Short-range Order Parameters Research Articles

Thermal Quenching of Intrinsic Photoluminescence in Amorphous and Monoclinic HfO2 Nanotubes

Nanotubular hafnia arrays hold significant promise for advanced opto- and nanoelectronic applications. However, the known studies concern mostly the luminescent properties of doped HfO2-based nanostructures, while the optical properties of nominally pure hafnia with optically active centers of intrinsic origin are far from being sufficiently investigated. In this work, for the first time we have conducted research on the wide-range temperature effects in the photoluminescence processes of anion-defective hafnia nanotubes with an amorphous and monoclinic structure, synthesized by the electrochemical oxidation method. It is shown that the spectral parameters, such as the position of the maximum and half-width of the band, remain almost unchanged in the range of 7–296 K. The experimental data obtained for the photoluminescence temperature quenching are quantitatively analyzed under the assumption made for two independent channels of non-radiative relaxation of excitations with calculating the appropriate energies of activation barriers—9 and 39 meV for amorphous hafnia nanotubes, 15 and 141 meV for monoclinic ones. The similar temperature behavior of photoluminescence spectra indicates close values of short-range order parameters in the local atomic surrounding of the active emission centers in hafnium dioxide with amorphous and monoclinic structure. Anion vacancies VO− and VO2− appeared in the positions of three-coordinated oxygen and could be the main contributors to the spectral features of emission response and observed thermally stimulated processes. The recognized and clarified mechanisms occurring during thermal quenching of photoluminescence could be useful for the development of light-emitting devices and thermo-optical sensors with functional media based on oxygen-deficient hafnia nanotubes.

Alloying and Segregation in PdRe/Al2O3 Bimetallic Catalysts for Selective Hydrogenation of Furfural

X-ray absorption fine structure (XAFS) spectroscopy, temperature-programmed reduction (TPR), and temperature-programmed hydride decomposition (TPHD) were employed to elucidate the structures of a series of PdRe/Al2O3 bimetallic catalysts for the selective hydrogenation of furfural. TPR evidenced low-temperature Re reduction in the bimetallic catalysts consistent of the migration of [ReO4]− (perrhenate) species to hydrogen-covered Pd nanoparticles on highly hydroxylated γ-Al2O3. TPHD revealed a strong suppression of β-PdHx formation in the reduced catalysts prepared by (i) co-impregnation and (ii) [HReO4] impregnation of the reduced Pd/Al2O3, indicating the formation of Pd-rich alloy nanoparticles; however, reduced catalysts prepared by (iii) [Pd(NH3)4]2+ impregnation of calcined Re/Al2O3 and subsequent re-calcination did not. Re LIII X-ray absorption edge shifts were used to determine the average Re oxidation states after reduction at 400 °C. XAFS spectroscopy and high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) revealed that a reduced 5 wt.% Re/Al2O3 catalyst contained small Re clusters and nanoparticles comprising Re atoms in low positive oxidation states (~1.5+) and incompletely reduced Re species (primarily Re4+). XAFS spectroscopy of the bimetallic catalysts evidenced Pd-Re bonding consistent with Pd-rich alloy formation. The Pd and Re total first-shell coordination numbers suggest that either Re is segregated to the surface (and Pd to the core) of alloy nanoparticles and/or segregated Pd nanoparticles are larger than Re nanoparticles (or clusters). The Cowley short-range order parameters are strongly positive indicating a high degree of heterogeneity (clustering or segregation of metal atoms) in these bimetallic catalysts. Catalysts prepared using the Pd(NH3)4[ReO4]2 double complex salt (DCS) exhibit greater Pd-Re intermixing but remain heterogeneous on the atomic scale.

Tuning chemical short-range order for stainless behavior at reduced chromium concentrations in multi-principal element alloys

Single-phase multi-principal element alloys hold promise for improved mechanical properties as a result of multiple operative deformation modes. However, the use of many of these alloys in structural applications is limited as a consequence of their poor aqueous corrosion resistance. Here we introduce a new approach for significantly improving the passivation behavior of alloys by tuning the chemical short-range order parameter. We show that the addition of only 0.03 to 0.06 mole fraction of Al to a (FeCoNi)0.9Cr0.1 alloy changed both the magnitude and sign of the Cr-Cr order parameter resulting in passivation behavior similar to 304L stainless steel containing twice the amount of Cr. Our analysis is based on comparing electrochemical measures of the kinetics of passive film formation with chemical short-range order characterizations using time-of-flight neutron scattering, cluster expansion methods, density functional theory and Monte Carlo techniques. Our findings are interpreted within the framework of a recently proposed percolation theory of passivation that examines how selective dissolution of the non-passivating alloy components and short-range order results in excellent passive films at reduced levels of the passivating component.

Thermodynamic, Structural and Surface Properties of Liquid Al-Sr Alloy at Different Temperatures

Thermodynamic functions like excess Gibbs free energy of mixing (∆GxsM) and activity (a) of liquid Al-Sr alloy were studied in the temperature range 1323-1623 K in the frame work of Redlich-Kister (R-K) polynomial. The compositional and temperature dependence of structural functions like concentration fluctuation in long wavelength limit (Scc(0)), Warren-Cowley short range order parameter (α) and ratio of mutual to intrinsic diffusion coefficients (Dm/Did) were computed and analysed using the same approach. The surface tension (σ) and surface segregation (xsi) tendencies of the atoms in the liquid mixture were computed and studied using Butler’s model. Present investigations revealed that association tendency among the atoms of metallic mixture gradually decreased with increase in temperature.

Thermodynamic, structural, surface and transport properties of Au-Cu melt

Thermodynamic, structural, surface and transport properties of Au-Cu liquid alloy were calculated on the basis of different theoretical modeling equations. The thermodynamic properties, such as excess Gibbs free energy of mixing, enthapy of mixing and activity were estimated and compared with the available experimental and literature data on the basis of simple theory of mixing (STM). The best fit values of model parameters were estimated using the experimental values of excess Gibbs free energy of mixing and enthalpy of mixing of the system at 1550 K. Using the same model parameters and frame, the concentration fluctuation in long wave-length limit and Warren-Cowley short range order parameter were calculated and analysed to understand the local arrangement of atoms in the liquid mixture. The surface tension and extent of surface segregation of atoms in the initial melt were computed using Butler’s model. In transport properties, the ratio of mutual to intrinsic diffusion coefficients was calculated using STM and viscosity was calculated using Kaptay equation. The system showed complete ordering tendency at its melting temperature.

Unraveling the collinearity in short-range order parameters for lattice configurations arising from topological constraints.

In multicomponent lattice problems, for example, in alloys and at crystalline surfaces and interfaces, atomic arrangements exhibit spatial correlations that dictate the kinetic and thermodynamic phase behavior. These correlations emerge from interparticle interactions and are frequently reported in terms of the short-range order (SRO) parameter. Expressed usually in terms of pair distributions and other cluster probabilities, the SRO parameter gives the likelihood of finding atoms/molecules of a particular type in the vicinity of other atoms. This study focuses on fundamental constraints involving the SRO parameters that are imposed by the underlying lattice topology. Using a data-driven approach, we uncover the interrelationships between different SRO parameters (e.g., pairs, triplets, and quadruplets) on a lattice. The main finding is that while some SRO parameters are independent, the remaining are collinear, i.e., the latter are dictated by the independent ones through linear relationships. A kinetic and thermodynamic modeling framework based on these constraints is introduced.

Defects Act in an “Introverted” Manner in FeNiCrCoCu High-Entropy Alloy under Primary Damage

High-entropy alloys (HEAs) attract much attention as possible radiation-resistant materials due to their several unique properties. In this work, the generation and evolution of the radiation damage response of an FeNiCrCoCu HEA and bulk Ni in the early stages were explored using molecular dynamics (MD). The design, concerned with investigating the irradiation tolerance of the FeNiCrCoCu HEA, encompassed the following: (1) The FeNiCrCoCu HEA structure was obtained through a hybrid method that combined Monte Carlo (MC) and MD vs. the random distribution of atoms. (2) Displacement cascades caused by different primary knock-on atom (PKA) energy levels (500 to 5000 eV) of the FeNiCrCoCu HEA vs. bulk Ni were simulated. There was almost no element segregation in bulk FeNiCrCoCu obtained with the MD/MC method by analyzing the Warren–Cowley short-range order (SRO) parameters. In this case, the atom distribution was similar to the random structure that was selected as a substrate to conduct the damage cascade process. A mass of defects (interstitials and vacancies) was generated primarily by PKA departure. The number of adatoms grew, which slightly roughened the surface, and the defects were distributed deeper as the PKA energy increased for both pure Ni and the FeNiCrCoCu HEA. At the time of thermal spike, one fascinating phenomenon occurred where the number of Frenkel pairs for HEA was more than that for pure Ni. However, we obtained the opposite result, that fewer Frenkel pairs survived in the HEA than in pure Ni in the final state of the damage cascade. The number and size of defect clusters grew with increasing PKA energy levels for both materials. Defects were suppressed in the HEA; that is to say, defects were “cowards”, behaving in an introverted manner according to the anthropomorphic rhetorical method.

Short-range ordering mechanics in FCC materials

Short-range order (SRO) has a crucial impact on the mechanical strength of metallic alloys. Recent atomistic investigations defined an average SRO and attempted to correlate it with the yield strength. We propose that the local change in SRO upon slip advance must dictate the strengthening, and we elaborate the methodology to establish the “SRO change” on a slip plane considering the Wigner-Seitz cell. The model captures the variation of lattice resistance (Critical Resolved Shear Stress; CRSS) in the crystal as the SRO changes depending on the probability of neighboring atoms. The methodology was applied to Ni-V binary alloys for a wide range of compositions and stacking fault widths. Dislocation core widths were determined as a function of SRO and energy parameters (unstable and intrinsic stacking fault energies; γus, γisf). The complex variation of CRSS with compositional variations shows good agreement with limited experimental results. The compositions corresponding to the transition from partial to full dislocations at higher vanadium contents are found depending on the SRO and the intrinsic energy levels.

Refining short-range order parameters from the three-dimensional diffuse scattering in single-crystal electron diffraction data.

Our study compares short-range order parameters refined from the diffuse scattering in single-crystal X-ray and single-crystal electron diffraction data. Nb0.84CoSb was chosen as a reference material. The correlations between neighbouring vacancies and the displacements of Sb and Co atoms were refined from the diffuse scattering using a Monte Carlo refinement in DISCUS. The difference between the Sb and Co displacements refined from the diffuse scattering and the Sb and Co displacements refined from the Bragg reflections in single-crystal X-ray diffraction data is 0.012 (7) Å for the refinement on diffuse scattering in single-crystal X-ray diffraction data and 0.03 (2) Å for the refinement on the diffuse scattering in single-crystal electron diffraction data. As electron diffraction requires much smaller crystals than X-ray diffraction, this opens up the possibility of refining short-range order parameters in many technologically relevant materials for which no crystals large enough for single-crystal X-ray diffraction are available.

Influence of long- and short-range chemical order on spontaneous magnetization in single-crystalline Fe0.6Al0.4 compound thin films

We systematically investigate the long- and short-range chemical order, lattice volume, and spontaneous magnetization in single-crystalline Fe0.6Al0.4 compound thin films. The vapor-quenching method based on a molecular beam epitaxy technique is utilized to fabricate the single-crystalline Fe0.6Al0.4 compound with the different B2 long-range order parameter S. S was varied by the deposition temperature T d, and it increases with increasing T d. The lattice volume V decreased with increasing T d, while the tetragonal distortion, ∼4%, due to epitaxial strain were observed. The changes in S and V were accompanied with the change in the magnetic moment per Fe, μ Fe. μ Fe showed the monotonic decrease as a function of S whereas μ Fe monotonically increases with V. With considering tetragonal distortion, μ Fe–V relationship has a good agreement with the previous reports. The μ Fe–S relationship showed the steep decrease of μ Fe around S∼ 0.6. In contrast to μ Fe–V relationship, μ Fe–S relationship does not match only from ours to previous studies but also among other reports. It implies the statistical number of the nearest-neighbor Fe–Fe bonds, i.e. S, cannot be an enough explanatory parameter. To clarify the structural origin of change in μ Fe, the short-range order (SRO) parameter inferred from the analysis of superlattice diffractions were introduced. They showed the clear difference for the films with high and low μ Fe. The results suggest that the transition from the long- to the SRO state plays the significant role on μ Fe.

ХАЛЬКОГЕНІДНІ СТЕКЛА: СТРУКТУРНІ І ОПТИЧНІ ВЛАСТИВОСТІ (ОГЛЯД)

Structural properties of chalcogenide glasses mainly on the example of binary As-S(Se) and Ge-S(Se) systems and ternary Ge-As-S(Se) systems, structural models, parameters of short range order of glasses obtained using diffraction methods, EXAFS and Raman spectroscopy are considered. Raman spectra of binary As-S(Se) and Ge-S(Se) systems and ternary Ge-As-S(Se) systems, structural models that are used for interpretation of Raman spectroscopy results are considered. Optical properties of chalcogenide glasses and optical absorption edge in binary and multicomponent systems are discussed. The refractive index and its wavelength dependence, other optical properties are among important parameters that determine the suitability of materials as optical media. Refractive and absorption indexes, optical band gap of chalcogenide glasses can be changed by doping of different elements. The results suggest a combined effect of chemical ordering and topological in such glasses (parameters dependence on average coordination number, composition, nanophase separation, etc.). Importance of study of interrelation of structural and physico- chemical properties is stated. As frequently pointed out by various researchers, chalcogenide glasses are promising materials for various applications because they are transparent over a wide range of wavelengths in the infrared region, they possess high linear and non-linear refractive indices, number of photoinduced effects, low phonon energies and are easy to fabricate. Applications of chalcogenide glasses cover wide range, among them: IR optics, recording and storage of information, xerography, thermoplastic and holographic media, inorganic resists, optical filters, diffraction optical elements, non-linear elements, fiber and integrated optics, etc. Composition-structure-properties correlations are convenient to tailor the physical, optical and other properties of chalcogenide glasses and provide an important reference for the further development of new chalcogenide glasses taking into account their possible applications.

Probing the Behavior of Composition‐Tunable Ultrathin PtNi Nanowires for CO Oxidation and Small‐Molecule Electrocatalytic Reactions

AbstractWe have successfully synthesized ultrathin nanowires of pure Pt, Pt99Ni1, Pt9Ni1, and Pt7Ni3 using a modified room‐temperature soft‐template method. Analysis of both methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) results found that the Pt7Ni3 samples yielded the best performance with specific activities of 0.36 and 0.34 mA/cm2 respectively. Additionally, formic acid oxidation reaction (FAOR) tests noted that both Pt and PtNi nanowires oxidize small organic molecules (SOMs) via an indirect pathway. CO oxidation data suggests little measurable performance without any pre‐reduction treatment; however, after annealing in H2, we detected significantly improved CO2 formation for both Pt9Ni1 and Pt7Ni3 motifs. These observations highlight the importance of pre‐treating these nanowires under a reducing atmosphere to enhance their performance for CO oxidation. To explain these findings, we collected extended x‐ray adsorption fine structure (EXAFS) spectroscopy data, consistent with the presence of partial alloying with a tendency for Pt and Ni to segregate, thereby implying the formation of a Pt‐rich shell coupled with a Ni‐rich core. We also observed that the degree of alloying within the nanowires increased after annealing in a reducing atmosphere, a finding deduced through analysis of the coordination numbers and calculations of Cowley's short range order parameters.

Solute-strengthening in metal alloys with short-range order

Recent surging interest in strengthening of High Entropy Alloys (HEAs) with possible chemical ordering motivates the development of new theory. Here, an existing theory for random alloys that accounts for solute-dislocation and solute–solute interactions is extended to include strengthening due to short-range order (SRO). Closed form expressions are presented for the yield strength and energy barrier of dislocation motion in alloys with SRO based on inputs of atomic misfit volumes, average lattice and elastic constants, the SRO parameters, and effective pair interactions between solutes. The theory shows both the long-established athermal strengthening effect of SRO as well as a notable effect of SRO on the misfit volume strengthening, which can increase or decrease strength. The generalized solute-strengthening theory is the most comprehensive to date that is applicable to macroscopically homogeneous single-phase alloys using inputs that can be measured, computed, or estimated.

Thermodynamics of compound forming CdHg liquid alloys at 600 K

The quasi-lattice model has been applied to analyze the alloying behavior of the compound forming CdHg liquid alloys at 600 K. In solid state, CdHg liquid alloys exhibit the existence of Cd2Hg and CdHg2 complexes. In present study, it is considered that Cd2Hg complex also exists in liquid state near the melting temperature. The investigative equations for the thermodynamic and structural functions have been derived for a binary molten alloy having the existence of A2B complex where A and B are constituent elements. The theoretical data for the excess Gibbs free energy of mixing, Gibbs free energy of mixing, thermodynamic activity, entropy of mixing, enthalpy (i.e. heat) of mixing, concentration fluctuations and short-range order (SRO) parameter are compared with the corresponding experimental values. A well agreement have been observed between the theory and experiment. The present study reveals that CdHg system at 600 K in molten state is a weakly interacting ordered system. Further, the order energy parameters are temperature dependent.

Ising nematic in <i>J</i><sub>1</sub>–<i>J</i><sub>2</sub> square-lattice Heisenberg model within self-consistent spin-wave theory

We study the Ising nematic ordering, the Ising nematic phase transition, and the temperature behavior of the short-range order parameters in spin 1/2 quantum Heisenberg model on two-dimensional square-lattice with exchange parameters of nearest and next-nearest neighbors. Two versions of the self-consistent spin-wave theory with and without auxiliary pseudofermions are considered.

Atomistic simulations reveal strength reductions due to short-range order in alloys

A theory for strengthening for multicomponent non-dilute alloys possessing short-range order (SRO) has recently been developed. The theory predicts that, in addition to a well-known athermal strengthening, there is a notable effect of SRO on the solute–dislocation interactions that can decrease or increase the strength relative to a random alloy. Here, carefully designed atomistic simulations in a model binary NbW alloy are used to demonstrate that alloy strength due to solute–dislocation interactions can be increased or decreased, depending on the SRO and consistent with theoretical predictions. Specifically, SRO is introduced using fictitious solute–solute interactions in an alloy system with very small true solute–solute interactions, and the Nudged Elastic Band (NEB) method is then used to compute the energy barriers for edge dislocation motion for various levels of SRO. Energy barriers, and hence alloy strengths, can be decreased when the Warren-Cowley SRO parameters are negative (attraction of unlike solutes). The theoretical predictions for the same system are in reasonable quantitative agreement with the simulation results. These findings demonstrate the unexpected possibility of reduced strength due to SRO and also further validate the analytical theory as a tool to guide alloy design.

Thermodynamic calculations using reverse Monte Carlo: Simultaneously tuning multiple short-range order parameters for 2D lattice adsorption problem.

Lattice simulations are an important class of problems in crystalline solids, surface science, alloys, adsorption, absorption, separation, catalysis, to name a few. We describe a fast computational method for performing lattice thermodynamic calculations that is based on the use of the reverse Monte Carlo (RMC) technique and multiple short-range order (SRO) parameters. The approach is comparable in accuracy to the Metropolis Monte Carlo (MC) method. The equilibrium configuration is determined in 5-10 Newton-Raphson iterations by solving a system of coupled nonlinear algebraic SRO growth rate equations. This makes the RMC-based method computationally more efficient than MC, given that MC typically requires sampling of millions of configurations. The technique is applied to the interacting 2D adsorption problem. Unlike grand canonical MC, RMC is found to be adept at tackling geometric frustration, as it is able to quickly and correctly provide the ordered c(2 × 2) adlayer configuration for Cl adsorbed on a Cu (100) surface.

Diffusion, atomic transport, and ordering in Al-Zr alloys: FCC and liquid phases

Additive manufacturing of materials with controlled microstructure demands knowledge of atomic scale properties near the solid-liquid transition state. Many of these properties are not affordable by experimental techniques and computer modeling is the possible solution to the problem. In this paper, we present the results of an extended atomistic study of intrinsic atomic transport due to vacancy diffusion in FCC and L12 solid phases and diffusion in the liquid phase of Al-Zr alloys. A deceleration of the overall self-diffusion was observed when Zr was added to Al. The effect was stronger in the solid and weaker in the liquid. Additionally, the effect was strongly temperature dependent in the solid phases, but not in the liquid. Atomic transport was chemically biased: transport of Zr atoms was significantly slower than that of Al atoms, and this bias effect was stronger in the solid phases. The overall diffusion and chemical ordering processes in the liquid state were five to six orders in magnitude faster than in the solid. Chemical short-range order parameters in the liquid saturated at values close to those in the ordered L12 structure of Al3Zr. Chemical and structural ordering in the solid phases was negligible over the modeled microsecond time scale. The results are discussed in view of optimizing additive manufacturing parameters for the controlled formation of metastable L12 precipitates.

Study of correlations between glass-transition temperature and local structure of Ge-As-Se, Ge-As-Se-S chalcogenide glasses

ABSTRACT The local structure and glass transition of Ge-As-Se, Ge-As-Se-Schalcogenide glasses have been studied by X-ray diffraction and differential scanning calorimetry. By applying the theory of topological constraints and considering chemically ordered network models, it has been determined that Ge10As20S10Se60 (average coordination number = 2.4), Ge25As10S25Se40 ( = 2.6) are the compositions associated with the more stable glasses. This result is related to the larger differences between the glass transition (Tg ) and crystallization-transition (Tcr ) temperatures (ΔTg-cr ∼ 38 and 43 K) at these compositions. Relatively high values of short-range (d∼5.7–6.34 Å) and medium-range (L∼26.59–33.74 Å) order parameters of the local structure in the glass compositions satisfying the mentioned conditions affect the glass-transition temperature (Tg ) and cause its value to increase (Tg ∼589–689 K). As a result of comparative analyses of X-ray diffraction and differential scanning calorimetry curves it has been determined that there is a correlation between the glass-transition temperature (Tg ) with the short range (d) and medium range order (L) parameters of the local structure.

Combined experiments and atomistic simulations for understanding the effect of ZnO on the sodium silicate and sodium borosilicate glass network

Combined experiments and molecular dynamics (MD) simulations were performed over wide range of glass compositions to understand the improved durability of borosilicate glass with ZnO addition. A significant change in glass structure was monitored from short range and intermediate range order parameters. The MD results were found to be in good agreement with the experimental data for Young's Modulus, glass transition temperature, and leaching. Both experiments and MD simulations report the enhanced chemical durability of glass with ZnO addition. Low R (Na2O/B2O3) and high K (SiO2/B2O3) of ZnO doped sodium borosilicate (Zn–NBS) glass surface compared to bare NBS represents the more stable structure of glass surface for Zn–NBS than NBS. The enhanced chemical resistivity of Zn–NBS was established from the increasing activation energy for diffusion of Na ions. The combined experiments and MD simulations disclose many interesting microstructure and dynamics due to the presence of ZnO in the glass.