Local Atomic Order Research Articles

Local atomic order and hierarchical polar nanoregions in a classical relaxor ferroelectric

The development of useful structure-function relationships for materials that exhibit correlated nanoscale disorder requires adequately large atomistic models which today are obtained mainly via theoretical simulations. Here, we exploit our recent advances in structure-refinement methodology to demonstrate how such models can be derived directly from simultaneous fitting of 3D diffuse- and total-scattering data, and we use this approach to elucidate the complex nanoscale atomic correlations in the classical relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN). Our results uncover details of ordering of Mg and Nb and reveal a hierarchical structure of polar nanoregions associated with the Pb and Nb displacements. The magnitudes of these displacements and their alignment vary smoothly across the nanoregion boundaries. No spatial correlations were found between the chemical ordering and the polar nanoregions. This work highlights a broadly applicable nanoscale structure-refinement method and provides insights into the structure of PMN that require rethinking its existing contentious models.

Heterogeneous structural changes correlated to local atomic order in thermal rejuvenation process of Cu-Zr metallic glass

ABSTRACT In this study, we investigated the atomistic mechanism of structural excitation in a thermal process (thermal rejuvenation) of metallic glass. In a molecular dynamics framework, Cu-Zr metallic glass was rejuvenated by a thermal process composed of an isothermal heat treatment at a temperature above the glass transition temperature , followed by fast cooling. Atomistic analyses of the local rearrangement, potential energy, and geometrical structure revealed structural changes correlating to the local atomic order in the rejuvenation process. In the early stage of the heat treatment for thermal rejuvenation, the structural excitation exhibited spatial heterogeneity at the nanometer scale. More-excited regions (i.e., regions with large atomic non-affine and affine transformations) exhibited low-ordered structures and vice versa, implying that the local structural excitation is significantly correlated with the local atomic order. The structural excitation transitioned from partial to whole as the isothermal process proceeded above . Although rejuvenation decreased the ordered structure, the calculation results suggested the formation of newly ordered local structures and newly disordered local structures correlated to local structural excitations and atomic dynamics in the thermal process. These results indicate that the heterogeneous structure evolution of the rejuvenation process induces a redistribution of the local atomic order in the microstructure of metallic glasses.

Tunable catalytic activity of cobalt-intercalated layered MnO2 for water oxidation through confinement and local ordering

The lowering of reaction overpotentials is a persistent and universal goal in the development of catalysts for (photo)electrochemistry, which can usually be facilitated by selectively stabilizing one reaction intermediate over another. In this mechanistic study of the oxygen evolution reaction (OER) catalyzed by cobalt-intercalated layered MnO2, we show that confinement effects and local cobalt atomic ordering in the interlayer space can be synergistically used to tune the adsorption energies of O, OH, and OOH reaction intermediates and the scaling relationship between them. In general, the interlayer confinement destabilizes the adsorption of intermediates for the OER, but clustering Co atoms can selectively stabilize the adsorption of OOH in particular. After considering both effects, our model predicts an overpotential of 0.30 V for the Co-intercalated MnO2 catalyzed OER, in excellent agreement with the experimental result of 0.36 V. These new insights explain the enhanced catalytic performance of MnO2 by intercalating atoms and illuminate a route for engineering non-toxic precious-metal-free catalysts through designed layered materials.

A Comparative Study of Experimental Configurations in Synchrotron Pair Distribution Function.

The identification and quantification of amorphous components and nanocrystalline phases with very small crystal sizes, smaller than ~3 nm, within samples containing crystalline phases is very challenging. However, this is important as there are several types of systems that contain these matrices: building materials, glass-ceramics, some alloys, etc. The total scattering synchrotron pair distribution function (PDF) can be used to characterize the local atomic order of the nanocrystalline components and to carry out quantitative analyses in complex mixtures. Although the resolution in momentum transfer space has been widely discussed, the resolution in the interatomic distance space has not been discussed to the best of our knowledge. Here, we report synchrotron PDF data collected at three beamlines in different experimental configurations and X-ray detectors. We not only discuss the effect of the resolution in Q-space, Qmax ins of the recorded data and Qmax of the processed data, but we also discuss the resolution in the interatomic distance (real) space. A thorough study of single-phase crystalline nickel used as standard was carried out. Then, selected cement-related samples including anhydrous tricalcium and dicalcium silicates, and pastes derived from the hydration of tricalcium silicate and ye’elimite with bassanite were analyzed.

Local-ordering mediated configuration stability and elastic properties of aluminum-containing high entropy alloys

Local atomic ordering has been demonstrated to be an effective approach to regulate the mechanical properties of aluminum-containing high-entropy alloys, but the underlying mechanism is not yet well understood. Taking the AlCoCrFeNi2.1 multi-component alloy as a prototype system, the effect of local ordering on its configuration stability and elastic properties were investigated from the aspects of lattice mismatch, modulus mismatch, and chemical bonding, through ab initio calculations. We highlighted that for the bcc phase at 0K, local ordering enhances configuration stability, while it has little influence on the elastic Young's modulus and shear modulus due to severe lattice distortion and elastic distortion in fully disordered configuration. Interestingly, with the increase of temperature, the structural stability and bulk modulus of the partially disordered configuration turns out to be higher than the fully disordered configuration, likely owing to the presence of strong chemical bonding between Al and X(Co, Cr, Fe, Ni). As for the fcc phase in the AlCoCrFeNi2.1 alloy, the degree of local ordering is low and thus results in little difference on the elastic properties between the partially disordered and fully disordered configurations. We believe that our results benefit to the understanding of stabilization mechanism and adjustable mechanical performance of multi-component alloys by considering local ordering.

Storage and transfer of optical excitation energy in GaInP epilayer: Photoluminescence signatures

GaInP alloy could be the most trusted key material for fabricating super-high-efficiency single- and multi-junction solar cells, especially for space applications. The storage and transfer of optical excitation energy in this key alloy is thus a key subject of the energy conversion from optical to electrical. In this article we present a study of the subject through investigating photoluminescence (PL) degradation in the GaInP epilayer at 4 K under the continuous optical excitations of ultraviolet (UV) 325 nm, visible 488.0 and 514.5 nm lasers. It is found that the decline of PL intensity with the irradiation time may be represented by I(t)/I0=1+tτ−1−1+C, where I0 is the luminescence intensity at the beginning of irradiation, τ a time constant, and C a background. Moreover, the PL degradation degree reduces with increasing the excitation wavelength. In addition, some red shift of the PL peak is observed accompanying with the intensity decline under the UV laser excitation. These PL signatures indicate that the localized carriers within the local atomic ordering domains play a major role in the storage and transfer of the excitation energy via photon recycling processes.

Synchrotron pair distribution function analyses of ye'elimite-based pastes

The study of nanocrystalline phase atomic ordering is intrinsically complicated. The presence of additional crystalline phase(s) exacerbates this challenge. Here, a synchrotron pair distribution function (PDF) is used to characterise the local atomic order of the nanocrystalline phases in ye'elimite-containing pastes. A multi r-range analysis approach has been used, where the 3–5 nm r-range allows the crystalline phase contents to be determined and the 0·16–3·5 nm is used to characterise the atomic ordering in the nanocrystalline components. Quantitative phase contents have been also derived from these analyses. Specifically, five stoichiometric ye'elimite pastes with variable anhydrite and water contents have been prepared. For the pastes without anhydrite addition, ettringite was not present and two calcium aluminium monosulfate phases were required for good PDF fits. In addition to crystalline calcium aluminium monosulfate (AFm) (c ≈ 2·86 nm), a nanocrystalline AFm phase with a significantly smaller c-unit cell parameter value, c ≈ 2·68 nm, was required, having about a 6 nm particle size. For a paste obtained with 31 wt% of anhydrite addition, the main crystalline phase was ettringite with the aluminium hydroxide having a nano-gibbsite local structure with quite a small average particle size, ≈3·5 nm.

Effects of cooling rate on the atomic structure and glass formation process ofCo90Zr10metallic glass investigated by molecular dynamics simulations

In this study, the atomic structure and the glass formation process of Co90 Zr10 metallic glass alloy were studied by molecular dynamic simulation based on the embedded atom method using four different cooling rates. The average atomic volume and the potential energy of the system were observed to be strongly dependent on the cooling rate during rapid solidification, and the glass transition temperature decreased with decreasing cooling rate. The radial distribution functions and the structure factors derived from molecular dynamics simulations at 300 K agreed well with the experimental and other molecular dynamics results. The coordination numbers calculated at 300 K were consistent with the experimental results in the literature. The local structural atomic orders of the system have been characterized by Honeycutt-Andersen indices and the Voronoi tessellation method. We realized that the FCC and icosahedral shortrange order increases with decreasing cooling rate, the icosahedral clusters are predominant in the Co90 Zr10 metallic glass, and the fraction of the icosahedral polyhedra increases with decreasing cooling rate and temperature. We have seen that Co-centered Voronoi polyhedrals such as , , and play a dominant role in the development of the icosahedral order of the Co90 Zr10 alloy.

Local atomic order of the amorphous TaO x thin films in relation to their chemical resistivity.

The experimental and theoretical studies of the local atomic order and chemical binding in tantalum oxide amorphous films are presented. The experimental studies were performed on thin films deposited at the temperature of 100 °C by atomic layer deposition on silicon (100) and glass substrates. Thin films of amorphous tantalum oxide are known to exhibit an extremely large extent of oxygen nonstoichiometry. Performed X-ray absorption and photoelectron studies indicated the oxygen over-stoichiometric composition in the considered films. Surplus oxygen atoms have 1s electron level with binding energy about 1 eV higher than these in reference Ta2O5 oxide. The density functional theory was applied to find the possible location of additional oxygen atoms. Performed calculation indicated that additional atoms may form the dumbbell defects, which accumulate the dangling oxygen bonds in orthorhombic structure and lead to increase of oxygen 1s level binding energy. The presence of this kind of oxygen–oxygen bonding may be responsible for increase of amorphous film chemical resistivity which is very important in many applications.

Graphitization of 13C enriched fine-grained graphitic material under high-pressure annealing

Hot pressing is one of the promising routes for graphitization of carbon materials. In the present work, high-pressure high-temperature behaviour of 13C-enriched fine-grained graphitic material has been studied by using a set of experimental tools such as scanning electron microscopy, Raman scattering, X-ray diffraction, X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies, and solid-state nuclear magnetic resonance (NMR) spectroscopy. The material has been annealed at a temperature of 1200 or 1500 °C using a stabilizing pressure of 5 GPa. It shown, that addition of small amount of water during the annealing at 1200 °C results in a better local atomic ordering of carbon material and increases the hydrocarbon fraction. Well-ordered graphite crystals enriched with 13C isotope are formed on the surface of nickel ampule at 1500 °C. An increase in processing time from 2 to 15 h decreases the defect density and improves the graphitization level of the material.

Thermally Induced Anomaly in the Shear Behavior of Magnetite at High Pressure

Understanding the crystal structure, phase stability, and elasticity of magnetite (Fe${}_{3}$O${}_{4}$) at high pressure and temperature will explain the behavior of the Fe-O system deep within the Earth, and promote applications and engineering of Fe-O under extreme conditions. Using ultrasonic interferometry plus $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ x-ray techniques, the authors discover a temperature-driven anomaly in magnetite's shear behavior and a pressure-induced softening of its shear modulus, due to local atomic distortions and charge ordering of Fe cations at the octahedral sites in the inverse-spinel structure. This work will interest researchers in physics, chemistry, materials science, and geoscience.

Precise implications for real-space pair distribution function modeling of effects intrinsic to modern time-of-flight neutron diffractometers.

Total scattering and pair distribution function (PDF) methods allow for detailed study of local atomic order and disorder, including materials for which Rietveld refinements are not traditionally possible (amorphous materials, liquids, glasses and nanoparticles). With the advent of modern neutron time-of-flight (TOF) instrumentation, total scattering studies are capable of producing PDFs with ranges upwards of 100-200 Å, covering the correlation length scales of interest for many materials under study. Despite this, the refinement and subsequent analysis of data are often limited by confounding factors that are not rigorously accounted for in conventional analysis programs. While many of these artifacts are known and recognized by experts in the field, their effects and any associated mitigation strategies largely exist as passed-down `tribal' knowledge in the community, and have not been concisely demonstrated and compared in a unified presentation. This article aims to explicitly demonstrate, through reviews of previous literature, simulated analysis and real-world case studies, the effects of resolution, binning, bounds, peak shape, peak asymmetry, inconsistent conversion of TOF to d spacing and merging of multiple banks in neutron TOF data as they directly relate to real-space PDF analysis. Suggestions for best practice in analysis of data from modern neutron TOF total scattering instruments when using conventional analysis programs are made, as well as recommendations for improved analysis methods and future instrument design.

Local atomic order of a metallic glass made visible by scanning tunneling microscopy

Exploring the atomic level structure in amorphous materials by STM becomes extremely difficult due to the localized electronic states. Here we carried out STM studies on a quasi-low-dimensional film of metallic glass Zr65Cu27.5Al7.5 which is ‘ultrathin’ compared with the localization length and/or the length scale of short range order. The local electronic structure must appear more inherent, having states at Ef available for tip-sample tunneling current. To enhance imaging contrasts between long-range and short-range orders, the highly oriented pyrolytic graphite was chosen as substrate, so that the structural heterogeneity arising from competition between the glass former ability and the epitaxy can be ascertained. A chemical order predicted for this system was observed in atomic ordered regimes (1–2 monolayers), accompanied with a superstructure with the period Zr–Cu(Al)–Zr along three hexagonal axes. The result implies a chemical short range order in disordered regimes, where polyhedral clusters are dominant with the solute atom Cu(Al) in the center. An attempt for the structural modelling was made based on high resolution STM images, giving icosahedral order on the surface and different Voronoi clusters in 3D space.

Impact of Stoichiometry on the Structure of van der Waals Layered GeTe/Sb2 Te3 Superlattices Used in Interfacial Phase-Change Memory (iPCM) Devices.

Van der Waals layered GeTe/Sb2 Te3 superlattices (SLs) have demonstrated outstanding performances for use in resistive memories in so-called interfacial phase-change memory (iPCM) devices. GeTe/Sb2 Te3 SLs are made by periodically stacking ultrathin GeTe and Sb2 Te3 crystalline layers. The mechanism of the resistance change in iPCM devices is still highly debated. Recent experimental studies on SLs grown by molecular beam epitaxy or pulsed laser deposition indicate that the local structure does not correspond to any of the previously proposed structural models. Here, a new insight is given into the complex structure of prototypical GeTe/Sb2 Te3 SLs deposited by magnetron sputtering, which is the used industrial technique for SL growth in iPCM devices. X-ray diffraction analysis shows that the structural quality of the SL depends critically on its stoichiometry. Moreover, high-angle annular dark-field-scanning transmission electron microscopy analysis of the local atomic order in a perfectly stoichiometric SL reveals the absence of GeTe layers, and that Ge atoms intermix with Sb atoms in, for instance, Ge2 Sb2 Te5 blocks. This result shows that an alternative structural model is required to explain the origin of the electrical contrast and the nature of the resistive switching mechanism observed in iPCM devices.

Advanced characterization technique for mechanochemically synthesized materials: neutron total scattering analysis

Materials that adopt the pyrochlore (A2B2O7) structure show promise for use in a variety of energy-related applications such as immobilization of actinide-rich nuclear waste and oxide fuel cells. Mechanochemical synthesis, a combination of milling and high-temperature treatment, has been successfully applied to fabricate many different pyrochlore compositions. High-resolution neutron total scattering experiments were used to gain fundamental insight into the structural details of milled Er2Ti2O7 pyrochlore and the subsequent evolution under high-temperature treatment. The milling process creates a highly disordered structure in which local atomic ordering is present that is significantly different than the observed long-range behavior. Thermal annealing leads to a complex defect recovery scheme with a gradual local rearrangement from a weberite-type atomic ordering to a pyrochlore phase independent of the sharp long-range crystallization process. Annealing of the milled sample up to 1200 °C does not reproduce the local structure of the same pyrochlore sample prepared by solid-state synthesis. This indicates that despite both samples possessing identical long-range structures, local defects induced by the milling process persist to very high temperatures. These findings provide a direct insight into the mechanochemical synthesis of pyrochlore oxides and help to better elucidate the structural properties of highly disordered complex oxides under extreme conditions from the local atomic arrangement to the macroscale.

Synchrotron Radiation Pair Distribution Function Analysis of Gels in Cements

The analysis of atomic ordering in a nanocrystalline phase with small particle sizes, below 5 nm, is intrinsically complicated because of the lack of long-range order. Furthermore, the presence of additional crystalline phase(s) may exacerbate the problem, as is the case in cement pastes. Here, we use the synchrotron pair distribution function (PDF) chiefly to characterize the local atomic order of the nanocrystalline phases, gels, in cement pastes. We have used a multi r-range analysis approach, where the ~4–7 nm r-range allows determining the crystalline phase contents; the ~1–2.5 nm r-range is used to characterize the atomic ordering in the nanocrystalline component; and the ~0.2–1.0 nm r-range gives insights about additional amorphous components. Specifically, we have prepared four alite pastes with variable water contents, and the analyses showed that a defective tobermorite, Ca11Si9O28(OH)2.8.5H2O, gave the best fit. Furthermore, the PDF analyses suggest that the calcium silicate hydrate gel is composed of this tobermorite and amorphous calcium hydroxide. Finally, this approach has been used to study alternative cements. The hydration of monocalcium aluminate and ye’elimite pastes yield aluminum hydroxide gels. PDF analyses show that these gels are constituted of nanocrystalline gibbsite, and the particle size can be as small as 2.5 nm.

EELS at very high energy losses.

Electron energy-loss spectroscopy (EELS) has been investigated in the range from 2 to >10 keV using an optimized optical coupling of the microscope to the spectrometer to improve the high loss performance in EELS. It is found that excellent quality data can now be acquired up until about 5 keV, suitable for both energy loss near edge structure (ELNES) studies of oxidation and local chemistry, and potentially useful for extended energy loss fine structure (EXELFS) studies of local atomic ordering. Examples studied included oxidation in Zr, Mo and Sn, and the ELNES and EXELFS of the Ti-K edge. It is also shown that good quality electron energy-loss spectroscopy can even be performed for losses above 9.2 keV, the energy loss at which the collection angle becomes 'infinite', and this is demonstrated using the tungsten L3 edge at about 10.2 keV.

X-ray absorption fine structure and X-ray photoelectron spectroscopy studies of nanocomposite systems based on ZnS:Cu deposited into porous anodic Al2O3 matrices

The results obtained via extended X-ray absorption fine structure and X-ray photoelectron spectroscopy studies of nanocomposite systems based on ZnS:Cu (5 at %) deposited into porous anodic alumina (AA) matrices, a promising material for electroluminescent light sources, are presented. The given results are compared with those corresponding to ZnS:Cu films on a smooth SiO2 surface. To implement the deposition process, we have pioneered the use of s method based on the thermal sputtering of ZnS and Cu powders mixed in a specified mass ratio. For the first time, ZnS:Cu single-crystal nanostructures are demonstrated to be formed in AA matrix pores. In this case, their size is assigned by the pore diameter and they possess better composition stoichiometry and better local ordering of atoms in the immediate environment of zinc.

The relationship between thermodynamic properties and local atomic structure of Al-TM (TM = Mn, Fe, Co, Ni, Cu) melts

ABSTRACTThe relationship between structure and thermodynamic properties of Al- transition metal (TM) (TM = Mn, Fe, Co, Ni, Cu) melts had been studied by using the results of X-ray diffraction experiments, reverse Monte Carlo simulations and the model of ideal associated solution (MIAS). It was considered that local atomic structure of melts is determined by dense non-crystalline atomic packing and energy of interatomic interactions. This allowed to choose the types of associates for the MIAS. It is shown that the prominent role of the atomic packing factor in the Al-Mn, Al-Fe melts leads to correlation between the local atomic ordering of melts and the one of relevant polytetrahedral phases and defines a set of associates. Strengthening of heteroatomic interactions in the Al-Co, Al-Ni melts leads to an increase of the energy factor role in determining their structure and properties that reduces the number of associates from three for the Al-Mn melts to one for the Al-Ni.

Orientational order of liquids and glasses via fluctuation diffraction.

Liquids, glasses and other amorphous matter lack long-range order, which makes them notoriously difficult to study. Local atomic order is partially revealed by measuring the distribution of pairwise atomic distances, but this measurement is insensitive to orientational order and unable to provide a complete picture of diverse amorphous phenomena, such as supercooling and the glass transition. Fluctuation scattering with electrons and X-rays is able provide this orientational sensitivity, but it is difficult to obtain clear structural interpretations of fluctuation data. Here we show that the interpretation of fluctuation diffraction data can be simplified by converting it into a real-space angular distribution function. We calculate this function from simulated diffraction of amorphous nickel, generated with a classical molecular dynamics simulation of the quenching of a high temperature liquid state. We compare the results of the amorphous case to the initial liquid state and to the ideal f.c.c. lattice structure of nickel. We show that the extracted angular distributions are rich in information about orientational order and bond angles. The diffraction fluctuations are potentially measurable with electron sources and also with the brightest X-ray sources, like X-ray free-electron lasers.