Short-range Structural Changes Research Articles

Structural Distortion in the Wadsley-Roth Niobium Molybdenum Oxide Phase Triggering Extraordinarily Stable Battery Performance.

Wadsley-Roth niobium oxide phases have attracted extensive research interest recently as promising battery anodes. We have synthesized the niobium-molybdenum oxide shear phase (Nb, Mo)13 O33 with superior electrochemical Li-ion storage performance, including an ultralong cycling lifespan of at least 15000 cycles. During electrochemical cycling, a reversible single-phase solid-solution reaction with lithiated intermediate solid solutions is demonstrated using in situ X-ray diffraction, with the valence and short-range structural changes of the electrode probed by in situ Nb and Mo K-edge X-ray absorption spectroscopy. This work reveals that the superior stability of niobium molybdenum oxides is underpinned by changes in octahedral distortion during electrochemical reactions, and we report an in-depth understanding of how this stabilizes the oxide structure during cycling with implications for future long-life battery material design.

Europium induced point defects in SrSnO3-based perovskites employed as antibacterial agents

The antibacterial activity of Eu3+-doped SrSnO3-type materials against Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) bacteria is described. Two Eu3+-doped SrSnO3 perovskites - (SrEu)SnO3 and Sr(SnEu)O3 - were synthesised by a modified-Pechini method and characterised by XRD, FT-IR, FE-SEM, HR-STEM/EDX, BET, Photoluminescence (PL), UV-Vis, Q-band EPR and XPS to understand the impact of Eu doping on the materials’ properties. Structural characterisations indicated that the desired perovskite phase completely crystallised after calcination at 700 °C. Long and short-range structural changes were observed as a function of the site-doping with Eu and calcination temperature. Small specific surface area, which varied from 8.09 to 13.28 m2/g, was observed for the samples. Nonetheless, the formation of nanoparticles under 10 nm, clusters of nanoparticles> 100 nm, and nanorods with 100–600 nm × 10–50 nm (length × width) was evidenced. Eu3+ doping led to an increase of Sn2+ and oxygen vacancies in SrSnO3 lattice, playing an essential role in the antibacterial activity. Reduced Eu2+ species were also observed. The samples had activity below 5 % against Escherichia coli, whereas (SrEu)SnO3 displayed an efficiency of 100 % after 24 h against Staphylococcus aureus at a concentration of 1 mg/mL. Our results demonstrate that a specific chemical doping with Eu induces the formation of distinct point defect (Sn2+, Eu2+ and VO•) in the materials, which promoted a negative surface charge that seems to have improved the redox ability and, therefore, enhanced the biocide property.

Pressure induced short-range structural changes in supercooled liquid Ge2Sb2Te5

Phase-change material such as Ge2Sb2Te5 is usually utilized to store data due to the pronounced contrast in optical and electrical properties between crystalline and amorphous phases. As the density differs in the two phases, it is necessary to explore the influence of pressure on the structures of Ge2Sb2Te5, especially for the supercooled liquid which is an inevitable state in the formation of the two phases. The short-range structures in supercooled liquid Ge2Sb2Te5 under compression have been investigated by using ab initio molecular dynamics simulation. The supercooled liquid eventually changes to a solid with an increase in pressure. During the process, tetrahedrons decrease slightly, revealing that tetrahedral structures are insensitive to the pressure. Octahedrons increase as the pressure is less than 3.7 GPa and then decrease, suggesting that a moderate pressure can promote the formation of octahedrons. The body-centered-cubic, face-centered-cubic and hexagonal closed-packed structures are observed at 9.9 GPa and then increase gradually. Additionally, it is noticed that Sb- and Te-centered clusters prefer Ge-centered clusters to forming the high-coordinated short-range structures during the compression process. Our research make us aware of the effects of pressure on short-range structures in supercooled liquid, which is of great importance in the application of phase-change materials.

Anomalous variation of Boson peak and fragility and their correlations with intermediate-range structure in PbO-B2O3 glasses

Understanding the microscopic origin of physical properties is fundamental to the study of glasses. In the present study, unique trends were observed in structural and dynamical properties in lead-borate [xPbO:(1-x)B2O3] glasses upon increasing the modifier. Properties driven by short-range structural changes like, glass transition temperature (Tg) and elastic modulii exhibit a broad maxima around x = 0.3, in congruence with the average coordination number <r>. Using Raman spectroscopy, this behaviour is attributed to the gradual conversion of three-coordinate (B3) to four-coordinate (B4) boron species. However, dynamic properties like the Boson peak and fragility behave similar to the above structural properties except an anomalous dip at x = 0.3. The corresponding dynamic correlation length, therefore, exhibits a decrease except a peak at x = 0.3 with increasing x. The static correlation lengths estimated from the first sharp diffraction peak (FSDP), did not follow a linear trend with the dynamic correlation lengths, indicating a different spatial origin. The analysis of the Boson peak and FSDP suggests that while the Boson peak arises from intermediate-range ordering within the network units, the FSDP arises due to ordering of voids between such units. Moreover, the Boson peak maximum showed a linear decrease with increasing <r>, while the Boson peak position and fragility were found to vary linearly. These correlations emerge from the evolution of the network packing and connectivity with the addition of modifier and changing role of lead oxide as a glass former at higher concentrations. The anomalous variation in dynamical properties reflects the critical role of Pb2+ in tuning the intermediate-range structure of the vitreous network.

Impact of hydrothermal treatment of FEBEX and MX80 bentonites in water, HNO3 and Lu(NO3)3 media: Implications for radioactive waste control

Engineered barriers of deep geological repositories (DGR) are commonly constructed with bentonite. FEBEX and MX80 bentonites have been selected by different countries as reference materials for the sealing of repositories; however, their chemical reactivity with high-level long-lived radioactive wastes (HLRW) under subcritical conditions had not been explored before. The hydrothermal stability in neutral and acid media and chemical reactivity in contact with an actinide analogous compound were both studied. The long-range and short-range structural changes were analyzed by X-ray diffraction, nuclear magnetic resonance and scanning electron microscopy. Both bentonites have exhibited a good stability in neutral and acid media and have generated a new phase immobilizing the actinide analogous compound. The extent of the chemical reaction is higher in MX80 bentonite than in FEBEX bentonite.

Local Structure and Dynamics in the Na Ion Battery Positive Electrode Material Na3V2(PO4)2F3

Na3V2(PO4)2F3 is a novel electrode material that can be used in both Li ion and Na ion batteries (LIBs and NIBs). The long- and short-range structural changes and ionic and electronic mobility of Na3V2(PO4)2F3 as a positive electrode in a NIB have been investigated with electrochemical analysis, X-ray diffraction (XRD), and high-resolution 23Na and 31P solid-state nuclear magnetic resonance (NMR). The 23Na NMR spectra and XRD refinements show that the Na ions are removed nonselectively from the two distinct Na sites, the fully occupied Na1 site and the partially occupied Na2 site, at least at the beginning of charge. Anisotropic changes in lattice parameters of the cycled Na3V2(PO4)2F3 electrode upon charge have been observed, where a (= b) continues to increase and c decreases, indicative of solid-solution processes. A noticeable decrease in the cell volume between 0.6 Na and 1 Na is observed along with a discontinuity in the 23Na hyperfine shift between 0.9 and 1.0 Na extraction, which we suggest is due to a rearrangement of unpaired electrons within the vanadium t2g orbitals. The Na ion mobility increases steadily on charging as more Na vacancies are formed, and coalescence of the resonances from the two Na sites is observed when 0.9 Na is removed, indicating a Na1–Na2 hopping (two-site exchange) rate of ≥4.6 kHz. This rapid Na motion must in part be responsible for the good rate performance of this electrode material. The 31P NMR spectra are complex, the shifts of the two crystallograpically distinct sites being sensitive to both local Na cation ordering on the Na2 site in the as-synthesized material, the presence of oxidized (V4+) defects in the structure, and the changes of cation and electronic mobility on Na extraction. This study shows how NMR spectroscopy complemented by XRD can be used to provide insight into the mechanism of Na extraction from Na3V2(PO4)2F3 when used in a NIB.

Long- and short-range structural changes of recrystallised cassava starch subjected to in vitro digestion

The conformational and structural order of resistant starch type III made from cassava starch was studied. The gelatinized starch was debranched using pullulanase and then recrystallised by annealing, temperature-cycling or heat-moisture treatment. Subsequently, the recrystallised products were subjected to in vitro digestion using porcine pancreatic α-amylase and amyloglucosidase. The undigested and digested products were analyzed for polymer chain distribution, crystallinity, molecular order, structural conformations and thermal stability using high performance anion exchange chromatography, wide angle X-ray diffraction, Fourier transform infrared spectroscopy, 13C CP/MAS nuclear magnetic resonance and differential scanning calorimetry, respectively. Average degree of polymerisation increased from 20 to 22 glucose units upon digestion. Both the undigested and digested starches comprised mixtures of A, B and V crystalline types. Percentage of crystallinities by X-ray diffraction were 40.9%, 50.7% and 56.2% in annealed, temperature-cycled and heat-moisture treated starches, respectively. These values increased to 47.9%, 54.4% and 58.2%, respectively, in the digested products. The ordered fractions in the undigested annealed, temperature-cycled and heat-moisture treated starches were 69.3%, 71.4% and 79.2%, respectively, as determined by 13C CP/MAS nuclear magnetic resonance. However, the disordered phase was indistinct in the digested products although the contents of non-crystalline conformations were significantly ( p < 0.01) higher. The melting enthalpies of the digested residues increased by factors of 2.50 in annealed, 2.53 in temperature-cycled and 2.06 in heat-moisture treated starches, suggesting molecular rearrangement in a manner related to the enzyme susceptibility of the initial materials.

Hydrothermal Stability of Layered Silicates in Neutral and Acidic Media: Effect on Engineered-Barrier Safety

AbstractMany environmental applications in the inorganic remediation field are based on the swelling and ion-exchange capacities of smectites, even though these can be affected by hydrothermal treatment in water and acidic media. Here a systematic study of the properties of layered silicates that could affect their hydrothermal stability at different pH is described: type of layers, octahedral occupancy, layer charge, and origin of the layer charge. The silicates studied were selected on the basis of their different characteristics associated with these properties. Kanemite (1:0 phyllosilicate), kaolinite (1:1 phyllosilicate), and pyrophyllite and talc (2:1 phyllosilicates with no-layer charge) were examined in order to determine the effect of layer structure, whereas the hydrothermal reactivity of silicates with different layer charge was analyzed by comparing the talc-hectorite-Laponite® and talc-saponite-trioctahedral vermiculite series. Samples were treated hydrothermally at 300ºC for 48 h in pure water and in a 0.01 M HNO3 solution and the final products were analyzed by X-ray diffraction, scanning electronic microscopy, and solid-state nuclear magnetic resonance spectroscopy. All layered silicates, except for kanemite, were found to remain intact after hydrothermal treatment in water and acidic media, with only minimal short-range structural changes observed. The extent of the structural changes depended on the octahedral sheet occupancy (greater extent) and the number of isomorphic substitutions (lesser extent), both of which weaken the structure.

Molar volume, excess enthalpy, and Prigogine-Defay ratio of some silicate glasses with different (P,T) histories

Structural relaxation in silicate glasses with different (p,T) histories was experimentally examined by differential scanning calorimetry and measurements of molar volume under ambient pressure. Temperature and pressure-dependent rates of changes in molar volume and generation of excess enthalpy were determined for sodium trisilicate, soda lime silicate, and sodium borosilicate (NBS) compositions. From the derived data, Prigogine-Defay ratios are calculated and discussed. Changes of excess enthalpy are governed mainly by changes in short-range structure, as is shown for NBS where boron coordination is highly sensitive to pressure. For all three glasses, it is shown how the relaxation functions that underlie volume, enthalpy, and structural relaxation decouple for changes in cooling rates and pressure of freezing, respectively. The magnitude of the divergence between enthalpy and volume may be related to differences in structural sensitivity to changes in the (p,V,T,t) space on different length scales. The findings suggest that the Prigogine-Defay ratio is related to the magnitude of the discussed decoupling effect.

Dynamic EXAFS study of discharging nickel hydroxide electrode with non-integer Ni valency

A dynamic in situ X-ray absorption study, in the energy dispersive mode, has revealed short-range structural changes taking place during the discharge of a nickel hydroxide film electrode. A Jahn–Teller distortion has been identified in the first Ni–O shell at overcharge, which diminishes during the discharge process. Throughout discharge, the first Ni–Ni shell moves antagonistically to Ni–O coordination. These structural changes and Ni K-edge shifts have been used as measures of Ni valency during film discharge. Correlation of the results with electrode charge leads to a simple mechanism, in terms of electron population at Ni sites, for the discharge process.

In Situ Studies of Iron under Pressure: New Windows on the Earth's Core

Experiments on iron with recently developed high-pressure techniques are providing new insights into the nature of the Earth's core. In situ high P-T synchrotron x-ray diffraction studies to above 160 GPa and 3000 K with double-sided, laser-heated diamond-anvil cells demonstrate that the hexagonal closed packed phase (ϵ-Fe) has a wide stability field extending from deep mantle to core conditions. Direct measurements of the melting line of ϵ-Fe to above 100 GPa with these techniques provide constraints on the temperature and density at the inner core boundary. Radial x-ray diffraction measurements carried out at room temperature to 220 GPa have constrained the elasticity, rheology, and acoustic velocities of ϵ-Fe at core pressures. Vibrational properties providing information on thermodynamic and elastic parameters have been measured to above 150 GPa using both synchrotron x-ray and laser Raman spectroscopies. X-ray diffraction studies of liquid iron reveal changes in short-range structure as functions of pressure and temperature. Insight into the problem of the identity and abundance of other (e.g., light) elements in the core has been obtained from direct studies of the Fe-O, Fe-S, and Fe-H binary systems as well as observations for several other key elements. Progress using various new techniques is reviewed, and new directions and questions are examined. We also compare recent experimental findings with a variety of theoretical predictions.

Temperature induced structural changes and transport mechanisms in borate, borosilicate and boroaluminate liquids: high-resolution and high-temperature NMR results

Temperature dependent structural rearrangements in glass-forming liquids and associated changes in the configurational entropy have been hypothesized to be closely related to the transport processes such as viscous flow. The nature of such structural rearrangements is, however, not well understood. In this study the temperature dependence of the short-range structural changes involving borate, silicate and aluminate speciation reactions in sodium borate, sodium borosilicate and lithium boroaluminate liquids have been investigated with high-resolution and high-temperature (up to 1793 K) 11B, 29Si and 27Al nuclear magnetic resonance spectroscopy. It is shown that such structural changes are the principal source of configurational entropy in binary alkali borate liquids. However, silicate speciation in the borosilicate and borate speciation in the boroaluminate liquids are found to be insensitive to temperature changes. It is speculated that in these liquids the temperature induced changes in the intermediate-range structural order are likely to be an important source of configurational entropy. The inter-connection between temperature dependent speciation reactions and atomic scale mechanisms for viscous flow in these liquids is also established.

99/02512 Fuel-bound nitrogen evolution during biomass gasification

Four series of zinc-based mixed oxides doped with Fe, Cu or Ti, as potential regenerable desulphurization sorbents of coal gas at high temperature were characterized using vibrational spectroscopies, in conjunction with previous studies in which the same sorbents were studied by XRD, SEM-EDX and XPS. For each series, the influence of the composition, within a broad range of concentration in their component oxides, and the temperature of calcination of the fresh sorbents, as well as the structural modifications undergone during sulphidation/regeneration cycles were investigated. It is shown that by analysing the end-process materials, vibrational spectroscopies can reveal fairly well the formation of spinel-structure compounds, sulphides and probable sulphates during calcination, sulphidation and regeneration, respectively. Additionally, vibrational spectroscopies can detect short-range structural changes, not evidenced by XRD, and can be used with advantage to study the evolution of these compounds under real operating conditions.

Vibrational spectroscopy study of zinc-containing mixed oxides as regenerable sulphur sorbents at high temperature

Four series of zinc-based mixed oxides doped with Fe, Cu or Ti, as potential regenerable desulphurization sorbents of coal gas at high temperature were characterized using vibrational spectroscopies, in conjunction with previous studies in which the same sorbents were studied by XRD, SEM-EDX and XPS. For each series, the influence of the composition, within a broad range of concentration in their component oxides, and the temperature of calcination of the fresh sorbents, as well as the structural modifications undergone during sulphidation/regeneration cycles were investigated. It is shown that by analysing the end-process materials, vibrational spectroscopies can reveal fairly well the formation of spinel-structure compounds, sulphides and probable sulphates during calcination, sulphidation and regeneration, respectively. Additionally, vibrational spectroscopies can detect short-range structural changes, not evidenced by XRD, and can be used with advantage to study the evolution of these compounds under real operating conditions.

Short-range structure in (partially) crystalline potato starch determined with attenuated total reflectance Fourier-transform IR spectroscopy

A fast and direct method, based on infrared spectroscopy, for quantitative determination of starch short-range structure has been developed. The IR spectrum of starch is sensitive to changes in short-range structure in the C—C and C—O stretching region at 1300-800 cm −1. The IR absorbance band at 1047 cm −1 is sensitive to the amount of ordered or crystalline starch and the band at 1022 cm −1 is characteristic of amorphous starch. The ratios ( R) of the heights of the bands at 1047 and 1022 cm −1, which expresses the amount of ordered starch to amorphous starch, and 1047 and 1035 cm −1, which is a measure of the amount of ordered starch, showed a linear regression with the amount of potato starch having B-type crystallinity as determined with wide-angle X-ray diffractometry. The IR spectrum and thus the short-range order is also sensitive to water content. In particular, the band at 994 cm −1, which is related to intramolecular hydrogen bonding of the hydroxyl group at C-6, is water sensitive. It is possible to quantify the IR data in terms of short-range order ( S IR) over a range of 10–50% water. The method has been applied to quantify the changes in short-range structure during the melting of potato starch with 18 or 26% (w/w) water. The amount of short-range structure and the changes during melting in the (partially) destructured starch samples concur with differential scanning calorimetry and wide-angle X-ray diffractometry measurements.

A Method Based on Radiative Cooling for Detecting Structural Changes in Undercooled Metallic Liquids

Abstract We introduce a structure-sensitive parameter for undercooled melts which can be measured in containerless processing experiments. We have established that the ratio, R(T), of hemispherical total emissivity eT(T) to constant-pressure specific heat cp(T) can serve as an indicator which is sensitive to any changes in short range atomic order in undercooled metallic melts. R(T) ≡ eT(T)/cp(T) values for nickel, zirconium, and silicon have been obtained using the high temperature electrostatic levitator while the levitated melts were undergoing purely radiative cooling into the deeply undercooled region. R(T) plots for undercooled liquid nickel and zirconium indicate no significant change in short-range structure from their melting temperatures to 15% undercooling. In contrast, liquid silicon shows marked shortrange structural changes beginning above its melting temperature and extending throughout the undercooled region. The short-range structure of liquid silicon is related to the highly-directional covalent bonding which characterizes its solid form. The nickel and zirconium data show that eT varies linearly with T, in support of metal emissivity theories.

Effects of High Temperature on Silicate Liquid Structure: A Multinuclear NMR Study

The structure of a silicate liquid changes with temperature, and this substantially affects its thermodynamic and transport properties. Models used by geochemists, geophysicists, and glass scientists need to include such effects. In situ, high-temperature nuclear magnetic resonance (NMR) spectroscopy on (23)Na, (27)A1, and (29)Si was used to help determine the time-averaged structure of a series of alkali aluminosilicate liquids at temperatures to 1320 degrees C. Isotropic chemical shifts for (29)Si increase (to higher frequencies) with increasing temperature, probably in response to intermediate-range structural changes such as the expansion of bonds between nonbridging oxygens and alkali cations. In contrast, isotropic chemical shifts for (27)Al decrease with increasing temperature, indicating that more significant short-range structural changes take place for aluminum, such as an increase in mean coordination number. The spectrum of a sodium aluminosilicate glass clearly indicates that at least a few percent of six-coordinated aluminum was present in the liquid at high temperature.

Solid-state n.m.r. study of the transformation of VPI-5/MCM-9 into AIPO 4-8/SAPO-8

The removal of water at 100 °C (or under specific conditions) from VPI-5/MCM-9 materials results in significant structural changes. The VPI-5/MCM-9 products transform into AIPO 4 -8/SAPO-8. This transformation is partially reversible and strongly depends on the evacuation and rehydration treatments, the heating rate, and probably on lattice defect concentrations. From the knowledge of the VPI-5 structure and of the structural model recently proposed for AIPO 4 -8, we have reassigned all the 31 P n.m.r. lines characterizing both materials and explained further the effect of water on the dramatic changes occurring in their 31 P and 27 Al n.m.r. spectra. 31 P and 27 Al n.m.r. have been used to investigate systematically the transformation of VPI-5 into AIPO 4 -8 and to determine which specific T sites undergo modifications. A mechanism accounting for the transformation is proposed together with a model allowing the determination of the degree of VPI-5/AIPO 4 -8 conversion by computing the relative intensities of five specific 31 P n.m.r. lines that characterize the transition intermediate structures. 31 P n.m.r. proved more sensitive than did conventional X-ray diffraction to detect short-range structural changes in these open structures.

Exafs studies on the local structure of transition metal-based amorphous alloy: -Structural rearrangement of amorphous CU 60ZR 40 alloy during the crystallization process

Short range structural changes of amorphous Cu60Zr40 alloy during the crystallization process have been investigated by EXAFS (Extended X-ray Absorption Fine Structure spectroscopy) at both the Cu and Zr K-absorption edges. The Cu K-edge EXAFS spectra were reconstructed well by using combinations of four subshells. A weak increase of the Cu K-edge EXAFS spectrum observed in the alloy heated at 623 K is due to the increase of the amount of neighboring atoms in the 3rd and 4th shells, which was caused by the reduction of structural disorder and the short range structural rearrangement accompanied by the structural relaxation. While, the increase of the Zr K-edge EXAFS spectrum in the alloy heated at 773 K suggests that the aggregation of zirconium atoms occurs and induces the formation of the multi-phases.