H. Graafsma

Improving the performance of high-resolution X-ray spectrometers with position-sensitive pixel detectors

A dispersion-compensation method to remove the cube-size effect from the resolution function of diced analyzer crystals using a position-sensitive two-dimensional pixel detector is presented. For demonstration, a resolution of 23 meV was achieved with a spectrometer based on a 1 m Rowland circle and a diced Si(555) analyzer crystal in a near-backscattering geometry, with a Bragg angle of 88.5 degrees . In this geometry the spectrometer equipped with a traditional position-insensitive detector provides a resolution of 190 meV. The dispersion-compensation method thus allows a substantial increase in the resolving power without any loss of signal intensity.

Refinement of framework disorder in dehydrated CaA zeolite from single-crystal synchrotron data

An accurate knowledge of zeolite structure is required for understanding their selective sorption capacities and their catalytic properties. In particular, the positions of the exchangeable cations and their interactions with the framework are essential. The present study deals with the accurate crystal structure determination of a fully exchanged and fully dehydrated CaA zeolite (Ca(48)Al(96)Si(96)O(384), Fm3c, a = 24.47 A) using single-crystal high-resolution synchrotron X-ray diffraction [(sin straight theta/lambda)(max) = 1.4 A(-1)]. It is shown that cation exchange severely distorts the skeleton, especially around the O2 atom. The high-resolution synchrotron data reveal that this latter O atom is disordered and lies out of the mirror plane it occupies in other A-type zeolites. This feature is related to that observed for Ca(2+) cations.

Pulsed laser deposition chamber for in situ X-ray diffraction

A sample chamber has been constructed for studying the growth of thin films by pulsed laser deposition in situ with surface X-ray diffraction. The achievable temperature ranges from room temperature to 1073 K in a controlled oxygen environment. The partial pressure of the oxygen background gas covers the range from 0.1 to 10(5) Pa. The first results, showing intensity oscillations in the diffracted signal during homoepitaxial deposition of SrTiO(3), are presented.

Deconvolution of the two-dimensional point-spread function of area detectors using the maximum-entropy algorithm

The maximum-entropy method (MEM) has been applied for the deconvolution of the point-spread function (PSF) of two-dimensional X-ray detectors. The method is robust, model and image independent, and only depends on the correct description of the two-dimensional point-spread function and gain factor of the detector. A significant enhancement of both the spatial resolution and the contrast ratio has been obtained for two phase-contrast images recorded with an ultra-high-resolution X-ray imaging detector. The method has also been applied to a Laue diffraction image of a protein crystal, showing an important improvement in both the peak separation of closely spaced diffraction peaks and the signal-to-noise ratio of medium and weak peaks. The principle of the method is explained and examples of its application are presented.

Polarization switching in BaTiO3 thin films measured by X-ray diffraction exploiting anomalous dispersion

Films of BaTiO 3 ranging from 20 nm to 300 nm in thickness were grown with pulsed laser deposition on Nb:SrTiO 3 . The quality of the layers was investigated using atomic force microscopy, X-ray reflectivity and X-ray diffraction. Both the micrographs and the X-ray reflectivity spectra indicate a smooth surface of the layers. The X-ray diffraction profiles measured using synchrotron radiation show features characteristic for highly crystalline thin films. The application of an external electric field parallel to the c axis changes an hkl reflection of BaTiO 3 to an hkl reflection. Due to the anomalous dispersion, the intensities of these two reflections are not equal and the atomic displacements can be determined from the intensity differences. The electric field-induced intensity changes can be as large as a few percent, which makes data collection from a 100 nm film using Cu Kα radiation from an X-ray tube feasible. The ΔI/I values of a number of reflections from the 20 and 50 nm films were measured using synchrotron radiation. The observed ΔI/I values were in good agreement with the intensity changes expected for polarization switching in the bulk.

A 1000°C furnace for in situ X-ray diffraction

A furnace covering the temperature range from 25 to 1000°C has been designed and constructed to study in situ solid-state reactions and melting and crystallization processes, with X-ray diffraction in transmission geometry using a two-dimensional-detector system. The oven can work in low vacuum and under a controlled atmosphere.

Electric-field-induced structural changes measured with a CCD-coupled X-ray image intensifier

The conventional method to measure small induced changes in integrated intensity utilizes a zero-dimensional detector, which greatly limits the data collection speed. This paper shows that the use of an area detector in combination with an X-ray chopper decreases the data collection time significantly. A monochromatic diffraction setup using a charge-coupled device (CCD) detector coupled to an X-ray image intensifier was constructed and tested. The setup proved to be sufficiently stable to measure changes in integrated intensity well below 0.1%. Subsequently, a data set of a piezoelectric KD2PO4 crystal in an external electric field was collected. The data were merged to yield 77 unique reflections. The induced structural changes were determined by a least-squares refinement. The results agree very well with experiments in which a zero-dimensional detector was used. The major improvement is the decrease in data collection time by one order of magnitude, without any degradation of the data quality, offering new possibilities for this type of perturbation study.

Time-resolved x-ray diffraction study of the relaxation process of electric-field-induced strain in KD2PO4

In this article, we report on measurements of the strain relaxation process in KD2PO4. The application of a static electric field to a piezoelectric crystal generates stresses, which are released by the introduction of strain. Time-resolved x-ray diffraction experiments on square platelike samples indicated that the strain relaxation process strongly depends on the rise time of the electric field. In the case of an instantaneously applied electric field (rise time 30 ns, i.e., much shorter than the response time of the crystal), the induced strain waves were only weakly damped and still present 400 μs after activating the external electric field (which corresponds to roughly 50 round trips of the strain wave). The strain waves can be suppressed completely by employing a rise time equivalent to the intrinsic response time of the crystal. Measurements on a bar-shaped crystal indicated that the strain waves are generated at the edges of the crystal and propagate along the length of the crystal at the speed o...

Towards extinction-free experimental diffraction data on Al2O3

Using 58 keV (0.214 A) synchrotron radiation we have obtained virtually extinction-free data on Al2O3. Refinement of a multipole model against these data produced deformation densities of high quality, which compare well with theoretical maps.

Texture Analysis of Coated Conductors by Micro-Raman and Synchrotron x-ray Diffraction

Two novel complementary and non-destructive techniques for texture analysis of YBCO coated conductors are presented. Micro-Raman (μ-Raman) spectroscopy enables an easy analysis of the film homogeneity by determining the distribution of aand c-oriented grains within the tape with a 1 μm spatial resolution and acquisition times of 5 min/spot. In addition, synchrotron x-ray diffraction analysis in transmission geometry and 2D detectors enables, with acquisition times of 100 ms, simultaneous observation of aand ccrystallites and in-plane textures of substrates, buffers and superconducting layer. Thus, opening the field of this technique to in-situ analysis of epitaxial growth of coated conductors prepared by ex-situ processes. Results from these two techniques for stainless steel tapes buffered with IBAD-YSZ and coated with PLD-YBCO are analyzed.