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Molten inorganic salts are attracting resurgent attention because of their unique physicochemical properties, making them promising media for next-generation concentrating solar power systems and molten salt reactors. The dynamics of these highly disordered ionic media is largely studied by theoretical simulations, while the robust experimental techniques capable of observing local dynamics are not well-developed. To provide fundamental insights into the atomic-scale transport properties of molten salts, we report the real-space dynamics of molten magnesium chloride at high temperatures employing the Van Hove correlation function obtained by inelastic neutron scattering. Our results directly depict the distance-dependent dynamics of a molten salt on the picosecond time scale. This study demonstrates the capability of the developed approach to describe the locally correlated- and self-dynamics in molten salts, significantly improving our understanding of the interplay between microscopic structural parameters and their dynamics that ultimately control physical properties of condensed matter in extreme environments.

The structure beyond the nearest neighbor atoms in liquid and glass is characterized by the medium-range order (MRO). In the conventional approach, the MRO is considered to result directly from the short-range order (SRO) in the nearest neighbors. To this bottom–up approach starting with the SRO, we propose to add a top–down approach in which global collective forces drive liquid to form density waves. The two approaches are in conflict with each other, and the compromise produces the structure with the MRO. The driving force to produce density waves provides the stability and stiffness to the MRO, and controls various mechanical properties. This dual framework provides a novel perspective for description of the structure and dynamics of liquid and glass.

Chiral symmetry breaking of phonons plays an essential role in emergent quantum phenomena owing to its strong coupling to spin degree of freedom. However, direct experimental evidence of the chiral phonon-spin coupling is lacking. In this study, we report a chiral phonon-mediated interlayer exchange interaction in atomically controlled ferromagnetic metal (SrRuO3)-nonmagnetic insulator (SrTiO3) heterostructures. Owing to the unconventional interlayer exchange interaction, we have observed rotation of magnetic moments as a function of nonmagnetic insulating spacer thickness, resulting in a spin spiral state. The chiral phonon-spin coupling is further confirmed by phonon Zeeman effects. The existence of the chiral phonons and their interplay with spins along with our atomic-scale heterostructure approach open a window to unveil the crucial roles of chiral phonons in magnetic materials., 40 pages

Zr-based metallic glasses are prepared by quenching supercooled liquid under pressure. These glasses are stable in ambient conditions after decompression. The High Pressure Quenched glasses have a distinct structure and properties. The pair distribution function shows redistribution of the Zr-Zr interatomic distances and their shift towards smaller values. These glasses exhibit higher density, hardness, elastic modulus, and yield stress. Upon heating at ambient pressure, they show volume expansion and distinct relaxation behavior, reaching an equilibrated state above the glass transition. These experimental results are consistent with an idea of pressure-induced low to high density liquid transition in the supercooled melt.

A wide range of behaviors, including non-monotonic rejuvenation and relaxation, and the ability to qualitatively change the effect by varying the structural state of the glass was observed during thermal cycling of bulk metallic glasses. For this, we considered various bulk metallic glasses, Zr44Ti11Cu10Ni10Be25, Pd43Cu27Ni10P20, Pt57.5Cu14.7Ni5.3P22.5, and La55Al25Ni20, at various fictive temperatures to study the effect of thermal cycling on structure, thermal signature, and fracture toughness. For some BMGs and conditions considered here, thermal cycling results in a looser structure and an increase in fracture toughness. We found that for certain other BMGs and conditions, thermal cycling results in relaxation, reflected in a denser structure, and a decrease in fracture toughness. All these responses are non-monotonic and reveal a pronounced extremum with fracture toughness values of ± 50% of the original value, before approaching a value similar to the original value prior to thermal cycling. Such richness in response to thermal cycling suggests incompleteness of the previous picture where monotonically decreasing local stresses resulting in a homogenization of the structure with increasing cycle number. Our finding suggests that relative comparisons of various contributions including activation barriers for α-relaxation have to be considered which are also constantly changing, to decide if further cycling results in an increase or a decrease in fracture toughness. The fracture toughness’ response to thermal cycling can be correlated with the average atomic structures’ response to thermal cycling, while the thermal response does not exhibit an obvious correlation.

Whereas exceptional mechanical and radiation performances have been found in the emergent body-centered cubic (BCC) refractory multi-principal element alloys (RMPEAs), the importance of their complex atomic environment, reflecting diversity in atomic size and chemistry, has been largely unexplored at the atomic level. Here, we adopt a local structure approach based on the atomic pair distribution function measurements in combination with density functional theory (DFT) calculations to investigate a series of BCC RMPEAs. Our results demonstrate that all the analyzed RMPEAs exhibit local lattice distortions (LLD) to some extent, but a severe LLD, a breakdown of the 15% atomic size difference in Hume-Rothery rules, occurs only in the Zr- and/or Hf-containing RMPEAs. In addition, through the DFT calculations we show that charge transfer among the elements profoundly reduces the size mismatch effect in average to stabilize this energy-unfavorable severe LLD. The observed competitive coexistence between LLD and charge transfer demonstrates the importance of the electronic effects on the local environments in RMPEAs.

Discovery of anti-sintering noble metal catalysts is challenging, as supported noble metal species tend to aggregate at high temperatures, leading to severely deteriorated catalytic performances. Here we show that 1 wt% of noble metal species including Au, Pd and Ru can be incorporated into high-entropy oxides (HEOs) through entropy stabilization at 900 °C in air. A reversible temperature-dependent dissolution-exsolution process is observed for Au-HEO. Further correlation with distinct CO oxidation capabilities demonstrates the potential to utilize the entropy effect to access self-regenerative catalysts for catalytic reactions.

The coherence length of the medium-range order (MRO) in metallic liquids is known to display a Curie-Weiss temperature dependence; its inverse is linearly related to temperature, and when extrapolated from temperatures above the glass transition, the coherence length diverges at a negative temperature with a critical exponent of unity. We propose a mean-field pseudospin model that explains this behavior. Specifically, we model the atoms and their local environment as Ising spins with antiferromagnetic exchange interactions. We further superimpose an exchange interaction between dynamical heterogeneities, or clusters of atoms undergoing cooperative motion. The coherence length in the metallic liquid is thus the correlation length between dynamical heterogeneities. Our results reaffirm the idea that the MRO coherence length is a measure of point-to-set correlations, and that local frustrations in the interatomic interactions are prominent in metallic liquids.

Metallic glasses exhibit fast mechanical relaxations at temperatures well below the glass transition, one of which shows little variation with temperature known as nearly constant loss (NCL). Despite the important implications of this phenomenon to in aging and deformation, the origin of the relaxation is unclear. Through molecular dynamics simulations of a model metallic glass, Cu_64.5Zr_35.5, we implement dynamic mechanical analysis with system stress decomposed into atomic-level stresses to identify the group of atoms responsible for NCL. This work demonstrates that NCL relaxation is due to fully transient groups of atoms that become normal over picosecond timescales. They are spatially distributed throughout the glass and have no outstanding features, rather than defect-like as previously reported.

We present the results of a structural study of metallic alloy liquids from high temperature through the glass transition. We use high energy X-ray scattering and electro-static levitation in combination with molecular dynamics simulation and show that the height of the first peak of the structure function, S(Q) − 1, follows the Curie-Weiss law. The structural coherence length is proportional to the height of the first peak, and we suggest that its increase with cooling may be related to the rapid increase in viscosity. The Curie temperature is negative, implying an analogy with spin-glass. The Curie-Weiss behavior provides a pathway to an ideal glass state, a state with long-range correlation without lattice periodicity, which is characterized by highly diverse local structures, reminiscent of spin-glass.

Physical properties of liquids and glasses are controlled not only by the short-range order (SRO) in the nearest-neighbor atoms but also by the medium-range order (MRO) observed for atoms beyond the nearest neighbors. In this article the nature of the MRO as the descriptor of point-to-set atomic correlation is discussed focusing on simple liquids, such as metallic liquids. Through the results of x-ray diffraction and simulation with classical potentials we show that the third peak of the pair-distribution function, which describes the MRO, shows a distinct change in temperature dependence at the glass transition, whereas the first peak, which represents the SRO, changes smoothly through the glass transition. The result suggests that the glass transition is induced by the freezing of the MRO rather than that of the SRO, implying a major role of the MRO on the viscosity of supercooled liquid.

The spatial atomic correlations in liquids and glasses extend often significantly beyond the nearest neighbors. Such correlations, called the medium-range order (MRO), affect many physical properties, but their nature is not well understood. In this article the variation of the MRO with temperature is calculated based upon the concept of the atomic-level pressure, focusing on simple liquids, such as metallic liquids. It is shown that the structural coherence length that characterizes MRO follows the Curie-Weiss law with a negative Curie temperature as observed by experiment and simulation. It is also shown that the glass transition is induced by freezing of the MRO, rather than the freezing of the nearest-neighbor shell. The implications of these results are discussed.

We present molecular-simulation-based calculations of the Van Hove correlation function (VHF) of water using multiple modeling approaches: classical molecular dynamics with simple three-site nonpolarizable models, with a polarizable model, and with a reactive force field; density functional tight-binding molecular dynamics; and ab initio molecular dynamics. Due to the many orders of magnitude difference in the computational cost of these approaches, we investigate how small and short the simulations can be while still yielding sufficiently accurate and interpretable results for the VHF. We investigate the accuracy of the different models by comparing them to recently published inelastic X-ray scattering measurements of the VHF. We find that all of the models exhibit qualitative agreement with the experiments, and in some models and for some properties, the agreement is quantitative. This work lays the foundation for future simulation approaches to calculating the VHF for aqueous solutions in bulk and under nanoconfinement.

The peculiar characteristics of relaxors, viz., a frequency-dependent dielectric permittivity peak and good functional properties (dielectric, electromechanical, electrocaloric, etc.), are attributed to nanoscale regions with correlated dipoles, or polar nanoregions (PNRs). However, the exact nature of PNRs and their contribution to relaxor behavior remains debatable. In recent years, solid solutions of $\mathrm{BaTi}{\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Bi}\mathrm{Me}{\mathrm{O}}_{3}$ (where Me is a metal), have emerged as an interesting system with characteristics in between that of relaxors and dipole glasses. Here, we have examined the atomistic origins of weakly coupled relaxor behavior, specifically with regard to formation of PNRs, in Sn-doped $(1\ensuremath{-}x)(\mathrm{Ba},\mathrm{Ca})\mathrm{Ti}{\mathrm{O}}_{3}\text{\ensuremath{-}}x\mathrm{BiSc}{\mathrm{O}}_{3}$ using macroscopic polarization and neutron dynamic pair distribution function measurements. We show that the short-range atomic correlations observed within the PNRs dynamically fluctuate with frequencies of the order of THz. Furthermore the composition-dependent dielectric and polarization behaviors are critically influenced by the relative stability of the atomic correlations near \ensuremath{\sim}1 THz, while the instantaneous atomic correlations are largely independent of $x$. The current results are discussed based on a model of intrinsic local modes distributed in a dielectrically soft matrix.

Owing to marked advances in instrumentation in X-ray and neutron scattering the time-dependent pair correlation function, the Van Hove function, can now be determined by inelastic X-ray and neutron...

Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility., Designing complex concentrated alloys with targeted properties for high performance remains challenging because of their complex local atomic environments. Here, the authors show how to engineer atomic-level pressure to customize complexity-induced properties such as solid-solution strengthening.

A synthesis method of pure low valence nickelates using a custom built H2 circulation apparatus is described. Pure infinite layer LnNiO2 (Ln = La, Nd, Pr) and La3Ni2O6 nickelates have been successfully prepared using this method and characterized by x-ray diffraction. Resistivity of La3Ni2O6 was measured as a function of temperature and pressure up to ∼2 GPa and revealed significant pressure-induced changes in both magnitude and pressure dependence of resistivity. The existence of a hidden insulator-metal transition in La3Ni2O6 is proposed at pressures above 100 GPa.

Liquid fragility characterizes how steeply the viscosity of a glass-forming liquid decreases with increasing temperature above the glass transition. It is one of the most fundamental properties of a liquid, with high importance for science and application. Yet, its origin is unclear. Here we show that it is directly related to the structural coherence of the medium-range order (MRO) in liquid defined by the decay of the pair-distribution function with distance. The MRO can also be evaluated from the first peak of the structure function determined by x-ray or neutron diffraction, and it is a measure of the cooperativity of atomic motion in a diffusive event in supercooled liquids. These findings shed light on the mechanism of atomic transport in supercooled liquids.

We show that the self-part of the Van Hove function---the correlation function describing the dynamics of a single molecule---of water can be determined through a high-resolution inelastic x-ray scattering experiment. The measurement of inelastic x-ray scattering up to $10\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$ makes it possible to convert the inelastic x-ray scattering spectra into the Van Hove function, and its self-part is extracted from the short-range correlations. The diffusivity estimated from the short-range dynamics of water molecules is different from the long-range diffusivity measured by other methods. This approach using the experimentally determined self-part of the Van Hove function will be useful to the study of the local dynamics of atoms and molecules in liquids.

With their brilliance and temporal structure, X-ray free-electron laser can unveil atomic-scale details of ultrafast phenomena. Recent progress in split-and-delay optics (SDO), which produces two X-ray pulses with time-delays, offers bright prospects for observing dynamics at the atomic-scale. However, their insufficient pulse energy has limited its application either to phenomena with longer correlation length or to measurement with a fixed delay-time. Here we show that the combination of the SDO and self-seeding of X-rays increases the pulse energy and makes it possible to observe the atomic-scale dynamics in a timescale of picoseconds. We show that the speckle contrast in scattering from water depends on the delay-time as expected. Our results demonstrate the capability of measurement using the SDO with seeded X-rays for resolving the dynamics in temporal and spatial scales that are not accessible by other techniques, opening opportunities for studying the atomic-level dynamics., 22 pages, 5 figures

It is difficult to characterize by experiment the structural features of liquids and glasses which lack long-range translational periodicity in the structure. Here, we suggest that the height and shape of the first peak of the structure function $S(Q)$ carry significant information about the nature of the medium-range order and the coherence of density correlations. It is further proposed that they indicate how ideal the liquid structure is. Here, the ideal state is defined by long-range density correlations, not by structural coherence at the atomic level. The analysis is applied to the $S(Q)$ of metallic alloy liquids determined by x-ray diffraction and simulation. The ideality index defined here may provide a common parameter to characterize structural coherence among various disparate groups of liquids and glasses.

Electrolyte solutions are ubiquitous in materials in daily use and in biological systems. However, the understanding of their molecular and ionic dynamics, particularly those of their correlated motions, are elusive despite extensive experimental, theoretical, and numerical studies. Here we report the real-space observations of the molecular/ionic-correlated dynamics of aqueous salt (NaCl, NaBr, and NaI) solutions using the Van Hove functions obtained by high-resolution inelastic X-ray scattering measurement and molecular dynamics simulation. Our results directly depict the distance-dependent dynamics of aqueous salt solutions on the picosecond time scale and identify the changes in the anion-water correlations. This study demonstrates the capability of the real-space Van Hove function analysis to describe the local correlated dynamics in aqueous salt solutions.

The phase stability of high entropy alloy (HEA), Al 0.5 TiZrPdCuNi, under fast electron irradiation was studied by in-situ high voltage electron microscopy (HVEM). The initial phase of this alloy quenched from the melt was dependent on cooling rate. At high cooling rates an amorphous phase was obtained, whereas a body-centered cubic ( b.c.c. ) phase were obtained at low cooling rates. By thermal crystallization of the amorphous phase b.c.c. phase nano-crystals were formed. Upon fast electron irradiation solid state amorphization (SSA) was observed in b.c.c. phase regardless of the initial microstructure (i.e., “coarse crystalline structure” or “nano-crystalline structure with grain boundaries as a sink for point defects”). SSA behavior in the Al 0.5 TiZrPdCuNi HEAs was investigated by in-situ transmission electron microscopy observations. Because the amorphization is very rarely achieved in a solid solution phase under fast electron irradiation in common metallic materials, this result suggests that the Al 0.5 TiZrPdCuNi HEA from other common alloys and the other HEAs. The differences in phase stability against the irradiation between the Al 0.5 TiZrPdCuNi HEA and the other HEAs were discussed. This is the first experimental evidence of SSA in HEAs stimulated by fast electron irradiation.