No abstract

A new mother wavelet function for extended X-ray absorption fine-structure (EXAFS) data analysis has been designed, combining a model EXAFS function derived from the ab initio EXAFS code FEFF8.20 and the complex Morlet wavelet. This new FEFF-Morlet mother wavelet routine allows the generation of wavelets well adapted to specific EXAFS problems. A substantial gain in resolution of the wavelet ridges in k and r space is achieved. The method is applied to a structural problem of Zn-Al double-layer hydroxides, demonstrating unequivocally the homogeneity of the metal cation distribution in the hydroxide layers.

Conventional x-ray transmission tomography provides the spatial distribution of the absorption coefficient inside a sample. Other tomographic techniques, based on the detection of photons coming from fluorescent emission, Compton and Rayleigh scattering, are used for obtaining information on the internal elemental composition of the sample. However, the reconstruction problem for these techniques is generally much more difficult than that of transmission tomography, mainly due to self-absorption effects in the sample. In this article an approach to the reconstruction problem is presented, which integrates the information from the three types of signals. This method provides the quantitative spatial distribution of all elements that emit detectable fluorescent lines (Z⩾15 in usual experimental conditions), even when the absorption effects are strong, and the spatial distribution of the global density of the lighter elements. The use of this technique is demonstrated on the reconstruction of a grain of the martian meteorite NWA817, mainly composed of low Z elements not measured in fluorescence and for which this method provides a unique insight. The measurement was done at the ID22 beamline of the European Synchrotron Radiation Facility.

The technological success of phase-change materials in the field of data storage and functional systems stems from their distinctive electronic and structural peculiarities on the nanoscale. Recently, superlattice structures have been demonstrated to dramatically improve the optical and electrical performances of these chalcogenide based phase-change materials. In this perspective, unravelling the atomistic structure that originates the improvements in switching time and switching energy is paramount in order to design nanoscale structures with even enhanced functional properties. This study reveals a high-resolution atomistic insight of the [GeTe/Sb 2 Te 3 ] interfacial structure by means of Extended X-Ray Absorption Fine Structure spectroscopy and Transmission Electron Microscopy. Based on our results we propose a consistent novel structure for this kind of chalcogenide superlattices.The need for fast and efficient management of information stimulates research on materials that can be switched on nanometer length scales and sub-nanosecond time scales. Phase-Change materials (PCMs) possess a unique property portfolio, which is ideally suited for memory device applications [1][2][3][4][5][6] . A PCM is identified by its ability of switching rapidly and reversibly between a crystalline and an amorphous state, where the amorphous state is obtained by melting the crystalline state followed by rapid quenching. These two states significantly differ in their properties, such as the optical reflectivity as well as the electrical conductivity. The phase transformation is in general triggered by thermal heating, or by either electrical and optical pulses of different time duration and amplitude. The large contrast in reflectivity between these two states lays at the base of already working PCM-based optical rewritable media devices-like DVDs or Blu-Ray Disc-where information is encoded as amorphous marks in a crystalline background. The contrast in resistivity could be exploited in the next generation of electronic solid-state memories based on PCMs, which might replace the current leading storage technologies, namely FLASH and magnetic disks. Furthermore, these materials could be employed in displays or data visualization applications by combining both their optical and electronic property modulations 7 . Hence, a lot of interest and effort is currently devoted to uncover the complex physical origin of the high contrast between the two phases [8][9][10]

Uptake of iodine in hydrotalcite-like minerals is a potential retardation mechanism for dose-relevant 129 I in the near-field of a deep repository for radioactive waste. The location of iodide in (Zn/Mg)Al layered double hydroxides (LDH) was investigated using a combination of advanced atomic-scale techniques. Wavelet transform analysis of Zn Kedge extended X-ray absorption fine structure (EXAFS) spectra and geometry optimization based on ab initio density functional calculations allowed the distribution of Al 3+ in the cationic layer to be determined. Using Rietveld refinement of synchrotron X-ray powder diffraction data (XRD) and EXAFS at the I K-edge enabled the average location of iodide in the interlayer to be established. Additional short-and medium-range structural information was also obtained from the pair distribution function analysis of the XRD data in support of the findings obtained with the long-and short-range techniques. By combining the results, a local order of Al 3+ in Zn 2 Al−I and Zn 3 Al−I LDHs was shown generating hexagonal and orthorhombic supercells, respectively. Furthermore, an uncorrelated distribution between I − anions and Zn 2+ /Al 3+ cations was demonstrated, resulting from a dynamic disorder of water and iodide position in the interlayer space.