Adriano Filipponi

An experimental station for advanced research on condensed matter under extreme conditions at the European Synchrotron Radiation Facility - BM29 beamline

We describe state-of-the-art experimental techniques using the beamline BM29 of the European Synchrotron Radiation Facility (ESRF). This station exploits the unique characteristics of an ESRF bending magnet source to provide a tunable, collimated, x-ray beam to perform high quality x-ray absorption spectroscopy within the energy range of E=5–75 keV using Si(111), Si(311), and Si(511) crystal pairs. Energy scans can be performed over this wide energy range with excellent reproducibility, stability and resolution, usually better than ΔE/E≃5×10−5. The experimental setup has been exploited to study condensed matter under extreme conditions. We describe here two sample environment devices; the L’ Aquila–Camerino oven for high-temperature studies up to 3000 K in high vacuum and the Paris–Edinburgh press suitable for high-pressure high-temperature studies in the range 0.1–7 GPa and temperatures up to 1500 K. These devices can be integrated in an experimental setup which combines various control and detection sys...

An extended x‐ray absorption fine structure study of aqueous solutions by employing molecular dynamics simulations

Bromine–oxygen radial distribution functions [g(r)] have been calculated by means of molecular dynamics simulations for aqueous solutions of rubidium bromide, 2‐bromopropane and bromoethane. X‐ray absorption spectra at the bromine K edge have been recorded for these solutions. The water contribution to the extended x‐ray absorption fine structure spectra has been calculated starting from the gBr,O(r) distribution function. Fits of the x‐ray absorption spectra have been performed directly on the raw experimental data, allowing the reliability of the g(r) distribution functions to be verified. The agreement between theoretical and experimental spectra is satisfactory. A procedure to improve model g(r) functions on the basis of the short‐range structural information provided by extended x‐ray absorption fine structure data is proposed.

EXAFS for liquids

Extended x-ray absorption fine structure (EXAFS) spectroscopy exploits the quantum interference resulting from the scattering of a photoelectron, generated by the excitation of a core level, by the potential of the surrounding atoms. From the interference pattern it is possible to determine the distance and average distribution of the nearest neighbours from the photoabsorbing atomic species. This spectroscopy therefore provides a unique site-selective local structural probe in condensed matter which is ideal for investigating the average environment of specific elements in a liquid. In the last 20 years we have seen substantial developments in the experimental techniques which nowadays allow scientists to perform EXAFS experiments under extreme conditions of high pressure and temperature that were not even conceivable just a few years ago. These techniques have been applied to the investigations of metals, semiconductors, molecular fluids and solutions, as a function of pressure and temperature and through phase transitions, attracting a wide scientific community towards this spectroscopy. The complete understanding of the x-ray absorption signal is however a challenging theoretical problem involving the many-electron response of the system. Our current theoretical framework for the interpretation of the EXAFS spectra is based on the solution of an effective one-electron problem. This theory is nevertheless accurate enough to be used in a quantitative data analysis able to retrieve valuable structural information.

AB-INITIO MODELLING OF X-RAY ABSORPTION SPECTRA

Abstract A new method for analyzing X-ray absorption spectra using multiple-scattering theory is presented. The basic principles and some applications are briefly discussed focusing the presentation on the general relevance of this approach. The GNXAS set of programs, especially designed to perform structural analysis on absorption data of any system, is described showing the practical applicability of the theory. The applications presented show that reliable and unique structural information on pair and higher-order distribution functions can be obtained by using this method which takes proper account of the multiple-scattering contributions.

Evidence of four-body contributions in the EXAFS spectrum of Na2Co[Fe(CN)6]

Abstract This contribution deals with an EXAFS investigation of Na 2 Co[Fe(CN) 6 ] at the K-edges of both Fe and Co. Na 2 Co[Fe(CN) 6 ], a well known and easily prepared mixed hexacyanoferrate, is characterised by a unit cell containing linear “FeCNCo” chains where a super-focusing effect may be observed at the K-edge of both transition metals. The multiple scattering (MS) signal intensity associated with the linear chain (four body contribution) is comparable with the first shell contributions. The combined fit over two edges, the intensity of the MS signal and the large number of experimental points included in the fit, permit an accurate extraction of the structural parameters at the limit of the EXAFS sensitivity.

Hydration properties and ionic radii of actinide(III) ions in aqueous solution

Ionic radii of actinide(III) cations (from U(III) to Cf(III)) in aqueous solution have been derived for the first time starting from accurate experimental determination of the ion-water distances obtained by combining extended X-ray absorption fine structure (EXAFS) results and molecular dynamics (MD) structural data. A strong analogy has been found between the lanthanide and actinide series concerning hydration properties. The existence of a contraction of the An-O distance along the series has been highlighted, while no decrease of the hydration number is evident up to Cf(III).

Triplet correlations in the hydration shell of aquaions

Abstract The presence of angular O—ion—O correlations in the hydration shell of dilute water solutions of Cu 2+ and Br − is investigated by means of an X-ray absorption experiment. An advanced data analysis including multiple-scattering effects poroduced quantitative information on these triplet correlations; the complementarity with X-ray and neutron diffraction is discussed. The experimental spectra indicate that Cu 2+ has a Jahn—Teller distorted octahedral coordination, while the hydration shell of Br − is more disordered and substantially dominated by excluded volume effects.

Hydration Properties of the Zn2+ Ion in Water at High Pressure

The structure and dynamics of water in ionic solutions at high pressure have been investigated using a combined approach based on extended X-ray absorption fine structure (EXAFS) spectroscopy and Molecular Dynamics (MD) simulations. Modification of the hydration properties of the Zn(2+) ion induced by a pressure increase from ambient condition up to ∼6.4 GPa has been revealed and accurately analyzed. With increasing pressure the first hydration shell of the Zn(2+) ion has been found to retain an octahedral symmetry with a shortening of the Zn-O distance up to 0.09 Å and an increased width associated with thermal motion, as compared to the ambient condition hydration complex. A very interesting picture of the dynamic behavior of the first hydration shell has emerged from the analysis of the simulations: up to 2.5 GPa no exchange events between first and second shell water molecules occurred, while above this pressure value several exchange events take place in the solution following an associative interchange mechanism. This result can be explained by the very high compression and packing of the solvent which force second shell water molecules to enter the Zn(2+) first hydration shell. MD simulations indicate a strong pressure effect also on the structure of the second coordination shell which is compressed and becomes more disordered and less structured with increasing pressure. The water mobility and the ion diffusion coefficient have been found to increase in the high density conditions, as a consequence of the rupture of the hydrogen bond network caused by pressure.

Development of an oven for X-ray absorption measurements under extremely high temperature conditions

Abstract We developed a high-temperature oven suitable for X-ray absorption studies that can operate from room temperature up to about 3000 K under high vacuum conditions or possibly controlled atmosphere. The main characteristic of the oven is its simplicity and quick operation. Typical heating and cooling rates can be faster than 100 K s . This is obtained by heating only a very small graphite crucible inside a glass vessel. The sample size is compatible with typical X-ray beams available and very good temperature homogeneity can be obtained. The oven can be useful to study structural properties of highly anharmonic solids, liquid elements and alloys, as well as to probe the structural effects of temperature-induced chemical reactions. Preliminary measurements on liquid germanium, liquid platinum and molybdenum carbide are presented.

Single-energy x-ray absorption detection: a combined electronic and structural local probe for phase transitions in condensed matter

We describe the general principle and prototypical applications of a largely unexploited x-ray absorption technique based on single-energy detection during temperature scans. The present developments take advantage of the highly automated experimental set-ups and unique x-ray radiation characteristics available at third-generation synchrotron radiation sources. The sensitivity of the x-ray absorption cross-section to the local structural order makes it possible to investigate the occurrence of phase transitions. We show that by tuning the photon energy to a spectral feature that is highly sensitive to the sample phase it is possible to follow the occurrence and the characteristics of the phase transition. The sensitivity to the atomic fraction of the sample in a certain phase can be in the range. The energy tunability can be used to enhance the sensitivity to structural or electronic transformations. Several applications to prototypical cases are discussed in detail, including examples of solid-solid phase transitions, melting and undercooling, melting of binary alloys, impurity melting, and non-bulk phenomena.