Role of Static Displacements in Stabilizing Body Centered Cubic High Entropy Alloys.

The configurational entropy of high entropy alloys (HEAs) plays little role in the stabilization of one particular crystal structure over another. We show that disorder-induced atomic displacements help stabilize body centered cubic (bcc) structure HEAs with average valences <4.7. These disorder-induced atomic displacements mimic the temperature-induced vibrations that stabilize the bcc structure of group IV elemental metals at high temperatures. The static displacements are significantly larger than for face centered cubic HEAs, approaching values associated with the Lindemann criterion for melting. Chemical disorder in high entropy alloys have a previously unidentified, nonentropic energy contribution that stabilizes a particular crystalline ground state.

Hydrogen interactions with low-index surface orientations of tungsten.

We report on density functional theory calculations that have been performed to systematically investigate the hydrogen-surface interaction as a function of surface orientation. The interactions that were analyzed include stable atomic adsorption sites, molecular hydrogen dissociation and absorption energies, migration pathways and barriers on tungsten surfaces, and the saturation coverage limits on the (1 1 1) surface. Stable hydrogen adsorption sites were found for all surfaces. For the reconstructed W(1 0 0), there are two primary adsorption sites: namely, the long-bridge and short-bridge sites. The threefold hollow site (3F) was found to be the most stable for W(1 1 0), while the bond-centered site between the first and second layer was found to be most stable for the W(1 1 1) surface. No bound adsorption sites for H2 molecules were found for the W surfaces. Hydrogen (H) migration on both the (1 0 0) and (1 1 0) surfaces is found to have preferred pathways for 1D motion, whereas the smallest migration barrier for net migration of H on the W(1 1 1) surface leads to 2D migration. Although weaker H interactions are predicted for the W(1 1 1) surface compared to the (1 0 0) or (1 1 0) surfaces, we observe higher H surface concentrations of Θ = 4.0 at zero K, possibly due to the corrugated surface structure. These results provide insight into H adsorption, surface saturation coverage and migration mechanisms necessary to describe the evolution from the dilute limit to concentrated coverages of H.

Impact of Short-Range Forces on Defect Production from High-Energy Collisions.

Primary radiation damage formation in solid materials typically involves collisions between atoms that have up to a few hundred keV of kinetic energy. During these collisions, the distance between two colliding atoms can approach 0.05 nm. At such small atomic separations, force fields fitted to equilibrium properties tend to significantly underestimate the potential energy of the colliding dimer. To enable molecular dynamics simulations of high-energy collisions, it is common practice to use a screened Coulomb force field to describe the interactions and to smoothly join this to the equilibrium force field at a suitable interatomic spacing. However, there is no accepted standard method for choosing the parameters used in the joining process, and our results prove that defect production is sensitive to how the force fields are linked. A new procedure is presented that involves the use of ab initio calculations to determine the magnitude and spatial dependence of the pair interactions at intermediate distances, along with systematic criteria for choosing the joining parameters. Results are presented for the case of nickel, which demonstrate the use and validity of the procedure.

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.

Tailoring the physical properties of Ni-based single-phase equiatomic alloys by modifying the chemical complexity.

Equiatomic alloys (e.g. high entropy alloys) have recently attracted considerable interest due to their exceptional properties, which might be closely related to their extreme disorder induced by the chemical complexity. In order to understand the effects of chemical complexity on their fundamental physical properties, a family of (eight) Ni-based, face-center-cubic (FCC), equiatomic alloys, extending from elemental Ni to quinary high entropy alloys, has been synthesized, and their electrical, thermal, and magnetic properties are systematically investigated in the range of 4-300 K by combining experiments with ab initio Korring-Kohn-Rostoker coherent-potential-approximation (KKR-CPA) calculations. The scattering of electrons is significantly increased due to the chemical (especially magnetic) disorder. It has weak correlation with the number of elements but strongly depends on the type of elements. Thermal conductivities of the alloys are largely lower than pure metals, primarily because the high electrical resistivity suppresses the electronic thermal conductivity. The temperature dependence of the electrical and thermal transport properties is further discussed, and the magnetization of five alloys containing three or more elements is measured in magnetic fields up to 4 T.

Thermodynamic approach to the stability of multi-phase systems: application to the Y2O3-Fe system.

Oxide-metal systems are important in many practical applications, and they are undergoing extensive study using a wide range of techniques. The most accurate theoretical approaches are based on density functional theory (DFT), which is limited to ~10(2) atoms. Multi-scale approaches, e.g. DFT + Monte Carlo, are often used to model oxide metal systems at the atomic level. These approaches can qualitatively describe the kinetics of some processes but not the overall stability of individual phases. In this article, we propose a thermodynamic approach to study equilibrium in multi-phase systems, which can be sequentially enhanced by considering different defects and microstructures. We estimate the thermodynamic equilibrium by minimization of the free energy of the whole multi-phase system using a limited set of defects and microstructural objects for which the properties are calculated by DFT. As an example, we consider Y2O3 + bcc Fe with vacancies in both the Y2O3 and bcc Fe phases, Y substitutions and O interstitials in Fe, Fe impurities, and antisite defects in Y2O3. The output of these calculations is the thermal equilibrium concentration of all the defects for a particular temperature and composition. The results obtained confirmed the high temperature stability of yttria in iron. Model development toward more accurate calculations is discussed.

The influence of transition metal solutes on the dislocation core structure and values of the Peierls stress and barrier in tungsten.

Several transition metals were examined to evaluate their potential for improving the ductility of tungsten. The dislocation core structure and Peierls stress and barrier of 1/2<111> screw dislocations in binary tungsten-transition metal alloys (W(1-x)TM(x)) were investigated using density functional theory calculations. The periodic quadrupole approach was applied to model the structure of the 1/2<111> dislocation. Alloying with transition metals was modeled using the virtual crystal approximation and the applicability of this approach was assessed by calculating the equilibrium lattice parameter and elastic constants of the tungsten alloys. Reasonable agreement was obtained with experimental data and with results obtained from the conventional supercell approach. Increasing the concentration of a transition metal from the VIIIA group, i.e. the elements in columns headed by Fe, Co and Ni, leads to reduction of the C' elastic constant and increase of the elastic anisotropy A = C(44)/C'. Alloying W with a group VIIIA transition metal changes the structure of the dislocation core from symmetric to asymmetric, similarly to results obtained for W(1-x)Re(x) alloys in the earlier work of Romaner et al (2010 Phys. Rev. Lett. 104 195503). In addition to a change in the core symmetry, the values of the Peierls stress and barrier are reduced. The latter effect could lead to increased ductility in a tungsten-based alloy. Our results demonstrate that alloying with any of the transition metals from the VIIIA group should have a similar effect to alloying with Re.

Quantum oscillations of nitrogen atoms in uranium nitride.

The vibrational excitations of crystalline solids corresponding to acoustic or optic one-phonon modes appear as sharp features in measurements such as neutron spectroscopy. In contrast, many-phonon excitations generally produce a complicated, weak and featureless response. Here we present time-of-flight neutron scattering measurements for the binary solid uranium nitride, showing well-defined, equally spaced, high-energy vibrational modes in addition to the usual phonons. The spectrum is that of a single atom, isotropic quantum harmonic oscillator and characterizes independent motions of light nitrogen atoms, each found in an octahedral cage of heavy uranium atoms. This is an unexpected and beautiful experimental realization of one of the fundamental, exactly solvable problems in quantum mechanics. There are also practical implications, as the oscillator modes must be accounted for in the design of generation IV nuclear reactors that plan to use uranium nitride as a fuel.

Three- to two-dimensional transition of the electronic structure in CaFe2As2: a parent compound for an iron arsenic high-temperature superconductor.

We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of CaFe2As2-parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three- to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature (T_{s}) Fermi surfaces around Gamma and X points are cylindrical and quasi 2D. Below T_{s}, the Gamma pocket forms a 3D ellipsoid, while the X pocket remains quasi 2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides.

Unconventional London penetration depth in single-crystal Ba(Fe0.93Co0.07)2As2 superconductors.

The London penetration depth lambda(T) has been measured in single crystals of Ba(Fe0.93Co0.07)2As2. The observed low-temperature variation of lambda(T) follows a power law, Deltalambda(T) approximately T(n) with n approximately 2.4+/-0.1, indicating the existence of normal quasiparticles down to at least 0.02T(c). This is in contrast with previous penetration depth measurements on single crystals of NdFeAsO1-xFx and SmFeAsO1-xFx, which indicate an anisotropic but nodeless gap. We discuss possible explanations of the observed power law behavior.

Anisotropic three-dimensional magnetism in CaFe2As2.

Inelastic neutron scattering measurements of the magnetic excitations in CaFe2As2 indicate that the spin wave velocity in the Fe layers is exceptionally large and similar in magnitude to the cuprates. However, the spin wave velocity perpendicular to the layers is at least half as large that in the layer, so that the magnetism is more appropriately categorized as anisotropic three-dimensional, in contrast to the two-dimensional cuprates. Exchange constants derived from band structure calculations predict spin wave velocities that are consistent with the experimental data.

K-doping dependence of the Fermi surface of the iron-arsenic Ba1-xKxFe2As2 superconductor using angle-resolved photoemission spectroscopy.

We use angle-resolved photoemission spectroscopy to investigate the electronic properties of the newly discovered iron-arsenic superconductor Ba_(1-x)K_(x)Fe_(2)As_(2) and nonsuperconducting BaFe_(2)As_(2). Our study indicates that the Fermi surface of the undoped, parent compound BaFe_(2)As_(2) consists of hole pocket(s) at Gamma (0,0) and larger electron pocket(s) at X (1,0), in general agreement with full-potential linearized plane wave calculations. Upon doping with potassium, the hole pocket expands and the electron pocket becomes smaller with its bottom approaching the chemical potential. Such an evolution of the Fermi surface is consistent with hole doping within a rigid-band shift model. Our results also indicate that the full-potential linearized plane wave calculation is a reasonable approach for modeling the electronic properties of both undoped and K-doped iron arsenites.

Thermal expansion and magnetostriction of pure and doped RAgSb(2) (R = Y, Sm, La) single crystals.

Data on temperature-dependent, anisotropic thermal expansion in pure and doped RAgSb(2) (R = Y, Sm, La) single crystals are presented. Using the Ehrenfest relation and heat capacity measurements, uniaxial pressure derivatives for long range magnetic ordering and charge density wave transition temperatures are evaluated and compared with the results of the direct measurements under hydrostatic pressure. In-plane and c-axis pressure have opposite effects on the phase transitions in these materials, with in-plane effects being significantly weaker. Quantum oscillations in magnetostriction were observed for the three pure compounds, with the possible detection of new frequencies in SmAgSb(2) and LaAgSb(2). The uniaxial (along the c-axis) pressure derivatives of the dominant extreme orbits (ß) were evaluated for YAgSb(2) and LaAgSb(2).

Role of disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN.

We present calculations of magnetic exchange interactions and critical temperature T(c) in Ga1-xMnxAs, Ga1-xCrxAs, and Ga1-xCrxN. The local spin-density approximation is combined with a linear-response technique to map the magnetic energy onto a Heisenberg Hamiltonion, but no significant further approximations are made. We show the following: (i) configurational disorder results in large dispersions in the pairwise exchange interactions; (ii) the disorder strongly reduces T(c); (iii) clustering in the magnetic atoms, whose tendency is predicted from total-energy considerations, further reduces T(c), while ordering the dopants on a lattice increases it. With all the factors taken into account, T(c) is reasonably predicted by the local spin-density approximation in Mn:GaAs without the need to invoke compensation by donor impurities.

Anisotropy of the magnetoresistance in Gd5Si2Ge2.

The observed magnetoresistance of single crystalline Gd5Si2Ge2 is negative and strongly anisotropic. The absolute values measured along the [100] and [010] directions exceed those parallel to the [001] direction by more than 60%. First principles calculations demonstrate that a structural modification is responsible for the anisotropy of the magnetoresistance, and that the latter is due to a significant reduction of electronic velocity in the [100] direction and the anisotropy of electrical conductivity.