Synchrotron X-ray Study Research Articles

Cation distribution and magnetic structure of M-type BaTiMnFe10O19 examined by synchrotron X-ray and neutron studies

Combinational studies of synchrotron X-rays and neutrons have been performed to determine the site occupancy, valence state and magnetic structure of M-type BaTiMnFe10O19. X-ray resonant scattering studies have revealed the site preference, where Ti and Mn ions are distributed evenly over the Fe1, Fe2 and Fe3 sites of five independent Fe sites. X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) results for BaTiMnFe10O19 are similar to those of BaFe12O19 at the Fe K absorption edge. Clear chemical shifts exist at both the Mn and Fe edges, suggestive of a mixed valence of Mn and Fe ions. The Mn K XANES resembles Mn–Zn ferrite, where the observed fourfold peak is explained as the presence of four-coordinated Mn ions, on the basis of the self-consistent calculation of relativistic density functional theory. This can be explained by the presence of spinel substructures within the S block layer. Satellite reflections of 002 ± τ and 004 − τ (τ = 2/3) on 00l were observed at a temperature of 8 K in neutron powder diffraction, due to magnetic scattering on the helicoidal arrangement of ordered spins with a propagation period of c/τ. The magnetic structure observed in the neutron powder diffraction and the XMCD results explain the decreasing uniaxial magnetization from BaFe12O19.

Structural Analysis of Novel Antiferromagnetic Material CeRu2Al10 and Its Related Compounds under Pressure

We performed synchrotron X-ray studies of orthorhombic CeRu2Al10 and its related compounds CeT2Al10 (T = Fe, Os) and RRu2Al10 (R = La, Yb, Lu). The results of the X-ray studies indicate that all of the compounds retain their orthorhombic crystal structures and that their lattice parameters all monotonically decrease with pressure; that is, no structural changes or distortions occur below 10 GPa within the experimental accuracy of this study. In addition, we evaluated the bulk moduli of the compounds, which are mainly affected by the transition metal in RT2Al10 systems. All of the compounds exhibited crystal structures that were harder along the b-axis than along the a- or c-axes. Furthermore, anisotropic shrinkage induced a difference between the physical pressure of CeRu2Al10 and the chemical pressure of Ce(Ru1−xFex)2Al10 (0 ≤ x ≤ 1). Thus, the physical properties of CeRu2Al10 under pressure were observed to be strongly affected by crystal structure shrinkage along the a- and c-axes.

Metastable phases in Zr-Excel alloy and their stability under heavy ion (Kr2+) irradiation

Zr-Excel alloy (Zr-3.5Sn-0.8Nb-0.8Mo, wt.%) has been proposed as a candidate material of pressure tubes in the CANDU-SCWR design. It is a dual-phase alloy containing primary hcp α-Zr and metastable bcc β-Zr. Metastable hexagonal ω-Zr phase could form in β-Zr as a result of aging during the processing of the tube. A synchrotron X-ray study was employed to study the lattice properties of the metastable phases in as-received Zr-Excel pressure tube material. In situ heavy ion (1 MeV Kr2+) irradiations were carried out at 200 °C and 450 °C to emulate the stability of the metastable phase under a reactor environment. Quantitative Chemi-STEM EDS analysis was conducted on both un-irradiated and irradiated samples to investigate alloying element redistribution induced by heavy ion irradiation. It was found that no decomposition of β-Zr was observed under irradiation at both 200 °C and 450 °C. However, ω-Zr particles experienced shape changes and shrinkage associated with enrichment of Fe at the β/ω interface during 200 °C irradiation but not at 450 °C. There is a noticeable increase in the level of Fe in the α matrix after irradiation at both 200 °C and 450 °C. The concentrations of Nb, Mo and Fe are increased in the ω phase but decreased in the β phase at 200 °C. The stability of metastable phases under heavy ion irradiation associated with elemental redistribution is discussed.

In situ synchrotron X-ray studies of the coupled effects of thermal and solutal supercoolings on the instability of dendrite growth

Special growth pattern representing unique growth conditions is a vital clue to investigate the morphology evolution mechanism in metallic alloy solidification. The dendrite pattern and growth orientation of dendritic doublons in the hypoeutectic Al-Cu alloy have been studied by synchrotron radiation imaging technology and electron back scatter diffraction. The results show that this kind of doublon is two-dimensional and the secondary arms grow perpendicularly to the primary stem. This doublon morphology can appear as an equiaxed grain, columnar dendrite, or coexist with a regular dendrite. The growth directions of the dendritic doublon tips and the secondary arms are <110> and <001>, respectively. Rising cooling rate or Cu concentration in the alloys facilitates the formation of the doublonic structure. According to the kinetic morphology diagram of Al-Cu alloys (pattern-dimensionless supercooling-anisotropy) obtained from the experimental data, the dendritic doublon forming region was above the boundary of fractal dendrite and compact dendrite.

In-situ synchrotron X-ray studies of crystallization of β-nucleated iPP subjected to a wide range of shear rates and shear temperatures

Previous study focused on the shear-induced crystallization of β-nucleated iPP under fixed shear rate and temperature [1]. In this work, in-situ synchrotron X-ray was used to investigate the crystallization behavior of well-selected β-nucleated iPP under thermo-mechanical conditions in a wide range of shear rates from 2 to 50 s−1 and shear temperatures from 133 to 150 °C. At lower shear rates, we found that the overall crystallization rate of the β-nucleated iPP was reduced compared with that under quiescent condition. This is mainly ascribed to variation of nucleation-dependent process after application of shear pulse, causing strong reduction of β-nucleating efficiency. As the shear rate increases from 2 to 50 s−1, the dimensionality of crystal growth basically decreased while the long period of lamellar structure tended to slightly increase. During the shear-induced crystallization, a decreasing temperature was significantly associated with decreased overall dimensionality of crystal growth but increased crystallization rate. Finally, we observed an abnormal increase of long period with time for β-nucleated iPP sheared at 133 °C.

A real-time synchrotron X-ray study of primary phase nucleation and formation in hypoeutectic Al–Si alloys

This work reports the results of real-time X-ray radiography of grain refined and unrefined Al–Si alloys solidified at the SPring-8 synchrotron. The nucleation events were observed and the grain density and growth rate following nucleation were measured. Nucleation of the grain refined alloy samples occurred from the coolest to the hottest parts of the field of view in a sequence that mimicked a forward moving wave. No additional nucleation events occurred between the first nucleated grains and there was no evidence of grains being generated by fragmentation. Measurements of both grain density and growth rate show the effect of Si content and grain refiner (Al3Ti1B master alloy) additions on grain size. While the total number of grains increases in the alloys with added Al3Ti1B master alloy, the growth rates tend to be slower. Furthermore, the growth rate for all alloy compositions fluctuates between slow and fast velocities during the initial stages of growth and then tends towards a low steady state value. This decreasing trend is explained in terms of thermal and solutal field interactions between adjacent growing grains and subsequent grain impingement. These measurements result in a better understanding of the role of nucleation and solute content in influencing further nucleation and the subsequent change in the solid–liquid growth rate.

Study on the Relaxation Process in 4H-SiC Homoepitaxy Growth

BPD half loops arrays (HLAs) present in the homoepitaxial layers of 4H-SiC are responsible for introducing an increase in the forward voltage drop via the electron-hole Recombination Enhanced Dislocation Glide (REDG) and therefore are harmful to the device operation. Different sources are observed to form HLAs and interfacial dislocations (IDs) including surface, scratch, micropipe, 3C inclusion. Recent observation was made in a 100mm 4H-SiC wafer where a short edge-oriented BPD segment in the substrate was drawn towards the interface during the epitaxy growth producing screw-oriented segments intersecting the growth surface. These screw segments of BPDs subsequently glide under the mismatch stress when the critical thickness is reached. HLAs and IDs are subsequently formed. We believe that the formation of HLAs and IDs is driven by the mismatch stress brought by the doping concentration difference between the substrate and the epilayer. To verify this, we first estimate the amount of mismatch strain caused by the epi/substrate doping concentration difference according to Sasaki’s work [1]. Then Matthews-Blakeslee’s model [2, 3] is used to predict the value for critical thickness corresponded to such amount of mismatch strain. The predict value is then compared to the critical thickness that we measured via Synchrotron X-ray study and they are found to have a good agreement with each other. This indicates that mismatch strain brought by the doping concentration is sufficient for relaxation process to occur.[1] S. Sasaki, J. Suda, and T. Kimoto, Materials Science Forum 717-720, 481 (2012).[2] J. W. Matthews, Journal of Vacuum Science and Technology 12(1), 126 (1975).[3] J. Matthews and A. Blakeslee, Journal of Crystal Growth 27, 118 (1974).

(Invited) Mechanism Study of Reversible Resistivity Change in Oxide Thin Film

The field of applied physics is driven by the need to understand materials structure and electronic properties with the ultimate goal of harnessing them in functional devices such as nonvolatile memories. One needs to understand the mechanism, which dictates the behavior of the charged defects and electrons under external stimuli such as electrostatic potential, thermal gradient and photons, to obtain a more microscopic understanding of the system. Then, one arrives at a predictive description of designing functional devices. In the first part of the talk, we report the x-ray-induced reversible resistance change in 40-nm-thick TiO2 films sandwiched by Pt top and bottom electrodes, and propose the physical mechanism behind the emergent phenomenon. Our findings indicate that there exists a photovoltaic-like effect, which modulates the resistance reversibly by a few orders of magnitude, depending on the intensity of impinging x-rays. We found that this effect, combined with the x-ray irradiation induced phase transition confirmed by transmission electron microscopy, triggers a non-volatile reversible resistance change. Understanding x-ray-controlled reversible resistance changes can provide possibilities to control initial resistance states of functional materials, which could be useful for future nonvolatile memories. In the second part, we cover the investigation of Ta2O5 powder and oxygen deficient thin films using synchrotron x-ray studies at the Advanced Photon Source, combining x-ray diffraction, extended x-ray absorption fine structure (EXAFS) and resonant x-ray scattering spectroscopy (RIXS) and Ab initio band structure calculations. Ta-based resistive switching devices have been extensively investigated due to their fast switching and reliable endurance among other materials. Despite extensive recent interests, however, there is still lack of fundamental understanding of electronic structure and local structure of the Ta-based device. Oxygen vacancies play essential roles in the electric functionality of resistive switching devices, which is based on the formation and rupture of conducting paths within the insulating matrix. We found that there are strong correlations among oxygen vacancy number and positions and energy gaps. Ab initio band structure calculations successfully explain the evolution of the electronic excitation spectrum as a function of oxygen vacancy number and positions and importantly provide a predictive description of the oxygen deficient Ta oxide that may improve the desired performance based on atomic level design rather than the traditional trial-error methods.

A sample chamber for in situ high-energy X-ray studies of crystal growth at deeply buried interfaces in harsh environments

We introduce a high pressure high temperature chamber for in situ synchrotron X-ray studies. The chamber design allows for in situ studies of thin film growth from solution at deeply buried interfaces in harsh environments. The temperature can be controlled between room temperature and 1073K while the pressure can be set as high as 50bar using a variety of gases including N2 and NH3. The formation of GaN on the surface of a Ga13Na7 melt at 1073K and 50bar of N2 is presented as a performance test.

Anomalies in magnetoelastic properties of DyFe11.2Nb0.8 compound

The structural and magnetic properties of DyFe11.2Nb0.8 compound have been investigated by high intensity, high resolution synchrotron x-ray diffraction, ac magnetic susceptibility, and dc magnetization measurements as well as Mössbauer spectroscopy (5–300 K). The easy magnetization direction at room temperature is along the c-axis. With decreasing temperature, the magnetocrystalline anisotropy changes from easy-axis to easy-cone at Tsr2 (∼202 K), then to easy plane at Tsr1 (∼94 K). The thermal expansion coefficient obtained from the synchrotron x-ray study exhibits clear anomalies around both of the spin reorientation transition temperatures. The 57Fe hyperfine interaction parameters and magnetic moments values have been determined for the 8i, 8j, and 8f sites from the Mössbauer spectra.

Dual relaxation and structural changes under uniaxial strain in main-chain smectic-C liquid crystal elastomer.

The relationship between strain-dependent macroscopic elastic behavior and the changes in microscopic structure of the smectic-C liquid crystal elastomer (LCE), C11MeHQSi8 were investigated using synchrotron X-ray studies. At very low strains ε ≤ 0.2, the smectic layers are randomly oriented. As the strain increases beyond 0.2, the smectic layers reorient and become parallel to the direction of the applied strain. The polydomain to monodomain (P-M) transition accompanied by the formation of chevron structure ensues for ε > 0.2 and is nearly complete for ε = 0.7. The chevron structure relaxes after the applied strain changes, with a time constant τα ∼ 45 min while the orientation order parameters of the mesogenic and elastomeric components gradually increase and saturate at 0.83 and 0.4, respectively at ε = 1.7 which is near the end of the plateau region. Relaxation rates τα for the tilt angle and τd corresponding to the smectic layer spacing both become about 10 times faster when the strain exceeds 0.7. The LCE remains "locked" into the monodomain state and retains 90% and 80% values of α and S, respectively for 24 hours after the applied strain is removed. The viscoelastic properties of the liquid crystal appear to dominate the equilibration process at low strains while the elastomeric properties control the system's response at high strains.

Pressure-induced amorphization of a dense coordination polymer and its impact on proton conductivity

The proton conductivity of a dense coordination polymer (CP) was investigated under high-pressure conditions. Impedance measurements under high pressures revealed that the proton conductivity of the CP decreased more than 1000-fold at pressures of 3–7 GPa and that the activation energy for proton conduction almost doubled compared with that at ambient pressure. A synchrotron X-ray study under high pressure identified the amorphization process of the CP during compression, which rationally explains the decrease in conductivity and increase in activation energy. This phenomenon is categorized as reversible pressure-induced amorphization of a dense CP and is regarded as a demonstration of the coupling of the mechanical and electrical properties of a CP.

Neutron and X-ray Diffraction from Exotic Magnets in Pulsed Magnetic Fields

Magnetic fields obtained by discharging a large capacitor bank through Helmholtz coils can be produced in excess of 25 T for relatively short periods of time, ~ 1 msec and longer. When combined with modern facilities for diffraction at modern neutron and synchrotron x-ray sources, one can study the structure and phase diagrams of new materials under extremes of magnetic field. I will present two such studies, each focussing on a new magnetic material which exhibit exotic low temperature states. I will show time-resolved neutron Laue diffraction on the multiferroic magnet MnWO4 [1], and time-resolved synchrotron x-ray studies of large magneto-elastic effects in the geometrically-frustarted pyrochlore magnet Tb2Ti2O7 [2], both in magnetic fields up to ~ 30 T. Such studies of new materials in extreme sample environments can be very revealing as to the nature of their exotic low temperature states.

Synchrotron X-ray Studies of Rapidly Evolving Morphology of Self-Assembled Nanoparticle Films under Lateral Compression

Interfacial nanostructures represent a class of systems that are highly relevant to studies of quasi-2D phases, chemical self-assembly, surfactant behavior, and biologically relevant membranes. Previous studies have shown that under lateral compression a Langmuir film of gold (Au) nanoparticles assembled at the liquid-air interface exhibits rich mechanical behavior: it undergoes a rapid structural and morphological evolution from a monolayer to a trilayer via an intermediate hash-like phase. We report the results of studying this structural evolution using grazing incidence X-ray off-specular scattering (GIXOS). We utilize GIXOS to obtain a quantitative mapping of electron density profile normal to the liquid surface with a subnanometer resolution and follow the structural evolution of the Au nanoparticle film under lateral compression with a subminute temporal resolution. As the surface pressure is increased, the self-assembled nanoparticle monolayer first crinkles into a double-layer phase before forming a trilayer. This study reveals the existence of a transient bilayer phase and provides a microscopic picture of the particle-level crinkling phenomena of ultrathin films. These studies were previously impossible due to the relatively short time scales involved in crinkling formation of these transient phases and their intrinsically inhomogeneous nature.

Synchrotron X-ray radioscopic in situ study of high-temperature polymer electrolyte fuel cells - Effect of operation conditions on structure of membrane

We present the first high-resolution synchrotron X-ray study on high temperature polymer electrolyte fuel cells (HT-PEFCs) in through-plane mode. Distribution and evolution of the phosphoric acid in the membrane electrode assembly was monitored in situ/in-operando under different operating conditions at steady states. At current densities of 350 mA cm−2 and 600 mA cm−2 significant changes in local media distribution were detected mainly beneath the channels of the flow field. We assign this effect to a varying degree of acid doping over the active region. Furthermore it was found that parts of the electrode structure were moved and partly irreversibly displaced after cell operation at load conditions. This effect might contribute to structural aging of the electrodes.

Structural analysis of diamond mosaic crystals for neutron monochromators using synchrotron radiation

The beams extracted from thermal neutron sources such as nuclear reactors are monochromatised by Bragg diffraction using imperfect single crystals with an angular mosaic spread of typically 0.2–0.8°. For neutron wavelengths below 1.5Å, the highest reflectivity of all crystalline materials is expected for diamond. Nowadays diamond single crystals with an appropriate mosaic spread exceeding a thickness of 1mm can be grown by heteroepitaxy on an Ir/yttria-stabilised zirconia bilayer deposited on a Si(001) single crystal.To explain the observed neutron reflectivity being below the theoretically expected value, we have studied the spatial distribution of the mosaic structure of two crystals by high resolution X-ray diffraction using a laboratory X-ray source and synchrotron radiation. The first sample (A) showed a uniform mosaic spread of 0.18°±0.02° across the 1cm wide sample. The peak shift of the X-ray rocking curves of 0.08° indicated a weak curvature of the crystal lattice. The measured absolute neutron peak reflectivity of 34% corresponded to 90% of the value predicted by theory. The peak width of the neutron rocking curve for the second sample (B) was twice as big, but here the peak reflectivity of 13% corresponded to only half of the theoretical value.This unfavourable behaviour could be assigned to a substantial spatial variation of the mosaic spread deduced from the synchrotron X-ray studies. X-ray diffraction with high spatial resolution indicated a mosaic block size below 50μm for sample A. This was consistent with chemical etching experiments on the surface of a comparable sample which showed both randomly distributed dislocations and others that are arranged in boundaries of several 10μm large domains.

High resolution synchrotron X-ray studies of phase separation phenomena and the scaling law for the threading dislocation densities reduction in high quality AlGaN heterostructure

High resolution X-ray coplanar (symmetric X-ray diffraction (SXRD) and asymmetric X-ray diffraction (ASXRD)) and non-coplanar diffraction (grazing incidence diffraction (GID)) have been used to investigate the quality of AlGaN epilayers with 20% Al content, grown on sapphire using SiNx interlayers. The measurement of reciprocal space maps (RSM) with higher orders of reflections of SXRD and ASXRD which is readily performed at the synchrotron with high resolution and intensity reveals the presence of several diffraction peaks originating from the occurrence of local differences in the lattice constants. Two distinguishable AlGaN phases having different crystalline parameters were clearly recognized from X-ray data and the corresponding densities of screw dislocations have been determined measured as function of the overgrowth thickness varying from 0.5μm to 3.5μm. The variation of the screw and edge type dislocation densities with the overgrowth thickness has been found to follow the scaling law h−n where h is the thickness of the buffer layer.

Insight into Thermal Instability of Charged Cathode Materials for Lithium-Ion Batteries: Combined in situ Synchrotron X-ray and Mass Spectroscopy Study

not Available.

Can a Formally Zwitterionic Rhodium(I) Complex Emulate the Charge Density of a Cationic Rhodium(I) Complex? A Combined Synchrotron X-ray and Theoretical Charge-Density Study

The molecular electron densities of structurally related cationic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indene)Rh(COD)](CF(3)SO(3)), [1c](CF(3)SO(3))) and formally zwitterionic ([(κ(2)-3-P(i)Pr(2)-2-NMe(2)-indenide)Rh(COD)], 1z) complexes were accurately determined using synchrotron bright-source X-ray radiation at 30 K followed by multipolar refinement (COD = η(4)-1,5-cyclooctadiene). The densities were also obtained from density functional theory calculations with a large, locally dense basis set. A 28-electron ([Ar]3d(10)) core of the Rh atom was modeled by an effective core potential to obtain a density that was then augmented with relativistic cores according to the Keith-Frisch approximation. Calculations were performed at the experimental geometry and after vacuum-phase geometry optimization starting from the experimental geometry. Experimental and calculated geometries and electron-density distributions show that the electron density and electronic structure in the region of the Rh center are not significantly altered by protonation of the aromatic ring and that formal removal of CF(3)SO(3)H from [1c](CF(3)SO(3)) affords a complex 1z possessing substantial zwitterionic character (with a charge separation of ca. 0.9 electronic charge) featuring a negatively charged aromatic indenide framework. Further, the molecular electrostatic potentials of 1c and 1z exhibit similar topography around the metal, despite being drastically different in the vicinity of the indene or indenide portion of the cation (1c) and zwitterion (1z), respectively. Collectively, these observations obtained from high-level experimental and theoretical electron-density analysis confirm, for the first time, that appropriately designed zwitterionic complexes can effectively emulate the charge distribution found within ubiquitous cationic platinum-group metal catalyst complexes, in keeping with recent catalytic investigations.

Synchrotron X-ray, Photoluminescence, and Quantum Chemistry Studies of Bismuth-Embedded Dehydrated Zeolite Y

For the first time, direct experimental evidence of the formation of monovalent Bi (i.e., Bi(+)) in zeolite Y is provided based on the analysis of high-resolution synchrotron powder X-ray diffraction data. Photoluminescence results as well as quantum chemistry calculations suggest that the substructures of Bi(+) in the sodalite cages contribute to the ultrabroad near-infrared emission. These results not only enrich the well-established spectrum of optically active zeolites and deepen the understanding of bismuth related photophysical behaviors, but also may raise new possibilities for the design and synthesis of novel hybrid nanoporous photonic materials activated by other heavier p-block elements.