Wolfgang Caliebe

Phase stability in ceria-zirconia binary oxide nanoparticles: The effect of the Ce3+ concentration and the redox environment

The stability of the cubic phase (c′) of Ce1−xZrxO2−y nanoparticles was studied by x-ray absorption near edge spectroscopy (XANES), time-resolved high temperature x-ray diffraction (XRD), and room temperature XRD. A clear relationship between the phase stability of the c′ phase and the Ce3+ concentration was found in zirconia-doped ceria, prepared in an oxidizing environment. The percentage of the 3+ oxidation state of cerium was measured from the relative Ce3+ peak intensity at the CeLIII edge in XANES. A concentration of 11% of the larger Ce3+ ions, among all cerium ions, helps to release the local stress induced by the smaller Zr4+ ions and stabilizes the c′ phase even under high zirconia concentrations of 40%–60%. A smaller particle size enhances this effect. XANES results at the ZrLIII edge supported the cubic phase stabilization. When the homogenization anneal was performed under a reducing environment instead of in air, the solubility limit of the cubic phase Ce1−xZrxO2−y was extended to above 90% ...

Lifetime-broadening removed X-ray absorption near edge structure by resonant inelastic X-ray scattering spectroscopy

A novel method to measure lifetime-broadening removed (LBR) X-ray absorption near edge structure (XANES) spectra is presented. It makes use of monochromatic incident X-rays near resonant condition as well as a high-resolution spectrometer. Fine structures observed in the resonant inelastic X-ray scattering spectra are compared to high-resolution fluorescent excitation XANES spectra for several copper compounds, demonstrating that the present method can be employed to obtain LBR-XANES.

HIGH-RESOLUTION X-RAY SPECTROMETER BASED ON A CYLINDRICALLY BENT CRYSTAL IN NONDISPERSIVE GEOMETRY

The results from a new crystal spectrometer for high‐resolution inelastic scattering and resonant Raman scattering spectroscopy are presented. It is based on a cylindrically bent silicon crystal in standard Rowland circle geometry where the energy dispersion is obtained nonconventionally by placing a position sensitive detector perpendicular to the scattering plane. The advantage of this geometry is that the energy dispersion is only about one order of magnitude larger than the intrinsic energy resolution of the instrument, allowing the optimization of the collection efficiency within a narrower energy range of interest while preserving the practical realization of a cylindrically bent crystal. A total energy resolution of 0.7 eV at 8 keV with an energy dispersion of only a few eV was reached. Potential applications of this new geometry as well as comparison with other focusing geometries are also discussed.

Optical, resonant X-ray scattering, and calorimetric investigations of two liquid crystal compounds exhibiting the SmA-SmCα*-SmC* transitions

Three experimental measurements have been conducted to investigate the nature of the SmA–SmCα*–SmC* phase transitions of two liquid crystal compounds. The SmA-SmCα* transition shows a continuous transition with XY-like critical exponents. The SmCα*-SmC* transition displays a rapid variation of the incommensurate helical pitch. One of the compounds shows a small jump in the helical pitch, which signals a first order transition and is consistent with high resolution calorimetric results.

Structure of the Co and Fe Kα3,4 satellite spectra

The Kα3,4 satellite spectra, originating in the [1s2p] −1 → [2p] −2 transition, were measured with high resolution for Co and Fe, using photoexcitation by tunable monochromatized synchrotron radiation. Fits to a phenomenological sum of Voigt functions and to ab initio relativistic Dirac–Fock-calculated spectra resolved the structure underlying the spectra. The dependence of the satellites’ intensity on the excitation energy yielded accurate values for the excitation thresholds. The shake-theory-based Thomas model was found to deviate from the measured intensities, indicating near-threshold major contributions from non-shake excitation processes, such as the knockout, or ‘two step one,’ effect.

Inelastic x‐ray scattering at the National Synchrotron Light Source (invited)

The research program at the inelastic x‐ray scattering beamline at the National Synchrotron Light Source is focused on the study of elementary excitations in condensed matter with total energy resolution on the order of 0.1 eV to 1.0 eV. Results from selected experiments are reported to demonstrate the capability of the beamline as well as the information that can be obtained from inelastic x‐ray scattering experiments.

Spin resolved resonant Raman scattering

Spin resolved resonant Raman scatteringmeasurements in Gd metal made by exciting x‐ray resonant Raman scattering with circularly polarized x rays near the L III edge of Gd are presented. The incident photon energy was fixed at the peak of the 2p→4f quadrupolar transition, and the scatteredphoton was energy analyzed around the 3d→2p fluorescent energy. Asymmetry ratios in the scattered intensity much larger than that of the L III XMCD effect (x‐ray magnetic circular dichroism) were observed upon reversal of the magnetization of the sample or the helicity of the photon. A detailed comparison of these results with XMCD results from both the M IV,V edges and L III edge of Gd metal will be discussed.

Use of deep reactive ion etching in the fabrication of high-efficiency high-resolution crystal x-ray analyzers

Spherically bent silicon crystal x-ray analyzers have been employed in high-resolution inelastic x-ray scattering experiments to increase the counting efficiency due to the small cross-section of the inelastic scattering processes of interest. [1] In these bent analyzers, strain causes a distribution of lattice spacing, limiting the achievable energy resolution. Hence, the silicon wafers were diced using precision diamond saws into an array of ~1x1 mm2 blocks, and then acid etched to remove the saw damage, leaving blocks ~0.6x0.6 mm2 glued to a spherical concave substrate. With this method, meV energy resolution has been demonstrated with a bending radius of 6.5 m. [2] We seek to optimize the dicing process using the technique of deep reactive ion etching (DRIE) to develop highly efficient crystal analyzers. Ideally, each individual block subtends an angle that matches the acceptance (Darwin width) of the silicon reflection. This requires block sizes of about 500 μm2. DRIE offers the flexibility of selecting the block size, with finely controlled groove widths (i.e., minimal loss of material), and hence the possibility of controlling the energy width. We have made a prototype analyzer using DRIE with block size of 470 μm2, groove widths of 30 μm, and about 500 μm deep. The wafer was then bent and glued to a glass substrate with 2-meter radius. Tests showed encouraging results, with the DRIE analyzer performing at the 100 meV level. Details of the process and further refinements will be discussed.

Optical design and performance of the phase II inelastic scattering beamline at the National Synchrotron Light Source

We report the optical design and performance of the phase II inelastic scattering beamline at the National Synchrotron Light Source. The new beamline consists of a four-crystal Si(220) monochromator followed by a bent cylinder mirror. The monochromator is tunable from 5 to 10 keV with about 0.2 eV energy resolution throughout the tuning range. The size of the focused beam is about 0.5 mm (H)×0.3 mm (V).

Multi-element Analyzer for Inelastic X-Ray Scattering

A nine‐element analyzer‐system for inelastic x‐ray scattering has been designed and built. Each individual analyzer crystal is aligned with an inverse joy‐stick goniometer. The energy of the scattered photons is measured with a conventional double‐circle goniometer mounted on a translation stage, which allows the maintenance of the sample, analyzers and detector, also mounted on a separate translation stage, on a Rowland‐circle.