated parameters of the alloys decreased with increasing of Si concentration except for Poisson’s ratio, anisotropy and B/G ratio. Furthermore, all of the materials investigated showed ductile nature, and Young’s modulus values are needed for further applications. Excitations from the quasi-harmonic Debye approximation’s vibrational part were applied to the 0[Formula: see text]K free energy calculated via ab-initio calculations. The influence of temperatures up to 800 K on phase stability was investigated. These findings can be utilized to help designers create alternative low-modulus alloys for biomedical applications.

Investigation of ultrasound propagation through dense glass bead packings immersed in water

Conventional ultrasound imaging generally relies on a single scattering assumption and a constant sound hypothesis. However, in dense granular sediments where the glass beads diameter is comparable to the wavelength (typically 500 μm), both hypothesizes are no longer valid, resulting in a loss of resolution and contrast, up to the situation where a target to image could totally vanish in the fog. To better characterize this strongly scattering medium, we have conducted several ultrasonic experiments in reflection and in transmission, either on the diffused wave or on the coherent one. Both the transport and scattering mean free paths are estimated in the imaging bandwidth (1–5 MHz), alongside with the phase velocity and extinction length of the coherent wave. These results are compared with an analytical model, i.e. Generalized Coherent Potential Approximation*, which considers a coated sphere buried in an effective medium. Finally, we show a dispersive beamformer which takes into account the measured phase and group velocity. This allows us to obtain a more resolved and contrasted image in such a scattering medium. * Jing X, Sheng P, Zhou M. Phys Rev Lett. 1991

Crystal structure and site preference, magnetic and elastic properties, and martensitic transformation of Ni- and Fe-doped Co2VGa alloys: A first-principles study

Using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, the crystal structure and site preference, magnetic and elastic properties, and martensitic transformation (MT) are systematically investigated with the three groups of Heusler alloys: (Co2−xMx)VGa (M1x), Co2(V1−xMx)Ga (M2x), and Co2V(Ga1−xMx) (M3x, M = Ni and Fe, 0≤x≤1.0). It is shown that instead of the L21 and XA structures, the fcc one is energetically preferred in the cubic M3x (x≥0.8) alloys. In L21-Ni2x (x≤0.6) and fcc-Ni3x (x=0.8), Ni atoms even prefer the Ga and Co anti-sites, respectively, and the replaced atoms move to the sublattices of the deficient ones. Their total magnetic moment is dominated by the magnetic exchange interactions corresponding to the pairs of two Co atoms on the different sublattices in M = Ni and Fe1x, Co and Fe in Fe2x and Fe3x (x<0.8), and Fe and Fe atoms in Fe3x (x≥0.8) alloys, respectively. These Ni1x, Ni2x, and Fe3x with x≥0.4 as well as Ni3x with x≥0.2 alloys are predicted having the MT behavior and also the better mechanical property relative to Co2VGa. A lower shear modulus (C′=(C11−C12)/2) generally corresponds to a higher MT temperature, and these alloys, which can undergo the MT are further evaluated with C′<36.50 GPa. Both considerable magnetocaloric and magnetovolume effects can be also expected during the MT of these Fe3x alloys (x=0.4 and 0.6). In the remaining Fe1x and Fe2x alloys, the Fe doping disfavors the MT and also improves their brittleness. The structural preference of these cubic alloys and also their stability relative to the tetragonal martensite can be mainly attributed to the number of their minority density of states at the Fermi level: the smaller they are, the more stable their system tends to be.

Ab-initio study of tuning the electronic and magnetic properties of [formula omitted] Heusler alloy by Co and Mn compound doping

We report the effects of Mn and Co doping on the electronic properties, magnetic exchange interaction, and Curie temperature of intermetallic Ni2MnGa by Green’s function based Korringa–Kohn–Rostoker method with coherent potential approximation(KKR-CPA). The effect of single and compound doping of Mn and Co at different crystallographic positions on these properties are evaluated by computing the electronic and magnetic structures of Ni2MnGa. The study revealed the possibility of tuning magnetic exchange interaction(Jij) and Curie temperature(TC) upon doping. Moreover, it is noted that doping can stabilize the Jahn–Teller distortion. It is also worth noting that TC responds in a different way with concentration as well as the site of the dopant. This study helps in understanding and realizing the cause for magnetic properties in Ni2MnGa, and experimental peers can also use it for further research on doped Ni2MnGa.

Magnetism and equation of states of fcc FeHx at high pressure

Abstract Hydrogen is a strong candidate for light alloying elements in terrestrial cores. Previous first-principles studies on non-stoichiometric hexagonal close-packed (hcp) and double hexagonal close-packed (dhcp) FeHx predicted a discontinuous volume expansion across the magnetic phase transition from non-magnetic (NM) or antiferromagnetic (AFM) to ferromagnetic (FM) state with increasing the hydrogen content, x at 0 K. However, previous high-pressure and -temperature neutron diffraction experiments on face-centered cubic (fcc) FeHx did not reveal such nonlinearity. The discrepancy between theory and experiment may be due to differences in the crystal structure, magnetism, or temperature. In this study, we computed the equation of states for fcc FeHx using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation (KKR-CPA). In addition to the four types of ground-state magnetism (FM, AFM-I, AFM-II, and NM), we calculated the local magnetic disorder (LMD) state, which approximates the paramagnetic (PM) state with local spin moment above the Curie temperature. Our results show that even though FM, AFM-I, AFM-II, and NM calculations predict a discontinuity in the volume at 0 K, the volume becomes continuous above the Curie temperature, consistent with the previous neutron experiment. From the enthalpy comparison at 0 K, FM fcc FeH (x = 1) becomes the NM state above ~48 GPa. The magnetic transition pressure decreases with decreasing hydrogen content. Therefore, below the magnetic transition pressure, local spin moments affect the density and elastic wave velocity of fcc FeHx, which may be important for small terrestrial bodies such as Mercury and Ganymede. By contrast, at the Earth’s core pressure above 135 GPa, fcc FeHx becomes NM. Thus, we calculated the density and bulk sound velocity as a function of pressure at 0 K for NM fcc FeHx. The density at 360 GPa decreases with increasing hydrogen content, with FeH0.5 best matching the preliminary reference Earth model (PREM) of the inner core. Since the density decreases with increasing temperature, this value constrains the upper limit of hydrogen content, assuming that the inner core is fcc FeHx. On the other hand, the bulk sound velocity at 360 GPa increases with increasing hydrogen content, with FeH0.3 best matching the PREM, which may give a lower bound. Assuming that Poisson’s ratio of the FeHx alloy is equal to that of the inner core, we examined the effects of temperature on density and bulk sound velocity. The results suggest that the fcc FeHx alloy alone cannot explain the inner core density and bulk sound velocity simultaneously unless the temperature is extremely low (T < 4000 K).

Elastic Properties of B2-NiAl with W addition: A first-principles study

The effect of tungsten alloying on the elastic properties of B2-NiAl at low-temperature and high-temperature distribution of W atoms on sublattices. By means of the exact muffin-tin orbitals method in conjunction with the coherent potential approximation, the constants C11,C12,C44, Young's modulus E, shear modulus G, Cauchy pressure values, G/B ratios are calculated. Using phenomenological criteria of the correlation between ductility and elastic properties of solution phases, it has been shown that the addition of tungsten could yield improved ductility for B2-NiAl in both types of alloys. It is established that with the high-temperature distribution of W atoms on the Al sublattice, there is a loss of mechanical stability and a decrease in mechanical properties with increasing W concentration. In alloys with a low-temperature distribution of tungsten atoms between Al and Ni sites a unique combination of properties occurs: in addition to the ductility enhancement, a simultaneous increase of the elastic constants C44 and C11 and C11, shear modulus G, Young's modulus E with increasing W content is observed. The differences in the behavior of elastic constants in alloys with different types of tungsten distribution on NiAl sublattices are analyzed by calculating the density of electronic states. Keywords: NiAl, alloying elements, tungsten, elastic properties, ductility, first-principles calculations.

Electronic and valleytronic properties of crystalline boron-arsenide tuned by strain and disorder.

Ab initio density functional theory (DFT) and DFT plus coherent potential approximation (DFT + CPA) are employed to reveal, respectively, the effect of in-plane strain and site-diagonal disorder on the electronic structure of cubic boron arsenide (BAs). It is demonstrated that tensile strain and static diagonal disorder both reduce the semiconducting one-particle band gap of BAs, and a V-shaped p-band electronic state emerges - enabling advanced valleytronics based on strained and disordered semiconducting bulk crystals. At biaxial tensile strains close to 15% the valence band lineshape relevant for optoelectronics is shown to coincide with one reported for GaAs at low energies. The role played by static disorder on the As sites is to promote p-type conductivity in the unstrained BAs bulk crystal, consistent with experimental observations. These findings illuminate the intricate and interdependent changes in crystal structure and lattice disorder on the electronic degrees of freedom of semiconductors and semimetals.