Average Local Structure Research Articles

X-ray absorption spectroscopy study of the local atomic structure in a ductile Cu-Al-Mn shape memory alloy

The martensitic transformation temperature and low-temperature aging treatment of Cu-Al-Mn (CAM) shape memory alloys play crucial roles in thermal- and stress-induced martensitic transformations. The local structural changes in Cu induced by low-temperature aging were investigated using X-ray absorption spectroscopy. The average local structure of Cu in the as-quenched CAM alloys is mainly composed of a D03-like structure, which is relatively close to the preparation composition. Furthermore, X-ray absorption near-edge structure spectral analysis of the CAM alloy showed no martensite phase (lattice angle change) was found after the low-temperature aging treatments at less than 150 °C. Instead, we found that the arrangement of Mn atoms changes in two stages during aging treatment.

Mapping nanocrystalline disorder within an amorphous metal–organic framework

Intentionally disordered metal–organic frameworks (MOFs) display rich functional behaviour. However, the characterisation of their atomic structures remains incredibly challenging. X-ray pair distribution function techniques have been pivotal in determining their average local structure but are largely insensitive to spatial variations in the structure. Fe-BTC (BTC = 1,3,5-benzenetricarboxylate) is a nanocomposite MOF, known for its catalytic properties, comprising crystalline nanoparticles and an amorphous matrix. Here, we use scanning electron diffraction to first map the crystalline and amorphous components to evaluate domain size and then to carry out electron pair distribution function analysis to probe the spatially separated atomic structure of the amorphous matrix. Further Bragg scattering analysis reveals systematic orientational disorder within Fe-BTC’s nanocrystallites, showing over 10° of continuous lattice rotation across single particles. Finally, we identify candidate unit cells for the crystalline component. These independent structural analyses quantify disorder in Fe-BTC at the critical length scale for engineering composite MOF materials.

Stepwise photoassisted decomposition of carbohydrates to H2

Stepwise photoassisted decomposition of carbohydrates to H2

Assessing diffusion model impacts on enstrophy and flame structure in turbulent lean premixed flames

Diffusive transport of mass occurs at small scales in turbulent premixed flames. As a result, multicomponent mass diffusion, which is often neglected in direct numerical simulations (DNS) of premixed combustion, has the potential to impact both turbulence and flame characteristics at small scales. In this study, we evaluate these impacts by examining enstrophy dynamics and the internal structure of the flame for lean premixed hydrogen-air combustion, neglecting secondary Soret and Dufour effects. We performed three-dimensional DNS of these flames by implementing the Stefan–Maxwell equations in the code NGA to represent multicomponent mass transport, and we simulated statistically planar lean premixed hydrogen-air flames using both mixture-averaged and multicomponent models. The mixture-averaged model underpredicts the peak enstrophy in the multicomponent simulation by up to 13% in the flame front. Comparing the enstrophy budgets of these flames, the multicomponent simulation yields larger peak magnitudes compared to the mixture-averaged simulation in the reaction zone, showing differences of 17% and 14% in the normalised vortex stretching and viscous effects terms. In the super-adiabatic regions of the flame, the mixture-averaged model overpredicts the viscous effects by up to 13%. To assess the effect of these differences on flame structure, we reconstructed the average local internal structure of the turbulent flame through statistical analysis of the scalar gradient field. Based on this analysis, we show that large differences in viscous effects contribute to significant differences in the average local flame structure between the two models.

Critical cation–anion radius ratio and two-dimensional antiferromagnetism in van der Waals TMPS3 (TM = Mn, Fe, Ni)

Two-dimensional TMPS3 antiferromagnets, transition metal (TM) = Mn, Fe, Ni, are studied by high-energy x-ray diffraction and atomic pair distribution analysis over a broad temperature range. Results show that the compounds exhibit common average but distinct local atomic structure, including distinct distortions of the constituent TM–S octahedra, magnitude and direction of atomic displacements, TM–TM distances and TM–S–TM bond angles. The differences in the local structure may be rationalized in terms of the Pauling’s rule for the critical ratio of TM2+ cation and S2− anion radii for octahedral coordination. We argue that the observed differences in the local structure are behind the differences in the antiferromagnetic properties of TMPS3 compounds, including different magnetic anisotropy and Neel temperature.

Synthesis and structural re-investigation of Fe2TeO6 using synchrotron x-rays

Fe2TeO6 (FTO), an antiferromagnetic magnetoelectric material which crystallizes in tetragonal (P42/mnm, 136) crystal structure, shows close to room temperature interesting magnetic interactions. To understand the complexity in its magnetism, a huge variation in its magnetic properties has been reported by various groups, which have been attributed to the presence of Fe2O3 (an impure phase) in the final FTO compound. Herein, synthesis using alternate ingredients and Synchrotron structural investigation of single phasic FTO are presented. Out of the several initial ingredients (Fe2O3, Fe3O4 with TeO2, Te(OH)6) in solid state reaction route, only the reaction between Fe3O4 and TeO2 results in the single phasic FTO (not reported so far), while the others (Fe2O3 with TeO2 or Te(OH)6) result in certain amount of Fe2O3 impurity. Rietveld refinement on Synchrotron X-ray Diffraction data and Fe K edge Extended X-ray Absorption Fine Structure data provide the overall similarity between the average crystalline structure and local structure of FTO, indicating the intrinsic nature of it’s complex magnetic behavior. Copyright © 2017 VBRI Press.

Origin and Absence of Giant Negative Thermal Expansion in Reduced and Oxidized Ca2RuO4

Negative thermal expansion (NTE) is an intriguing physical phenomenon. Layered Ca2RuO4 exhibits giant NTE over a wide temperature range from 200 to 400 K, which makes it attractive for fundamental research and industrial applications. However, a clear physical understanding is lacking for the appearance of NTE over such a wide temperature range and the oxygen-content-dependent switch from NTE to positive thermal expansion (PTE). Herein, we present insights into the average crystal structure, local structure, and electronic and orbital states of Ca2RuO4. Surprisingly, a previously overlooked monoclinic distortion is identified by electron diffraction and synchrotron X-ray diffraction (SXRD). X-ray absorption fine structure (XAFS) and synchrotron X-ray pair distribution function (PDF) analyses show large local distortions in monoclinic Ca2RuO4. Moreover, local stress on Ru cations is confirmed by the existence of over-bonding states, which relaxes along with NTE. Theoretical calculations indicate that dxy orbital ordering and disordering in the monoclinic structure are the origins of NTE. Moreover, interstitial oxygen plays a critical role in stabilizing elongated RuO6 and locally breaks the dxy orbital ordering, facilitating the occurrence of PTE. This work elucidates the electronic and orbital states in NTE materials with defective lattices and provides a different route to designing unconventional NTE materials.

Accelerated microwave-assisted synthesis and in situ X-ray scattering of tungsten-substituted vanadium dioxide (V1−xWxO2)

Vanadium dioxide (VO2) has been widely studied due to its metal-insulator phase transition at 68 °C, below which it is a semiconducting monoclinic phase, P21/c, and above it is a metallic tetragonal phase, P42/mnm. Substituting vanadium with transition metals allows transition temperature tunability. An accelerated microwave-assisted synthesis for VO2 and 5d tungsten-substituted VO2 presented herein decreased synthesis time by three orders of magnitude while maintaining phase purity, particle size, and transition character. Tungsten substitution amount was determined using inductively coupled plasma-optical emission spectroscopy. Differential scanning calorimetry, superconducting quantum interference device measurements, and in situ heating and cooling experiments monitored through synchrotron X-ray diffraction (XRD) confirmed the transition temperature decreased with increased tungsten substitution. Scanning electron microscopy analyzed through the line-intercept method produced an average particle size of 3–5 μm. Average structure and local structure phase purity was determined through the Rietveld analysis of synchrotron XRD and the least-squares refinement of pair distribution function data.

The origin of weak coupling between polar clusters in Ta2O5-doped 0.94Bi0.5Na0.5TiO3-0.06BaTiO3

Ta2O5-doped 0.94Bi0.5Na0.5 TiO3-0.06BaTiO3 [()(0.94BNT-0.06BT)-xTa ()] ceramics were prepared by a solid-state reaction technique. The single perovskite structure with space group R3c of the ceramics was identified by X-ray diffraction (XRD). Raman spectroscopy revealed the evolution of the local structure with Ta2O5 concentration. The temperature dependence of dielectric permittivity of the ceramics were deconvoluted by three Gaussian distribution functions which suggests that three dielectric anomalies exist in this system. The low-temperature dielectric anomaly exhibits a typical relaxor behavior with strong frequency dispersion (reentrant relaxor behavior). The activation energy derived from the V-F law is 0.298 eV, 0.338 eV, 0.412 eV and 0.449 eV, for , 0.005, 0.01 and 0.02, respectively. The mid-temperature and high-temperature anomalies are attributed to two structure phase transitions. The increase of the activation energies suggests that the coupling between polar clusters or polar nanoregions (PNRs) becomes weaker. The origin of the interaction between PNRs and phase transition behavior has been proposed according to the average structure, local structure and defect compensation mechanism of the system.

Understanding the Origin of the Breakdown of the Stokes-Einstein Relation in Supercooled Water at Different Temperature-Pressure Conditions.

A recent experiment has measured the viscosity of water down to approximately 244 K and up to 300 MPa. The correct viscosity and translational diffusivity data at various temperature-pressure (T-P) state points allowed for checking the validity of the Stokes-Einstein (SE) relation, which accounts for the coupling between translational self-diffusion and medium viscosity. The diffusion-viscosity decoupling increases with decreasing temperature, but the increasing pressure reduces the extent of the decoupling. Earlier simulation studies explained the breakdown of the SE relation in terms of the location of the Widom line, emanating from the liquid-liquid critical point (LLCP). Although these studies made a significant contribution to the current understanding of the above phenomena, a detailed molecular picture is still lacking. Recently, our group has explained the diffusion-viscosity decoupling from a jump-diffusion perspective. The jump-diffusion coefficient, emanating from the jump translation of water molecules, is calculated using a quantitative approach for different temperatures at ambient pressure. It has been observed that jump-diffusion is the key factor for diffusion-viscosity decoupling in supercooled water. The same method is adopted in the present work to estimate the jump-diffusion coefficient for different T-P state points and, thereby, explains the role of jump-diffusion for the different extents of the SE relation breakdown at different pressures. The residual diffusion coefficient, the other component of the total diffusion that originates from small step displacement and that is calculated by subtracting the jump-diffusion coefficient from the total diffusion, is seen to be fairly coupled to the viscosity at the entire range of temperature and pressure. Furthermore, we have calculated the average number of H-bonds per water molecule and the tetrahedral order for different T-P state points and investigated an approximate correlation between the average local structure and the contribution of the jump-diffusion to the total diffusion of water. This study, therefore, puts forward a new perspective for explaining the SE relation breakdown in supercooled water under different pressure conditions.

Bi4TaO8Cl Nano-Photocatalyst: Influence of Local, Average, and Band Structure

The average structure, local structure, and band structure of nanoparticles of photocatalyst Bi4TaO8Cl, an Aurivillius-Sillen layered material, has been studied by powder neutron Rietveld refinement, neutron pair distribution function technique, Raman scattering, and density functional theory calculations. A significant local structural deviation of nano-Bi4TaO8Cl was established in contrast to the local structure of bulk-Bi4TaO8Cl. Local structure was further supported by Raman scattering measurements. Through DFT calculations, we identify specific features in the electronic band structure that correlate lower secondary structural distortions in nano-Bi4TaO8Cl. Increased distortion of TaO6, decreased Ta-O-Ta bond angle, and increased octahedral tilt in the local structure of nano-Bi4TaO8Cl influence the band structure and the electron hole pair migration. Therefore, in addition to morphology and size, the local structure of a nanomaterial contributes to the photocatalytic performance. Trapping experiments confirm the role of superoxide radical in the photocatalysis mechanism of this material. Such studies help in developing new functional materials with better photocatalytic efficiency to address energy and environmental issues.

Correlation Between Local Structure and Boson Peak in Metallic Glasses

We made a systematic study of the boson peak for six different Zr-based metallic glasses and found a universal correlation between average local atomic structure and boson peak. It is found that the boson peak can be decomposed into six characteristic vibratory modes, i.e., Debye’s vibratory mode and five Einstein’s vibratory modes. By using the Ioffe–Regel condition over all studied Zr-based metallic glasses, we reveal that atomic pair correlation function exactly maps on the low-temperature dynamics and the origin of the boson peak, which is the sum of vibrations of local density fluctuation domains in the glasses. In addition, it is found that the Debye’s type oscillators are the major contributors to the low-temperature specific heat capacities. This study opens a new way of understanding the relationship of the physical properties with the atomic arrangements in glasses.

Lock-and-key dimerization in dense Brownian systems of hard annular sector particles.

We develop a translational-rotational cage model that describes the behavior of dense two-dimensional (2D) Brownian systems of hard annular sector particles (ASPs), resembling C shapes. At high particle densities, pairs of ASPs can form mutually interdigitating lock-and-key dimers. This cage model considers either one or two mobile central ASPs which can translate and rotate within a static cage of surrounding ASPs that mimics the system's average local structure and density. By comparing with recent measurements made on dispersions of microscale lithographic ASPs [P. Y. Wang and T. G. Mason, J. Am. Chem. Soc. 137, 15308 (2015)JACSAT0002-786310.1021/jacs.5b10549], we show that mobile two-particle predictions of the probability of dimerization P_{dimer}, equilibrium constant K, and 2D osmotic pressure Π_{2D} as a function of the particle area fraction ϕ_{A} correspond closely to these experiments. By contrast, predictions based on only a single mobile particle do not agree well with either the two-particle predictions or the experimental data. Thus, we show that collective entropy can play an essential role in the behavior of dense Brownian systems composed of nontrivial hard shapes, such as ASPs.

Local Structural Distortion Induced Uniaxial Negative Thermal Expansion in Nanosized Semimetal Bismuth.

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

Microscopic Structure of Contact Ion Pairs in Concentrated LiCl- and LiClO4-Tetrahydrofuran Solutions Studied by Low-Frequency Isotropic Raman Scattering and Neutron Diffraction with (6)Li/(7)Li Isotopic Substitution Methods.

Low-frequency isotropic Raman scattering and time-of-flight neutron diffraction measurements were carried out for (6)Li/(7)Li and H/D isotopically substituted *LiCl- and *LiClO4-tetrahydrofuran (*THF) solutions in order to obtain microscopic insight into solvated Li(+), Li(+)···Cl(-) and Li(+)···ClO4(-) contact ion pairs formed in concentrated THF solutions. Symmetrical stretching vibrational mode of solvated Li(+) in LiCl and LiClO4 solutions was observed at ν = 181-184 and 140 cm(-1), respectively. The stretching vibrational mode of Li(+)···Cl(-) and Li(+)···ClO4(-) solvated contact ion pairs formed in 4 mol % (6)LiCl-THF-h8 and (7)LiCl-THF-h8 solutions was found at ν = 469 and 435 cm(-1), respectively. Detailed structural properties of solvated Li(+) and the contact ion pairs were derived from the least-squares fitting analyses of the first-order difference function, ΔLi(Q), obtained from neutron diffraction measurements on (6)Li/(7)Li isotopically substituted THF-d8 solutions. It has been revealed that Li(+) takes 4-fold coordination in the average local structure of Li(+)X(-)(THF)3, X = Cl and ClO4. The nearest neighbor Li(+)···O(THF) distance was determined to be 2.21 ± 0.01 Å and 2.07 ± 0.01 Å for 4 mol % *LiCl- and 10 mol % *LiClO4-THF-d8 solutions, respectively. The Li(+)···anion distances for Li(+)···Cl(-) and Li(+)···O(ClO4(-)) contact ion pairs were determined to be 2.4 ± 0.1 Å and 2.19 ± 0.01 Å, respectively. The nearest neighbor Li(+)···THF interaction is significantly modified by the anion in the first solvation shell.

Role of Vanadium and Phosphorus in Substituted Keggin-Type Heteropolyoxo Molybdates Supported on Silica SBA-15 in Selective Propene Oxidation

Vanadium-containing Keggin-type heteropolyoxo molybdate ([PV2Mo10O40]5−) was supported on silica SBA-15 (denoted as PV2Mo10-SBA-15). The structural evolution and catalytic activity of PV2Mo10-SBA-15 and a suitable reference V2Mo10Ox-SBA-15 were investigated under selective propene oxidation conditions by using in situ X-ray absorption spectroscopy. 31P MAS NMR measurements of supported PV2Mo10-SBA-15 and reference H3PO4-SBA-15 were performed after the catalytic reaction. PV2Mo10-SBA-15 formed a mixture of mainly tetrahedral [MoOx] and [VOx] units during thermal treatment under propene oxidation conditions. Changes in the average local structure around V centers coincided with the changes in the average local structure around Mo centers and the onset of catalytic activity. In addition, mainly tetrahedral [MoOx] and [VOx] units seemed to be in close proximity and interacted under catalytic conditions. Conversely, in the reference material V2Mo10Ox-SBA-15 synthesized with individual V and Mo source precursors, Mo and V centers appeared to be more separated from each other. The structural environment of P in PV2Mo10-SBA-15 under catalytic conditions corresponded to a mixture of various species. P was connected to both the support material SBA-15 via POSi bonds and [MoOx] or [VOx] units. Apparently, the proximity of V and Mo in Keggin precursors was a prerequisite for obtaining (Mo,V) oxide species on the support material.

Local Structure of Pr, Nd, and Sm Complex Oxides and Their X-ray Absorption Near Edge Structure Spectra

Pr, Nd, and Sm L1-edge and L3-edge X-ray absorption near edge structure (XANES) spectra of their complex oxides were measured to investigate the relationship between the characteristic feature of the spectra and the local structure around the Pr, Nd, and Sm atoms. The pre-edge peak area of the L1-edge and the full width at half-maximum (fwhm) of the white line of the L3-edge XANES spectra are extracted by curve fitting analyses with one Gaussian and one cubic spline function as a background or with one arctangent and one pseudo-Voigt function, respectively. As a result, a clear correlation between the pre-edge peak area and the fwhm is found with another correlation with abstract physical parameters defined by bond angles formed with Pr, Nd, or Sm and two adjacent oxygen atoms, which represent an indicator of disorder of local configuration of the target element. This finding will open a way to estimate an average local structure of lanthanide elements by means of XANES spectroscopy.

Probing the Average Local Structure of Biomolecules Using Small-Angle Scattering and Scaling Laws

Small-angle neutron and x-ray scattering have become invaluable tools for probing the nanostructure of molecules in solution. It was recently shown that the definite integral of the scattering profile exhibits a scaling (power-law) behavior with respect to molecular mass. We derive the origin of this relationship, and discuss how the integrated scattering profile can be used to identify differing levels of disorder over local ≲30 Å length scales. We apply our analysis to globular and intrinsically disordered proteins.

Combining LBP difference and feature correlation for texture description.

Effective characterization of texture images requires exploiting multiple visual cues from the image appearance. The local binary pattern (LBP) and its variants achieve great success in texture description. However, because the LBP(-like) feature is an index of discrete patterns rather than a numerical feature, it is difficult to combine the LBP(-like) feature with other discriminative ones by a compact descriptor. To overcome the problem derived from the nonnumerical constraint of the LBP, this paper proposes a numerical variant accordingly, named the LBP difference (LBPD). The LBPD characterizes the extent to which one LBP varies from the average local structure of an image region of interest. It is simple, rotation invariant, and computationally efficient. To achieve enhanced performance, we combine the LBPD with other discriminative cues by a covariance matrix. The proposed descriptor, termed the covariance and LBPD descriptor (COV-LBPD), is able to capture the intrinsic correlation between the LBPD and other features in a compact manner. Experimental results show that the COV-LBPD achieves promising results on publicly available data sets.

Synthesis, EPR and optical spectroscopy of the Cr-doped tetraborate glasses

Electron paramagnetic resonance (EPR), optical absorption and photoluminescence spectra as well as luminescence kinetics of the Li2B4O7:Cr and KLiB4O7:Cr tetraborate glasses were investigated at T=300K. The Li2B4O7:Cr and KLiB4O7:Cr glasses containing 0.4 and 1.6mol.% Cr2O3 of high optical quality were obtained from polycrystalline compounds by fast cooling of the melts. The X-band EPR spectroscopy shows that the Cr impurity is incorporated in the tetraborate glass network as isolated Cr3+ centers and Cr3+–Cr3+ pairs coupled by magnetic dipolar and exchange interactions. The EPR spectral parameters (geff and ΔBpp) of both Cr3+ and Cr3+–Cr3+ centers in the Li2B4O7:Cr and KLiB4O7:Cr glasses were measured and analyzed. All transitions in optical absorption, luminescence excitation and emission spectra of these glasses are identified. Broad complex bands that peak near 615, 405, and 350nm in optical absorption and luminescence excitation spectra correspond to the 4A2g(F)→4T2g(F), 4A2g(F)→4T1g(F), and 4A2g(F)→4T1g(P) spin-allowed transitions of the Cr3+ centers in distorted octahedral sites of the tetraborate glass network. The octahedral (cubic) crystal field strength (10Dq) and Racach parameters (B and C) for Cr3+ centers in Li2B4O7:Cr and KLiB4O7:Cr glasses are estimated. Narrow and broad emission bands in red – NIR regions are assigned to the 2Eg(F)→4A2g(F) (R1 line) and 4T2g(F)→4A2g(F) (electron-vibration) transitions, which correspond to the Cr3+ centers in high-field and low-field sites, respectively. All observed emission bands are characterized by non-exponential decay. Measured average lifetimes and local structure of the Cr3+ centers in high-field and low-field sites of the Li2B4O7:Cr and KLiB4O7:Cr glass network have been discussed.