Self-consistent linear-muffin-tin-orbitals coherent-potential technique for bulk and surface calculations: Cu-Ni, Ag-Pd, and Au-Pt random alloys

High thermoelectric performance in metallic NiAu alloys via interband scattering

Thermoelectric materials seamlessly convert thermal into electrical energy, making them promising for power generation and cooling applications. Although historically the thermoelectric effect was first discovered in metals, state-of-the-art research focuses on semiconductors. Here, we discover unprecedented thermoelectric performance in metals and realize ultrahigh power factors up to 34 mW m-1 K-2 in binary NixAu1-x alloys, more than twice larger than in any bulk material above room temperature, reaching zTmax ∼ 0.5. In metallic NixAu1-x alloys, large Seebeck coefficients originate from electron-hole selective scattering of Au s electrons into more localized Ni d states. This intrinsic energy filtering effect owing to the unique band structure yields a strongly energy-dependent carrier mobility. While the metastable nature of the Ni-Au system as well as the high cost of Au pose some constraints for practical applications, our work challenges the common belief that good metals are bad thermoelectrics and presents an auspicious route toward high thermoelectric performance exploiting interband scattering.

Interplanar Ferromagnetism Enhanced Ultrawide Zero Thermal Expansion in Kagome Cubic Intermetallic (Zr,Nb)Fe2

A cubic metal exhibiting zero thermal expansion (ZTE) over a wide temperature window demonstrates significant applications in a broad range of advanced technologies but is extremely rare in nature. Here, enabled by high-temperature synthesis, we realize tunable thermal expansion via magnetic doping in the class of kagome cubic (Fd-3m) intermetallic (Zr,Nb)Fe2. A remarkably isotropic ZTE is achieved with a negligible coefficient of thermal expansion (+0.47 × 10-6 K-1) from 4 to 425 K, almost wider than most ZTE in metals available. A combined in situ magnetization, neutron powder diffraction, and hyperfine Mössbauer spectrum analysis reveals that interplanar ferromagnetic ordering contributes to a large magnetic compensation for normal lattice contraction upon cooling. Trace Fe-doping introduces extra magnetic exchange interactions that distinctly enhance the ferromagnetism and magnetic ordering temperature, thus engendering such an ultrawide ZTE. This work presents a promising ZTE in kagome metallic materials.

Alloying-induced spin Seebeck effect and spin figure of merit in Pt-based bimetallic atomic wires of noble metals

The chemical potential of electrodes can be tuned to generate pure thermal spin voltages in certain bimetallic wires of noble metals.

Systematic Quantum Cluster Typical Medium Method for the Study of Localization in Strongly Disordered Electronic Systems

Great progress has been made in recent years towards understanding the properties of disordered electronic systems. In part, this is made possible by recent advances in quantum effective medium methods which enable the study of disorder and electron-electronic interactions on equal footing. They include dynamical mean-field theory and the Coherent Potential Approximation, and their cluster extension, the dynamical cluster approximation. Despite their successes, these methods do not enable the first-principles study of the strongly disordered regime, including the effects of electronic localization. The main focus of this review is the recently developed typical medium dynamical cluster approximation for disordered electronic systems. This method has been constructed to capture disorder-induced localization and is based on a mapping of a lattice onto a quantum cluster embedded in an effective typical medium, which is determined self-consistently. Unlike the average effective medium-based methods mentioned above, typical medium-based methods properly capture the states localized by disorder. The typical medium dynamical cluster approximation not only provides the proper order parameter for Anderson localized states, but it can also incorporate the full complexity of Density-Functional Theory (DFT)-derived potentials into the analysis, including the effect of multiple bands, non-local disorder, and electron-electron interactions. After a brief historical review of other numerical methods for disordered systems, we discuss coarse-graining as a unifying principle for the development of translationally invariant quantum cluster methods. Together, the Coherent Potential Approximation, the Dynamical Mean-Field Theory and the Dynamical Cluster Approximation may be viewed as a single class of approximations with a much-needed small parameter of the inverse cluster size which may be used to control the approximation. We then present an overview of various recent applications of the typical medium dynamical cluster approximation to a variety of models and systems, including single and multiband Anderson model, and models with local and off-diagonal disorder. We then present the application of the method to realistic systems in the framework of the DFT and demonstrate that the resulting method can provide a systematic first-principles method validated by experiment and capable of making experimentally relevant predictions. We also discuss the application of the typical medium dynamical cluster approximation to systems with disorder and electron-electron interactions. Most significantly, we show that in the limits of strong disorder and weak interactions treated perturbatively, that the phenomena of 3D localization, including a mobility edge, remains intact. However, the metal-insulator transition is pushed to larger disorder values by the local interactions. We also study the limits of strong disorder and strong interactions capable of producing moment formation and screening, with a non-perturbative local approximation. Here, we find that the Anderson localization quantum phase transition is accompanied by a quantum-critical fan in the energy-disorder phase diagram.

Electrodeposited Ni-Based Magnetic Mesoporous Films as Smart Surfaces for Atomic Layer Deposition: An "All-Chemical" Deposition Approach toward 3D Nanoengineered Composite Layers

Mesoporous Ni and Cu-Ni (Cu₂₀Ni₈₀ and Cu₄₅Ni₅₅ in at. %) films, showing a three-dimensional (3D) porous structure and tunable magnetic properties, are prepared by electrodeposition from aqueous surfactant solutions using micelles of P-123 triblock copolymer as structure-directing entities. Pores between 5 and 30 nm and dissimilar space arrangements (continuous interconnected networks, circular pores, corrugated mesophases) are obtained depending on the synthetic conditions. X-ray diffraction studies reveal that the Cu-Ni films have crystallized in the face-centered cubic structure, are textured, and exhibit certain degree of phase separation, particularly those with a higher Cu content. Atomic layer deposition (ALD) is used to conformally coat the mesopores of Cu₂₀Ni₈₀ film with amorphous Al₂O₃, rendering multiphase "nano-in-meso" metal-ceramic composites without compromising the ferromagnetic response of the metallic scaffold. From a technological viewpoint, these 3D nanoengineered composite films could be appealing for applications like magnetically actuated micro/nanoelectromechanical systems (MEMS/NEMS), voltage-driven magneto-electric devices, capacitors, or as protective coatings with superior strength and tribological performance.

Quantum-mechanical analysis of effect of alloying elements on ε-martensite start temperature of steels

With regard to the transformation mechanism of austenitic high manganese steel, the prediction of the ε-martensite start temperature is a critical consideration in alloy design. Evaluation of the ε-martensite start temperature makes it possible to predict the microstructure and to understand the phase transformation occurring during deformation. Here we use the quantum mechanical calculation of random alloys to understand the physics for ε-martensitic transformation in steels. We could find the linear relationship between the measured ε-martensite start temperatures and the crystal structure stability for various compositions. We also could estimate the effect of several alloying elements. It is expected that the effect of decreasing the temperatures for the same amount of alloying elements addition will be larger moving farther from Group VIII. By creating a free-energy model that reflects the temperature effect, we were able to calculate the average driving force required for the ε-martensitic transformations.

Electron-phonon coupling in Ni-based binary alloys with application to displacement cascade modeling

Energy transfer between lattice atoms and electrons is an important channel of energy dissipation during displacement cascade evolution in irradiated materials. On the assumption of small atomic displacements, the intensity of this transfer is controlled by the strength of electron-phonon (el-ph) coupling. The el-ph coupling in concentrated Ni-based alloys was calculated using electronic structure results obtained within the coherent potential approximation. It was found that Ni0.5Fe0.5, Ni0.5Co0.5 and Ni0.5Pd0.5 are ordered ferromagnetically, whereas Ni0.5Cr0.5 is nonmagnetic. Since the magnetism in these alloys has a Stoner-type origin, the magnetic ordering is accompanied by a decrease of electronic density of states at the Fermi level, which in turn reduces the el-ph coupling. Thus, the el-ph coupling values for all alloys are approximately 50% smaller in the magnetic state than for the same alloy in a nonmagnetic state. As the temperature increases, the calculated coupling initially increases. After passing the Curie temperature, the coupling decreases. The rate of decrease is controlled by the shape of the density of states above the Fermi level. Introducing a two-temperature model based on these parameters in 10 keV molecular dynamics cascade simulation increases defect production by 10-20% in the alloys under consideration.

Density functional theory in materials science

Materials science is a highly interdisciplinary field. It is devoted to the understanding of the relationship between (a) fundamental physical and chemical properties governing processes at the atomistic scale with (b) typically macroscopic properties required of materials in engineering applications. For many materials, this relationship is not only determined by chemical composition, but strongly governed by microstructure. The latter is a consequence of carefully selected process conditions (e.g., mechanical forming and annealing in metallurgy or epitaxial growth in semiconductor technology). A key task of computational materials science is to unravel the often hidden composition-structure-property relationships using computational techniques. The present paper does not aim to give a complete review of all aspects of materials science. Rather, we will present the key concepts underlying the computation of selected material properties and discuss the major classes of materials to which they are applied. Specifically, our focus will be on methods used to describe single or polycrystalline bulk materials of semiconductor, metal or ceramic form.

Grain boundary segregation and interdiffusion effects in nickel-copper alloys: an effective means to improve the thermal stability of nanocrystalline nickel

Nanocrystalline (nc) Ni films show pronounced grain growth and suffer from concomitant deterioration of their mechanical and magnetic properties after annealing at relatively low temperatures (T(ANN) ≥ 475 K). This constitutes a drawback for their applicability as coatings or in components of miniaturized devices. This work reveals that the thermal stability of nc Ni is significantly improved by controllably alloying Ni with Cu, by means of electrodeposition, to form a Ni(1-x)Cu(x) solid solution. To tune the composition of such nc alloys, Ni(1-x)Cu(x) films are deposited galvanostatically using an electrolytic bath containing Ni and Cu sulfate salts as electroactive species, saccharine as grain-refining agent, and applying current densities ranging from -10 to -40 mA cm(-2). The enhanced thermal stability is ascribed to segregation of a Cu-rich phase at the Ni(1-x)Cu(x) grain boundaries, which acts as a shielding layer against grain growth. As a result, high values of hardness (in excess of 6 GPa) remain in nc Ni(1-x)Cu(x) for x ≥ 0.3, even after annealing at T(ANN) ≥ 575 K. From a magnetic point of view, Ni(1-x)Cu(x) films possess lower coercivity values than pure nc Ni films, both in the as-prepared and annealed states, thus offering potential advantages for certain soft magnetic applications.

Variation of equation of state parameters in the Mg2(Si(1-x)Sn(x)) alloys

Thermoelectric performance peaks up for intermediate Mg(2)(Si(1-x)Sn(x)) alloys, but not for isomorphic and isoelectronic Mg(2)(Si(1-x)Ge(x)) alloys. A comparative study of the equation of state parameters is performed using density functional theory, Green's function technique, and the coherent potential approximation. Anomalous variation of the bulk modulus is found in Mg(2)(Si(1-x)Sn(x)) but not in the Mg(2)(Si(1-x)Ge(x)) analogs. Assuming a Debye model, linear variations of the unit cell volume and pressure derivative of the bulk modulus suggest that lattice effects are important for the thermoelectric response. From the electronic structure perspective, Mg(2)(Si(1-x)Sn(x)) is distinguished by a strong renormalization of the anion-anion hybridization.

Complex magnetic interactions in off-stoichiometric NiMnGa alloys

Using first-principles density functional theory, the magnetic pair interactions between various pairs of chemical specie have been calculated and the trends in magnetism with varying compositions and chemical ordering are analyzed for three off-stoichiometric NiMnGa alloys in their austenite phases. The experimentally observed trend of decreasing magnetization with increasing Mn concentration is attributed to the antiferromagnetic interactions among Mn atoms occupying sublattices other than the original Mn one. The role of chemical ordering on magnetization is also analyzed by total energy results and exchange interactions. We are able to explain the recently published neutron scattering experiments with our theoretical analyses.

Dimensionality crossover in the induced magnetization of Pd layers

The magnetic ordering of a series of samples consisting of ultrathin Fe layers embedded in Pd was investigated using the magneto-optical Kerr effect. The samples consisted of a single Fe layer with nominal thickness 0.2≤d(Fe)≤1.6 monolayers sandwiched between two 20 monolayer Pd layers. A dimensionality crossover from two dimensions to three dimensions occurs as d(Fe) is increased from 0.4 to 1.0 monolayers. First-principles calculations were performed in order to determine the magnetic profile, and we used a spin-wave quantum well model for obtaining a qualitative description of the dimensionality crossover. The results clearly prove the existence of a dimensionality crossover in the induced magnetization, opening new routes for addressing the influence of extension on order.

Sample preserving deep interface characterization technique

We propose a nondestructive technique based on atomic core-level shifts to characterize the interface quality of thin film nanomaterials. Our method uses the inherent sensitivity of the atomic core-level binding energies to their local surroundings in order to probe the layer-resolved binary alloy composition profiles at deeply embedded interfaces. From an analysis based upon high energy x-ray photoemission spectroscopy and density functional theory of a Ni/Cu fcc (100) model system, we demonstrate that this technique is a sensitive tool to characterize the sharpness of a buried interface. We performed controlled interface tuning by gradually approaching the diffusion temperature of the multilayer, which lead to intermixing. We show that core-level spectroscopy directly reflects the changes in the electronic structure of the buried interfaces, which ultimately determines the functionality of the nanosized material.

Competing exchange interactions in magnetic multilayers

We have studied alloying of the nonmagnetic spacer layer with a magnetic material as a method of tuning the interlayer coupling in magnetic multilayers. We have specifically studied the Fe/V(100) system by alloying the spacer V with various amounts of Fe. For some Fe concentrations in the spacer, it is possible to create a competition between antiferromagnetic Ruderman-Kittel-Kasuya-Yoshida exchange and direct ferromagnetic exchange coupling. The exchange coupling and transport properties for a large span of systems with different spacer concentrations and thicknesses were calculated and measured experimentally and good agreement between observations and theory was observed. A reduction in magnetoresistance of about 50% was observed close to the switchover from antiferromagnetic to ferromagnetic coupling.

Oxidation of plutonium dioxide

The physics and chemistry of the actinide elements form the scientific basis for rational handling of nuclear materials. In recent experiments, most unexpectedly, plutonium dioxide has been found to react with water to form higher oxides up to PuO(2.27), whereas PuO(2) had always been thought to be the highest stable oxide of plutonium. We perform a theoretical analysis of this complicated situation on the basis of total energies calculated within density functional theory combined with well-established thermodynamic data. The reactions of PuO(2) with either O(2) or H(2)O to form PuO(2+delta) are calculated to be endothermic: that is, in order to occur they require a supply of energy. However, our calculations show that PuO(2+delta) can be formed, as an intermediate product, by reactions with the products of radiolysis of water, such as H(2)O(2).

Valence-band hybridization and core level shifts in random Ag-Pd alloys

First-principles calculations of the core-level binding energy shifts (CLS) for 3d inner-core electrons of Ag and Pd in fcc Ag-Pd alloy were carried out within the complete screening picture, which includes both initial and final state effects. These alloys show remarkable CLS that have the same sign for both alloy components, in contradiction to what would be expected from the potential model for core electron energies. We show that the main contribution to the core-level shift is due to the intra-atomic charge redistribution, which is related to the hybridization between the valence electron states of the alloy components. There is also a large contribution to the CLS from the core-hole relaxation energy.

First-principles predictions of yet-unobserved ordered structures in the Ag-Pd phase diagram

The complexity of first-principles total energy calculations limits the pool of structure types considered for a ground-state search for a binary alloy system to a rather small, O(10), group of "usual suspects." We conducted an unbiased search of fcc-based Ag(1)-(x)Pd(x) structures consisting of up to many thousand atoms by using a mixed-space cluster expansion. We find an unsuspected ground state at 50%-50% composition-the L1(1) structure, currently known in binary metallurgy only for the Cu(0.5)Pt(0.5) alloy system. We also provide predicted short-range-order profiles and mixing enthalpies for the high temperature, disordered alloy.