Finite-temperature properties of non-magnetic transition metals: Comparison of the performance of constraint-based semi and nonlocal functionals

We assess the performance of nonempirical, truly nonlocal and semi-local functionals with regard to structural and thermal properties of $3d$, $4d$, and $5d$ non-magnetic transition metals. We focus on constraint-based functionals and consider the new consistent-exchange van der Waals density functional version vdW-DF-cx [Phys. Rev. B 89, 035412 (2014)], the semi-local PBE [Phys. Rev. Lett. 77, 3865 (1996)] and PBEsol functionals [Phys. Rev. Lett. 100, 136406 (2008)] as well as the AM05 meta-functional [Phys. Rev. B 72, 085108 (2005)]. Using the quasi-harmonic approximation structural parameters, elastic response, and thermal expansion at finite temperatures are computed and compared to experimental data. We also compute cohesive energies explicitly including zero-point vibrations. It is shown that overall vdW-DF-cx provides an accurate description of thermal properties and retains a level of transferability and accuracy that is comparable to or better than some of the best constraint-based semi-local functionals. Especially, with regard to the cohesive energies the consistent inclusion of spin polarization effects in the atoms turns out to be crucial and it is important to use the rigorous spin-vdW-DF-cx formulation [Phys. Rev. Lett. 115, 136402 (2015)]. This demonstrates that vdW-DF-cx has general-purpose character and can be used to study systems that have both sparse and dense electron distributions.Comment: 10 pages, 5 figure

Optical absorption spectrum in disordered semiconductor multilayers

The effects of chemical disorder on the electronic and optical properties of semiconductor alloy multilayers are studied based on the tight-binding theory and single-site coherent potential approximation. Due to the quantum confinement of the system, the electronic spectrum breaks into a set of subbands and the electronic density of states and hence the optical absorption spectrum become layer-dependent. We find that, the values of absorption depend on the alloy concentration, the strength of disorder, and the layer number. The absorption spectrum in all layers is broadened because of the influence of disorder and in the case of strong disorder regime, two optical absorption bands appear. In the process of absorption, most of the photon energy is absorbed by the interior layers of the system. The results may be useful for the development of optoelectronic nanodevices.Comment: 6 pages, 6 EPS figures, revised versio

Mechanism of Re precipitation in irradiated W-Re alloys from kinetic Monte Carlo simulations

High-temperature, high-dose, neutron irradiation of W results in the formation of Re-rich clusters at concentrations one order of magnitude lower than the thermodynamic solubility limit. These clusters may eventually transform into brittle W-Re intermetallic phases, which can lead to high levels of hardening and thermal conductivity losses. Standard theories of radiation enhanced diffusion and precipitation cannot explain the formation of these precipitates and so understanding the mechanism by which nonequilibrium clusters form under irradiation is crucial to predict materials degradation and devise mitigation strategies. Here we carry out a thermodynamic study of W-Re alloys and conduct kinetic Monte Carlo simulations of Re cluster formation in irradiated W-2Re alloys using a generalized Hamiltonian for crystals containing point defects parameterized entirely with electronic structure calculations. Our model incorporates recently-gained mechanistic information of mixed-interstitial solute transport, which is seen to control cluster nucleation and growth by forming quasi-spherical nuclei after an average incubation time of 20 s at 1800 K. These nuclei are seen to grow by attracting more mixed interstitials bringing solute atoms, which in turns attracts vacancies leading to recombination and solute agglomeration. The clusters grow to a maximum size of approximately 4-nm radius, and are not fully dense with Re, containing 50% or less near the center. Our simulations are in reasonable agreement with recent atom probe examinations of ion irradiated W-2Re systems at 773 K

The role of interstitial binding in radiation induced segregation in W-Re alloys

Due to their high strength and advantageous high-temperature properties, tungsten-based alloys are being considered as plasma-facing candidate materials in fusion devices. Under neutron irradiation, rhenium, which is produced by nuclear transmutation, has been found to precipitate in elongated precipitates forming thermodynamic intermetallic phases at concentrations well below the solubility limit. Recent measurements have shown that Re precipitation can lead to substantial hardening, which may have a detrimental effect on the fracture toughness of W alloys. This puzzle of sub-solubility precipitation points to the role played by irradiation induced defects, specifically mixed solute-W interstitials. Here, using first-principles calculations based on density functional theory, we study the energetics of mixed interstitial defects in W-Re, W-V, and W-Ti alloys, as well as the heat of mixing for each substitutional solute. We find that mixed interstitials in all systems are strongly attracted to each other with binding energies of -2.4 to -3.2 eV and form interstitial pairs that are aligned along parallel first-neighbor strings. Low barriers for defect translation and rotation enable defect agglomeration and alignment even at moderate temperatures. We propose that these elongated agglomerates of mixed-interstitials may act as precursors for the formation of needle-shaped intermetallic precipitates. This interstitial-based mechanism is not limited to radiation induced segregation and precipitation in W-Re alloys but is also applicable to other body-centered cubic alloys.Comment: 8 pages, 7 figure

First principles study of tungsten-based alloys: From defect thermodynamics to phase diagrams

Nuclear fusion is a very attractive option for energy production as it is clean, safe and efficient. The major obstacle in construction of fusion power plants is the development of materials that can tolerate the extreme operational condition especially high-energy neutron flux. Tungsten is the prime candidate as armor material in fusion reactors due to its high strength and excellent high temperature properties. Tungsten alloys are interesting in this context because of the transmutation of tungsten to other elements upon neutron irradiation. In this thesis, properties of tungsten alloys were studied by means of first-principles calculations based on density functional theory (DFT). I investigated the thermodynamic and kinetic properties of intrinsic and extrinsic defects in tungsten. Ti, V and Re were shown to trap self-interstitials. Whereas mixed-interstitial migration was found to proceed via a non-dissociative mechanism with a lower barrier than that of vacancies.A closer look at trapping effects in W-Re, W-V, and W-Ti alloys showed that mixed-interstitials in all systems are strongly attracted to each other with large binding energies. They form interstitial pairs aligned along parallel first-neighbor ⟨111⟩ strings. This behavior is caused by the very large and anisotropic strain field of these interstitial defects. Low barriers for defect translation and rotation enable defect agglomeration and alignment even at moderate temperatures. Re-rich clusters that are observed in irradiated tungsten, at concentrations below the solubility limit, may eventually transform into intermetallic phases, which can affect materials hardness and embrittlement. To explain the formation of these precipitates, I carried out DFT calculations to characterize the landscape for diffusion. My results were subsequently used by collaborators to parametrize a kinetic Monte Carlo model. Simulations based on this model showed that both vacancy and interstitial mediated transports control cluster nucleation. Re-rich precipitates were seen to grow by attracting more mixed interstitials bringing solute atoms, which in turn attracted vacancies leading to recombination and solute agglomeration.The heat of mixing for the previously mentioned alloying elements was investigated further. For W-Ti, the equilibrium phase diagram below 1700 K is not known due to experimental difficulties. The present study revealed a negative heat of formation on the W-rich side, which is consistent with the experimentally observed asymmetric solubility. A revised solubility limit at low temperatures of the W-Ti phase diagram was derived on the basis of these calculations.Finally I assessed the performance of constraint-based functionals and considered the recently-developed consistent-exchange van der Waals density functional version vdW-DF-cx, the semi-local PBE and PBEsol functionals as well as the AM05 meta-functional. Structural and thermophysical properties of 3d, 4d, and 5d non-magnetic transition metals were computed at finite temperatures. It was shown that overall vdW-DF-cx provides an accurate description of thermophysical properties that is typically superior to the other functionals considered. It can thus be used to study systems that have both sparse and dense electron distributions

A first-principles investigation of interstitial defects in dilute tungsten alloys

The thermodynamic properties of intrinsic and extrinsic (Ti, V, Zr, Nb, Hf, Ta, Re) defects in tungsten have been investigated using density functional theory calculations. The formation energies of substitutional defects are discussed with respect to their thermodynamic solubility limits. Several different interstitial configurations have been identified as local minima on the potential energy surface. In addition to dumbbell configurations with orientations along < 111 > and < 110 >, a lower symmetry configuration is described, which is referred to as a bridge interstitial. This interstitial type is found to be the lowest energy configuration for mixed-interstitials containing Ti, V, and Re, and can be up to 0.2 eV lower in energy than the other configurations. According to the calculations Ti, V and Re also trap self-interstitial atoms, which can be produced in substantial numbers during ion irradiation, affecting the mobility of the latter. (C) 2015 Elsevier B.V. All rights reserved