Synchrotron-based Time-resolved X-ray Diffraction Research Articles

Self-stabilization of the equilibrium state in ferroelectric thin films

(K,Na)NbO3 is a lead-free and sustainable ferroelectric material with electromechanical parameters comparable to Pb(Zr,Ti)O3 (PZT) and other lead-based solid solutions. It is therefore a promising candidate for caloric cooling and energy harvesting applications. Specifically, the structural transition from the low-temperature Mc- to the high-temperature c-phase displays a rich hierarchical order of domains and superdomains, that forms at specific strain conditions. The relevant length scales are few tens of nanometers for the domain and few micrometers for the superdomain size, respectively. Phase-field calculations show that this hierarchical order adds to the total free energy of the solid. Thus, domains and their formation has a strong impact on the functional properties relevant for electrocaloric cooling or energy harvesting applications. However, monitoring the formation of domains and superdomains is difficult and requires both, high spatial and high temporal resolution of the experiment. Synchrotron-based time-resolved X-ray diffraction methods in combination with scanning imaging X-ray microscopy is applied to resolve the local dynamics of the domain morphology with sub-micrometer spatial and nanosecond temporal resolution. In this regime, the material displays a novel self-stabilization mechanism of the domain morphology, which may be a general property of first-order phase transitions.

Development of shock-dynamics study with synchrotron-based time-resolved X-ray diffraction using an Nd:glass laser system.

The combination of high-power laser and synchrotron X-ray pulses allows us to observe material responses under shock compression and release states at the crystal structure on a nanosecond time scale. A higher-power Nd:glass laser system for laser shock experiments was installed as a shock driving source at the NW14A beamline of PF-AR, KEK, Japan. It had a maximum pulse energy of 16 J, a pulse duration of 12 ns and a flat-top intensity profile on the target position. The shock-induced deformation dynamics of polycrystalline aluminium was investigated using synchrotron-based time-resolved X-ray diffraction (XRD) under laser-induced shock. The shock pressure reached up to about 17 GPa with a strain rate of at least 4.6 × 107 s-1 and remained there for nanoseconds. The plastic deformation caused by the shock-wave loading led to crystallite fragmentation. The preferred orientation of the polycrystalline aluminium remained essentially unchanged during the shock compression and release processes in this strain rate. The newly established time-resolved XRD experimental system can provide useful information for understanding the complex dynamic compression and release behaviors.

Time response demonstration of in situ lattice deformation under an applied electric field by synchrotron-based time-resolved X-ray diffraction in polar-axis-oriented epitaxial Pb(Zr,Ti)O3 film

The dependence of applied rectangular pulses with various widths on the crystal structure change was investigated by time-resolved synchrotron-based X-ray diffraction measurement. A (001)-oriented epitaxial Pb(Zr0.5Ti0.5)O3 film of 2.1 µm thickness grown on a (100)cSrRuO3//(100)LaNiO3//(100)CaF2 substrate by metal organic chemical vapor deposition was investigated. The crystal lattice increased almost linearly with increasing applied electric field up to 230 kV/cm in the case of a 0.3-µs-width pulse. This elongation with the application of an electric field was ascertained to be almost independent of the pulse width from 0.3 to 2000 µs at 190 kV/cm. These values were almost consistent with the macroscopic measurements obtained at 5 and 1000 Hz by piezoelectric force macroscopy. The present results show that the time-resolved XRD measurement is very useful for analyzing the frequency dependence of the piezoelectric response in view of the crystal structure change because the crystal structure change under an applied electric field can be systematically investigated by changing the applied pulse width.

Structure and Thermal Stability of (H2O)4 Tetrahedron and (H2O)6 Hexagon Adsorbed on NaY Zeolite Studied by Synchrotron-Based Time-Resolved X-ray Diffraction

Water adsorption and desorption features play an important role in determining the adsorptive and catalytic properties of zeolites. In this study, the extraframework structure (water and cations) in NaY faujasite zeolite was studied by synchrotron-based in situ time-resolved X-ray diffraction (TR-XRD) in combination with the Rietveld refinement method. It was observed that Na cations locating at different sites migrated between each other during the dehydration process. Water molecules at three separate sites showed interesting geometries: (H2O)4 tetrahedron W1, (H2O)6 ice-like hexagonal W2, and disordered triangle W3. Interactions between each form of water and Na cations were deeply investigated. The stability of the waters followed the sequence of W1 > W3 > W2. This work provides insights into both framework structure and cation positions of model zeolite under actual working conditions, which helps fully understand the properties of zeolites.

The kinetic effect on structural behavior of mixed LiMn 2O 4–LiNi 1/3Co 1/3Mn 1/3O 2 cathode materials studied by in situ time-resolved X-ray diffraction technique

How the structural changes of each active material in mixed cathode systems take place at different charge–discharge rates is quite important in the application of the system in which the mixed cathode materials with different rate capabilities are formed into one composite electrode. Here we report the results of the real time structural change studies of mixed LiMn 2O 4–LiNi 1/3Co 1/3Mn 1/3O 2 composite cathode in a Li-ion cell by using in situ synchrotron-based time resolved x-ray diffraction (TR-XRD) technique. The layer structured component in the mixed composite cathode system shows less utilization at fast discharge rate (high power mode) whereas the spinel structured component is fully utilized. This clearly demonstrates that the reduced capacity at fast discharge rate for this system is caused by the less utilization of the layer structured component. The real time monitoring of the structural behavior at various discharge rates is a great tool to design the best ratios of active materials with different rate capabilities in the mixed cathode systems for different applications.

Ca Doping of Nanosize Ce−Zr and Ce−Tb Solid Solutions: Structural and Electronic Effects

Ceria-based ternary oxides are novel materials with potential use in many areas of chemistry, physics, and materials science. Synchrotron-based time-resolved X-ray diffraction (TR-XRD), X-ray absorption near-edge spectroscopy (XANES), Raman and infrared spectroscopies (RS and IR), and density-functional (DF) calculations were used to study the structural and electronic properties of Ce−M−Ca (M = Zr, Tb) oxide nanoparticles. The nanoparticles were synthesized following novel microemulsion and citrate methods and had sizes in the range of 3−6 nm. The atoms in these nanoparticles adopted a Fluorite-type structure and exhibited cell parameters with deviations with respect to the values predicted by Vegard's rule for ideal solid solutions. The simultaneous presence of Zr/Tb and Ca creates strain in the Fluorite-type lattice which correlates with the presence and number of oxygen vacancies through the Ce−M−Ca samples. The oxygen vacancy and cation distributions of the nanoparticles are strongly affected by the ...

The structural and electronic properties of nanostructured Ce1−x−yZrxTbyO2 ternary oxides: Unusual concentration of Tb3+ and metal↔oxygen↔metal interactions

Ceria-based ternary oxides are widely used in many areas of chemistry, physics, and materials science. Synchrotron-based time-resolved x-ray diffraction, x-ray absorption near-edge spectroscopy (XANES), Raman spectroscopy, and density-functional calculations were used to study the structural and electronic properties of Ce-Zr-Tb oxide nanoparticles. The nanoparticles were synthesized following a novel microemulsion method and had sizes in the range of 4-7 nm. The Ce1-x-yZrxTbyO2 ternary systems exhibit a complex behavior that cannot be predicted as a simple extrapolation of the properties of Ce1-xZrxO2, Ce1-xTbxO2, or the individual oxides (CeO2, ZrO2, and TbO2). The doping of ceria with Zr and Tb induces a decrease in the unit cell, but there are large positive deviations with respect to the cell parameters predicted by Vegard's rule for ideal solid solutions. The presence of Zr and Tb generates strain in the ceria lattice through the creation of crystal imperfections and O vacancies. The O K-edge and Tb LIII-edge XANES spectra for the Ce1-x-yZrxTbyO2 nanoparticles point to the existence of distinctive electronic properties. In Ce1-x-yZrxTbyO2 there is an unexpected high concentration of Tb3+, which is not seen in TbO2 or Ce1-xTbxO2 and enhances the chemical reactivity of the ternary oxide. Tb<-->O<-->Zr interactions produce a stabilization of the Tb(4f,5d) states that is responsible for the high concentration of Tb(3+) cations. The behavior of Ce1-x-yZrxTbyO2 illustrates how important can be metal<-->oxygen<-->metal interactions for determining the structural, electronic, and chemical properties of a ternary oxide.

Time-resolved Studies for the Mechanism of Reduction of Copper Oxides with Carbon Monoxide: Complex Behavior of Lattice Oxygen and the Formation of Suboxides

Temperature-programmed reduction (TPR), synchrotron-based time-resolved X-ray diffraction (TR-XRD), extended X-ray absorption fine-structure (EXAFS), and X-ray absorption near-edge structure (XANES) were employed in this work to systematically study the reaction of CuO and Cu2O with CO gas molecules. Both TPR and isothermal reduction results showed that CuO was easier to reduce than Cu2O under the same reaction conditions. In situ measurements of XRD and XANES showed a direct transformation pathway for CuO reduction (CuO → Cu) when there was a large supply of CO, while they showed a sequential step pathway involving one intermediate (CuO → Cu2O → Cu) with a limited supply of CO. An induction period for CuO reduction was seen and increased with decreasing temperature. O vacancies in CuO were observed during the induction period that could foster formation of a nonstoichiometric metastable copper-oxide species. The metastable species can either react rapidly with CO to form metallic copper during high CO fl...

Physical and Chemical Properties of MoP, Ni2P, and MoNiP Hydrodesulfurization Catalysts: Time‐Resolved X‐Ray Diffraction, Density Functional, and Hydrodesulfurization Activity Studies.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The behavior of mixed-metal oxides: Structural and electronic properties of Ce1−xCaxO2 and Ce1−xCaxO2−x

Synchrotron-based time-resolved x-ray diffraction (TR-XRD), x-ray absorption near edge spectroscopy (XANES), Raman spectroscopy (RS), and first-principles density functional (DF) calculations were used to study the structural and electronic properties of Ce–Ca mixed-metal oxides. The XRD results and DF calculations show that doping with calcium induces relatively minor variations (&amp;lt;0.05 Å) in the cell dimensions of ceria. However, the presence of Ca leads to slightly distorted tetragonal structures, a substantial strain in the lattice of the oxide and a tendency to form O vacancies in an ideal Ce1−xCaxO2 solid solution. The two latter effects can be a consequence of the large number of oxygen neighbors that Ca is forced to have in Ce1−xCaxO2 and differences in the electronic charges of calcium and cerium cations. The Ce1−xCaxO2−x systems are not fully ionic. Cation charges derived from the DF calculations indicate that these systems obey the Barr model for charge redistribution in mixed-metal oxides. The Ca atoms in Ce1−xCaxO2−x are more electropositive than the cations in CaO, while the Ce cations of Ce1−xCaxO2−x are less electropositive than those of CeO2. These trends are consistent with XANES measurements at the Ca K- and Ce LIII-edges. The cation charge redistributions should be taken into consideration when explaining or predicting the chemical and catalytic properties of Ce1−xCaxO2−x. Ca induces structural and electronic perturbations on ceria quite different from those found after doping with Zr. The behavior of Ce1−xCaxO2−x illustrates the drastic effects that doping with an electropositive element can have on the properties of ceria.

Physical and Chemical Properties of MoP, Ni2P, and MoNiP Hydrodesulfurization Catalysts: Time-Resolved X-ray Diffraction, Density Functional, and Hydrodesulfurization Activity Studies

Synchrotron-based time-resolved X-ray diffraction was used to study in situ the crystalline phases present during the preparation of bulk and silica-supported MoP, Ni2P, and MoNiP by reduction of oxidic precursors in hydrogen. Independent of the type of oxidic precursor used or the presence of silica as a support, the formation of the metal phosphides occurs at temperatures between 600 and 800 °C. Since the common species in all the cases are phosphate-type groups (POx), it seems that their reduction by hydrogen is the final and determining step in the formation of MoP, Ni2P, and MoNiP. Silica-stabilized phosphide phases were detected during the synthesis of Ni2P/SiO2 and MoP/SiO2 catalysts. In the case of Ni2P/SiO2, before the appearance of the final phosphide, strong diffraction lines are observed for Ni12P5. First-principles density functional calculations for bulk MoP, Ni2P, and MoNiP indicate that the Ni−P and Mo−P bonds in these compounds have a small degree of ionic character. For MoP, Ni2P, and Mo...

Experimental and theoretical studies on the reaction of H(2) with NiO: role of O vacancies and mechanism for oxide reduction.

Reduction of an oxide in hydrogen is a method frequently employed in the preparation of active catalysts and electronic devices. Synchrotron-based time-resolved X-ray diffraction (XRD), X-ray absorption fine structure (NEXAFS/EXAFS), photoemission, and first-principles density-functional (DF) slab calculations were used to study the reaction of H(2) with nickel oxide. In experiments with a NiO(100) crystal and NiO powders, oxide reduction is observed at atmospheric pressures and elevated temperatures (250-350 degrees C), but only after an induction period. The results of in situ time-resolved XRD and NEXAFS/EXAFS show a direct NiO-->Ni transformation without accumulation of any intermediate phase. During the induction period, surface defect sites are created that provide a high efficiency for the dissociation of H(2). A perfect NiO(100) surface, the most common face of nickel oxide, exhibits a negligible reactivity toward H(2). The presence of O vacancies leads to an increase in the adsorption energy of H(2) and substantially lowers the energy barrier associated with the cleavage of the H-H bond. At the same time, adsorbed hydrogen can induce the migration of O vacancies from the bulk to the surface of the oxide. A correlation is observed between the concentration of vacancies in the NiO lattice and the rate of oxide reduction. These results illustrate the complex role played by O vacancies in the mechanism for reduction of an oxide. The kinetic models frequently used to explain the existence of an induction time during the reduction process can be important, but a more relevant aspect is the initial production of active sites for the rapid dissociation of H(2).

Studies on the Behavior of Mixed-Metal Oxides: Structural, Electronic, and Chemical Properties of β-FeMoO4

The structural and electronic properties of iron molybdate (FeMoO4) have been studied using synchrotron-based time-resolved X-ray diffraction (TR-XRD), X-ray absorption near-edge spectroscopy (XANES), and first-principles density functional (DFT-GGA) calculations. The β phase of FeMoO4 is stable in a large range of temperatures (25−800 °C). For β-FeMoO4, the results of Reitveld refinement and DFT-GGA calculations give a crystal structure in which Fe is in an almost octahedral environment while Mo is tetracoordinated. The Mo LII-edge XANES spectra of β-FeMoO4 exhibit a line shape that is typical of oxides in which Mo is in a tetrahedral environment. The valence electronic structure of β-FeMoO4 is characterized by an intense peak with Fe 3d character near the top of the valence band. The iron molybdate displays a good chemical affinity for H2, H2S, and SO2. H2-TPR (temperature programmed reduction) spectra for β-FeMoO4 show consumption of hydrogen and evolution of water (β-FeMoO4 + H2,gas → H2Ogas + FeMoOx)...

Phase transformations and electronic properties in mixed-metal oxides: Experimental and theoretical studies on the behavior of NiMoO4 and MgMoO4

Mixed-metal oxides play a relevant role in many areas of chemistry, physics, and materials science. We have examined the structural and electronic properties of NiMoO4 and MgMoO4 by means of synchrotron-based time-resolved x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES), and first-principles density functional theory (DFT) calculations. Nickel molybdate can exist in two phases (α and β). Mo is in a near tetrahedral environment in the β-phase, whereas in the α-phase the metal exhibits a pseudo-octahedral coordination with two very long Mo–O distances (2.3–2.4 Å). The results of DFT calculations indicate that the α-phase of NiMoO4 is ∼9 kcal/mol more stable than the β-phase. On the other hand, in the case of magnesium molybdate, an α-NiMoO4-type phase is ∼13 kcal/mol less stable than β-MgMoO4. These trends in stability probably result from variations in the metal–metal repulsion within the α-phases of the compounds. For the α→β transition in NiMoO4, the DFT calculations predict an energy barrier of ∼50 kcal/mol. An apparent activation energy of ∼80 kcal/mol can be derived from the time-resolved XRD experiments. The degree of ionicity in MgMoO4 is larger than that in NiMoO4. The nickel molybdate displays a large density of states near the top of the valence band that is not observed in the magnesium molybdate. This makes NiMoO4 more chemically active than MgMoO4. A similar type of correlation is found between the electronic and chemical properties of NiMoO4, CoMoO4, and FeMoO4. The DFT results and Mo LII-edge XANES spectra show big differences in the splitting of the Mo 4d orbitals in the α- and β-phases of the molybdates. The line shape in the O K-edge essentially reflects the behavior seen for the 4d orbitals in the Mo LII-edge (i.e., mainly O 1s→Mo 4d electronic transitions). The Mo LII- and O K-edges in XANES can be very useful for probing the local symmetry of Mo atoms in mixed-metal oxides.

Characterization of Mixed-Metal Oxides Using Synchrotron-Based Time-Resolved x-ray Diffraction and x-ray Absorption Spectroscopy

ABSTRACTExperiments are described showing the utility of synchrotron-based time-resolved x-ray diffraction (TR-XRD) and x-ray absorption near-edge spectroscopy (XANES) for characterizing the physical and chemical properties of mixed-metal oxides that contain Mo and a second transition metal (Fe, Co or Ni). TR-XRD was used to study the transformations that occur during the heating of a FeMoO4/Fe2(MoO4)3 mixture and the α⇒β phase transitions in CoMoO4 and NiMoO4. The Mo LII- and O K-edges in XANES are very useful for probing the local symmetry of Mo atoms in mixed-metal oxides. The results of XANES and density-functional calculations (DMo13, DFT-GGA) show large changes in the splitting of the empty Mo 4d levels when going from tetrahedral to octahedral coordinations. XANES is very useful for studying the reaction of H2, H2S and SO2 with the mixed-metal oxides. Measurements at the S K-edge allow a clear identification of S, SO2, SO3 or SO4 on the oxide surfaces. Changes in the oxidation state of molybdenum produce substantial shifts in the position of the Mo LII- and MIII-edges.