Francesco Rocca

Italian National Research Council | CNR · Institute for Photonics and Nanotechnologies IFN

Scandium fluoride (ScF3), having cubic ReO3-type structure, attracts much scientific attention due to its rather strong negative thermal expansion (NTE) in the broad temperature range from 10 to 1100 K. Here we use the results of diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy to interpret the influence of NTE on the t...

Scandium fluoride ScF3 has a simple cubic structure and attracts attention due to its large negative thermal expansion (NTE) over a wide range of temperatures (0-1100 K). In this study we present ab initio molecular dynamics (AIMD) simulations of ScF3 and their validation using the Sc K-edge EXAFS spectra in the temperature range from 300 K to 1000...

We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD data and for ab initio calculations of electronic and vibration properties of materials with negative thermal expansion. Ab initio electronic structure and lattice dynamics calculations for cubic and distorted ScF3 were performed using the linear combi...

Strontium titanate is a model quantum paraelectric in which, in the region of dominating quantum statistics, the ferroelectric instability is inhibited due to nearly complete compensation of the harmonic contribution into ferroelectric soft mode frequency by the zero- point motion contribution. The enhancement of atomic masses by the substitution o...

This chapter provides a brief description of X-ray absorption fine structure spectroscopy (XAFS) theory and analysis methods, with particular attention to glasses. Examples of state-of-the-art studies in different glassy systems are presented along with a critical assessment of XAFS potential for such systems. The chapter commences with a note on t...

Strontium titanate is a model quantum paraelectric in which in the region of dominating quantum statistics the ferroelectric instability is inhibited due to nearly complete compensation of the harmonic contribution into ferroelectric soft mode frequency by the zero-point motion contribution. The enhancement of atomic masses by the substitution of 1...

The thermal expansion of polycrystalline ReO3 is studied in vacuum by x-ray diffraction from 300 to 600 K and by extended x-ray absorption fine structure (EXAFS) at the Re L3 edge from 30 to 600 K. A detailed EXAFS analysis is presented up to the sixth coordination shell around Re. The crystal thermal expansion is weak throughout the explored range...

Silver iodide-silver molybdate glasses, (AgI)(x)(Ag2MoO4)(1 - x), with x = 0.75 and x = 0.67, exhibit high ionic conductivity, comparable to that of some ionic liquids at room temperature, and low values of the calorimetric glass transition temperature. The behaviour of the attenuation of acoustic waves as a function of temperature in silver iodomo...

In this work we report on the local structure of Ti in SrTi 16 O 3 (STO16) and SrTi 18 O 3 (STO18) investigated in the low temperature range (6–300 K) by extended x-ray absorption fine structure and x-ray absorption near edge structure (XANES) spectroscopy at Ti K-edge and by optical second harmonic generation (SHG). By comparing XANES of STO16 and...

The interaction at the interface between chromium and amorphous Silicon (a-Si) films in the presence of a sandwich layer of chromium oxide is investigated using X-ray absorption fine structure (XAFS) spectroscopy. The oxidized interface was created, in situ, prior to the deposition of a 400 nm tick a-Si layer over a 50 nm tick Cr layer. The entire...

Erbium doped tellurite glasses (TPE), having composition (70TeO(2)-(30 - x)ZnO-xPbO)(0.99)-(Er2O3)(0.01) (x = 5, 10, 15, and 20), were prepared by melt quenching technique and characterized by density measurements, Raman and optical spectroscopies and extended X-ray absorption fine structure (EXAFS) experiments. The main structural modifications of...

The evaluation of uncertainty in temperature-dependent EXAFS measurements is discussed, considering the specific case of a recent experiment performed on CdTe. EXAFS at both Cd and Te K -edges was measured at different times and at different beamlines in a temperature range from 5 to 300 K. Attention is focused on the nearest-neighbours parameters:...

The role of thin metallic layer (Chromium or Nickel) in the crystallization of a-Si film has been studied using X-ray absorption fine structure spectroscopy (XAFS). The films were grown at different substrate temperatures in two different geometrical structures : (a) a 200 nm metal layer (Cr or Ni) was deposited on fused silica (FS) followed by 400...

We investigate theoretically the sensitivity of Cu K-edge XANES and XMCD of Cu-doped ZnO (Cu:ZnO) to the local structure. We find that the sensitivity is high enough to enable to distinguish between geometries conceivable for Cu:ZnO. However, one cannot distinguish between Cu and Zn in further coordination shells. Likewise, presence of oxygen vacan...

We present a comparison of the local structure of erbium in zinc-lead tellurite glasses and in silica-hafnia glass ceramics waveguides prepared using the melt quenching or sol-gel and dip coating techniques respectively. The local environment of Er3+ions was probed using the extended x-ray absorption fine structure (EXAFS) at the L3-edge of erbium...

The effect of substrate temperature on amorphous Silicon crystallization, mediated by metal impurity is reported. Bilayers of Ni(200nm)/Si(400nm) are deposited on fused silica substrate by electron beam evaporator at 200 and 500 °C. Raman mapping shows that, 2 to 5 micron size crystalline silicon clusters are distributed over the entire surface of...

Cu doped ZnO polycrystalline pellets were synthesized with Cu concentrations varying from 2 to 10 wt% by a solid state reaction route (mixing of ZnO and CuO powders). Global magnetization measurements showed that all the samples were paramagnetic. Fitting the temperature-dependence of the magnetization to the Curie-Weiss law revealed the presence o...

Erbium activated PbO-ZnO tellurite glasses ((70TeO(2)-(30-x)ZnO-xPbO)(0.99)-(Er(2)O(3))(0.01) (TZPE), (x = 5, 10, 15, 20)) were prepared by a melt quenching process and studied by optical absorption, luminescence, Raman and x-ray absorption spectroscopy measurements as a function of the PbO/ZnO ratio. The glass structure, as monitored by Raman scat...

Nickel induced crystallization of amorphous Si (a-Si) films is investigated using transmission electron microscopy. Metal-induced crystallization was achieved on layered films deposited onto thermally oxidized Si(3 1 1) substrates by electron beam evaporation of a-Si (400 nm) over Ni (50 nm). The multi-layer stack was subjected to post-deposition a...

The mechanism involved in the metal induced a-Si crystallization has been investigated by X-ray absorption spectroscopy for Cr. The growth process involves the deposition on fused silica (FS) of FS/Cr(200nm)/a-Si(400nm) stacks (FCS) and FS/a-Si(400nm)/Cr(200nm) stacks (FSC) at different substrate temperatures by electron beam evaporation technique....

The extended x-ray absorption fine structure (EXAFS) has been measured at both the K edges of cadmium and tellurium in CdTe, from liquid helium to room temperature, in order to investigate the local thermodynamic behaviour. The temperature dependences of the structural parameters obtained from the separate analysis of the two edges are perfectly co...

Amorphous Si (a-Si) and Ni films were deposited by electron beam evaporation on to borosilicate glass (BSG) substrate maintained at ambient temperature. The BSG/a-Si/Ni stack was subjected to post deposition annealing in air at various temperatures from 200 to 500 °C for 1 h. Electron diffraction was employed to characterize the crystallographic ph...

Zn K edge and O K edge x-ray absorption near-edge structure (XANES) spectra of ZnO surfaces are calculated. The difference between theoretical XANES for ZnO surfaces and ZnO bulk is then compared to the earlier observed differences between experimental XANES for ZnO nanostructures and ZnO bulk as taken from the literature. It follows from our calcu...

A comparative review is presented of a range of physical and chemical properties of borate and silicate glasses. Most attempts to explain the difference between the properties of these glasses, in terms of the so-called borate (or boron oxide) anomaly, have focussed on the change in co-ordination number of some of the boron atoms from three to four...

In order to assess the usability of X-ray absorption near-edge structure (XANES) for studying the structure of BO(n)-containing materials, the dependence of theoretical XANES at the B K-edge on the way the scattering potential is constructed is investigated. Real-space multiple-scattering calculations are performed for self-consistent and non-self-...

CdTe is affected by a low-temperature negative thermal expansion (NTE) of the lattice parameter, whose strength and temperature interval are intermediate between those of the iso-structural crystals Ge and CuCl. EXAFS measurements have been performed on CdTe from 19 to 300 K. The first-shell analysis has led to an accurate evaluation of the bond th...

The analysis of the EXAFS signals from 70 Ge and 76 Ge has evidenced the low-temperature effect of isotopic mass difference on the amplitude of relative atomic vibrations. This effect is reflected in the difference of the Debye-Waller factors of the first three coordination shells, and on the difference of nearest-neighbour average interatomic dist...

Mechanical spectroscopy allows the investigation of the relaxation processes, associated with the hopping of mobile ions occurring in ionic glasses well below their glass transition temperature. In the present work we study the properties of the secondary mechanical relaxation in the fast ion conducting glass (AgI)x[(Ag2O)·n(B2O3)]1−x with n = 2 an...

In this work, we report on the spectroscopic characterization of erbium doped HfO2 nanocrystals in SiO2-HfO2 glass-ceramic waveguides, fabricated by sol-gel process. The results are compared with those of single crystals grown using the flux growth technique. XRD and Raman measurements showed that the single crystals are monoclinic, while in the gl...

Cu K-edge x-ray absorption near-edge structure (XANES) spectra of trigonal (3R) CuScO(2) and CuLaO(2) and of hexagonal (2H) CuScO(2) were investigated experimentally and theoretically, in order to study differences between spectra of isostructural and isoelectronic compounds. Significant differences were found in the Cu K-edge XANES of 3R CuScO(2)...

Ionic glasses obtained combining silver iodide and silver molybdate are characterized by quite low values of the glass transition temperature T(g) around 320-350 K, by high values of the dc ionic conductivity even at room temperature and by a peculiar behavior of the mechanical response at ultrasonic frequencies. In fact, at temperatures well below...

This paper describes the structure of Er3+-doped SiO2–HfO2 waveguides containing nanocrystals of HfO2. Pure and 1 mol% Er3+-doped 70SiO2–30HfO2 films were deposited by the sol–gel method on amorphous SiO2 substrates using the dip-coating technique. Each waveguide has experienced a single thermal treatment at temperatures ranging from 900 to 1200 °C...

Extended x-ray absorption fine structure has been measured at the K edge of copper in CuScO2 and CuLaO2 to investigate the local origin of negative thermal expansion (NTE) along the c axis. A positive expansion of the Cu-O bond has been measured within the entire temperature interval to be contrasted with the negative expansion of the distance betw...

A large number of AgI-based fast-ion-conducting glasses have been investigated by K-iodine extended x-ray absorption fine structure spectroscopy (EXAFS) measurements at liquid nitrogen temperature. A general correlation between the I-Ag distance measured by EXAFS and the glass activation energy for dc ionic conductivity has been found out: glasses...

EXAFS investigation about Metal Induced Crystallization (MIC) of a-Si thin films doped with Ni, has been carried out at the K edge of Ni. Several a-Si films deposited on quartz and annealed at different temperatures and a non-annealed sample have been analyzed in order to study the variation of the nickel surroundings as a function of temperature....

In the last few decades, X-ray Absorption Spectroscopy (XAS) has recorded a remarkable amount of interest in the study of glass structure for its sensitivity to the Short Range Order (SRO) around a selected atomic species. XAS greatest evolution started in the seventies of the last century thanks to an adequate and structurally useful theoretical i...

The structural, electronic and dynamical properties of crystals are mainly dependent on the atomic number of the constituent atoms. However, the isotopic composition has subtle but non-negligible influence on some basic properties, like density, phonon widths,and electronic energy gaps [1]. Isotopic effects are relevant not only for their basic sci...

Extended x-ray absorption fine structure has been measured on two powdered samples of $^{70}\mathrm{Ge}$ and $^{76}\mathrm{Ge}$ as a function of temperature from 20 to 300 K. The effect of isotopic mass difference on the amplitude of relative atomic vibrations is neatly evidenced by the temperature dependence of the difference of Debye-Waller facto...

Investigations of complex nanostructured materials used in modern technologies require special experimental techniques able to provide information on the structure and electronic properties of materials with a spatial resolution down to the nanometer scale. We tried to address these needs through the combination of x-ray absorption spectroscopy (XA...

The short-range order around molybdenum has been investigated in AgI-doped silver molybdate glasses (AgI)(x)(Ag2MoO4)(1-x) (with x = 0.67 and 0.75) by Mo-K edge EXAFS measurements as a function of temperature. The difference from crystalline Ag2MoO4 is weak. A softening of the Mo-O nearest-neighbours bond has been detected, but the MoO4 units still...

The first results obtained with the prototype system at the synchrotron beamline ID03 at ESRF are presented and illustrate the possibility to detect an element-specific contrast and to perform nanoscale x-ray absorption spectroscopy experiments at the Zn K and W L 3 absorption edges in mixed zinc oxide-zinc tungstate thin films.

Zn K-edge XANES in ZnO has been calculated within the full-multiple-scattering (FMS) and finite difference method (FDM) formalism using the ab initio FDMNES code. The influence of non-muffin-tin potential, bulk defects, surface termination and polarization effects on XANES has been analysed. The obtained theoretical results are compared with availa...

This paper describes and compares the local environments of Er3+ ions in pure SiO2, SiO2-Al2O3, SiO2-TiO2, and SiO2-HfO2 glasses, and in SiO2-HfO2 glass-ceramic waveguiding systems. This structural information is useful for selecting waveguides with optimised functional properties. This comparison has emphasised the peculiarity of the SiO2-HfO2 gla...

Er{sup 3+}-doped SiOâ-HfOâ glassy waveguides with HfOâ concentrations ranging from 10 to 50 mol % were prepared using the sol-gel route and deposited on v-SiOâ substrates using the dip-coating technique. The local environment around Er{sup 3+} ions was determined from Er Lâ-edge extended x-ray-absorption fine-structure (EXAFS) measurements. The fir...

Silicon-rich silicon oxide films deposited by plasma enhanced chemical vapor deposition with different total Si contents from 39 to 46 at. % have been annealed at increasing temperature up to 1250 ° C in order to study the Si nanocrystal Si-nc nucleation as well as the structural changes induced in the amorphous embedding matrix. The comparison bet...

Extended x-ray absorption fine structure (EXAFS) measurements have been performed at the K edge of iodine in (AgI)1−y(Ag2MoO4)y glasses, with y = 0.25 and 0.33, as a function of temperature from 25 K to the glass transition temperature Tg 323 K. EXAFS monitors the short-range component of the I–Ag distance distribution. At 25 K, iodine coordinates...

Extended x-ray absorption fine structure EXAFS has been measured from liquid helium to ambient tem-perature at the Cu K edge of copper chloride CuCl to investigate the local origin of negative thermal expansion. A quantitative analysis of the first coordination shell, performed by the cumulant method, reveals that the nearest-neighbor Cu-Cl interat...

The potential of x-ray absorption near-edge structure (XANES) spectroscopy for studying medium range order in borate glasses is assessed by theoretical modelling of the spectra. B K edge XANES is calculated in case that B atoms are located in isolated BO3 and BO4 units and in case that B atom are located in superstructural units of 9-15 atoms. It i...

EXAFS has been measured on ReO3 from 30 to 600 K; XRD has been contemporarily measured above 300 K. In this way, it has been possible to compare the expansion of the lattice parameter (XRD) and of the bond lengths (EXAFS), measured at the same time. EXAFS was interpreted by the cumulant approach, using ReO3 measured at low temperature as referen...

The local environment around iodine has been studied as a function of temperature by extended X-ray absorption fine structure (EXAFS) in fast-ion-conducting AgI doped glasses (borate and molybdate): silver borate glass (AgI)0.55(Ag2O:4·B2O3)0.45 has been investigated from 39 to 298 K, and silver molybdate glasses (AgI)x(Ag2MoO4)1−x, with x = .67 an...

We present a Kirkpatrick‐Baez optics (KB) system specially optimized to operate in the 6–13 keV X‐ray range, where valuable characteristic lines are present. The mirrors are coated with aperiodic laterally graded (Ru/B4C)35 multilayers to define a 15% energy bandpass and to gain flux as compared to total reflection mirrors. For any X‐ray energy se...

Purans, Juris Dalba, Giuseppe Fomasini, Paolo Kuzmin, Alexei De Panfilis, Simone Rocca, Francesco

Er 3+-doped SiO 2-HfO 2 glassy waveguides with HfO 2 concentrations ranging from 10 to 50 mol % were prepared using the sol-gel route and deposited on v-SiO 2 substrates using the dip-coating technique. The local environment around Er 3+ ions was determined from Er L 3-edge extended x-ray-absorption fine-structure EX-AFS measurements. The first coo...

Er 3+ -doped silica waveguides, co-doped with aluminium, titanium, and hafnium oxides, were prepared using the sol–gel method and dip-coating processing. Here, we present a characterisation of the local environment around Er 3+ ions, as determined from the Er L 3 -edge extended X-ray absorption fine structure (EXAFS) measurements performed at ESRF...

This paper reports a detailed structural study on the nucleation of t-HfO2 nanocrystals in thin films of 70SiO2–30HfO2 prepared by sol–gel route on v-SiO2 substrates. Thermal treatment was performed at different temperatures ranging from 900 to 1200°C for short (30min) or long (24h) time periods. Crystallisation and microstructure evolutions were t...

EXAFS measures the thermal variation of average interatomic distances, while Bragg diffraction and dilatometric techniques measure the variation of distances between average atomic positions. The difference not only gives information on vibrational motion perpendicular to the bond direction, but can be exploited to study the local behaviour in syst...

Copper and silver oxide share the rather unusual cuprite structure, formed by two interpenetrating networks of corner-sharing M4O tetrahedra (M = Cu, Ag). Negative thermal expansion of the cell parameter has been observed from 5 to about 200 K for Cu2O and up to 470 K for Ag2O by powder diffraction. In the case of Ag2O, it has been shown by EXAFS t...

Cuprite-type oxides (Cu2O and Ag2O) are framework structures composed by two interpenetrated networks of metal-sharing M4O tetrahedra (M=Cu, Ag). Both compounds exhibit a peculiar negative thermal expansion (NTE) behaviour over an extended temperature range (9–240 K for Cu2O, 30–470 K for Ag2O). High-accuracy synchrotron powder diffraction and EXAF...

EXAFS temperature dependent measurements at the I K-edge have been done on AgI rich silver borate glasses containing crystallites of AgI stabilised in the α-phase at room temperature, as obtained by fast roller quenching. Data analysis was performed in the cumulant expansion approximation, to distinguish between structural and vibrational disorder,...

The potential of XANES spectroscopy for studying the structure of boron-containing compounds is assessed by theoretical modelling of B K edge spectra of minerals. Spectra arising from three-fold coordinated boron sites differ considerably from spectra arising from four-fold coordinated sites. The characteristic shape of the spectra is governed firs...

The dielectric response of borate glasses Ag2O.nB2O3 has been investigated as a function of the composition using a broadband measurement technique in the frequency range 10−3 Hz-109 Hz. This approach reveals a d.c. plateau at low frequencies, a power law dispersive behaviour at intermediate frequencies, and a nearly constant loss regime in the hig...

An XAFS study on several samples of borate glasses with increasing PbO concentrations was performed in order to detect a possible clustering of Pb atoms or PbO structures. Samples with concentrations from 28.6 up to 75% of PbO were examined at the L3 edge of Pb. Analysis of data was carried out by experimental comparison with low temperature crysta...

High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure (EXAFS) measurements have been performed on the iso-structural framework crystals CuâO and AgâO as a function of temperature. According to diffraction, both compounds exhibit a negative thermal expansion (NTE) of the lattice parameter over extended temperature int...

EXAFS can represent a powerful probe of the local behaviour of negative thermal expansion (NTE) materials, thanks to the possibility of measuring the expansion of selected inter-atomic bonds and the perpendicular relative atomic displacements. The effectiveness of EXAFS for NTE studies is illustrated by a comparison of results recently obtained on...

The local structure around Ag ions in silver borate glasses g-Ag 2 O·nB 2 O 3 n =2,4 was studied by x-ray absorption spectroscopy at the Ag K edge for temperatures from 77 to 450 K. Extended x-ray absorption fine structure EXAFS analysis based on cumulant expansion or multishell Gaussian model fails for these systems. Therefore, the radial distribu...

Silica-hafnia glass-ceramics waveguides activated by Er3+ ions were fabricated by sol-gel route. X ray diffraction and optical spectroscopy showed that after an adapted heat treatment, the resulting materials showed a crystalline environment. Analysis of the luminescence properties has demonstrated that erbium ions are, at least partially, trapped...

The sensitivity of x-ray absorption spectroscopy (XAS) to the local structure around a selected atomic species symmetry, distances and angles is commonly exploited to quantitatively describe systems where all the configurations around the absorbing atoms may be approximated by the mean one. However, in many cases of disordered systems a simple desc...

Glasses from the Ge-rich region [Ge(0.5-0.33x Ag 0.33xSe0.5] of the Ge-Se-Ag phase diagram are synthesised for x=0.6, 0.7 and 0.8. For comparison a special sample Ge3Se 9Ag4 (Ge0.1875Se0.562Ag 0.25) from the Se-rich region has also been prepared. For the concentrations studied both the x-ray scattering curves as well as the radial electron distribu...

Copper and silver oxide share the rather unusual cuprite structure, formed by two interpenetrating networks of corner-sharing M4O tetrahedra (M = Cu, Ag). Negative thermal expansion of the cell parameter has been observed from 5 to about 200K for Cu2O and up to 470K for Ag2O by powder diffraction. In the case of Ag2O, it has been shown by EXAFS tha...

Accurate temperature-dependent EXAFS measurements can give original information on the correlation of thermal motion in crystals, not only along the bond direction (parallel Mean Square Relative Displacement, MSRD), but also within the perpendicular plane (perpendicular MSRD). This potential is demonstrated by the results obtained for copper and ge...

Extended x-ray-absorption fine-structure (EXAFS) of copper has been measured from 4 to 500 K and analyzed by the cumulant method, to check the effectiveness of EXAFS as a probe of local dynamics and thermal expansion. The comparison between parallel mean square relative displacements (MSRD) of the first four coordination shells has allowed detectin...

A great progress has been made in the last years in the XAFS studies of thermal and structural disorder, due to the increased quality of measurements allowed by the intense third generation synchrotron radiation sources, and the refinement of the analysis techniques. A description of XAFS strengths and limits is given and some paradigmatic investig...

The local coordination of Ag cations in silver borate glasses Ag2O⋅nB2O3 has been studied by comparing the experimental x-ray absorption near edge structure (XANES) at the Ag K edge with results of theoretical simulations. We demonstrate that simple models which describe the local structure around Ag with a single geometric configuration cannot be...

Si nanocrystals (Si-nc) embedded in amorphous silica matrix have been obtained by thermal annealing of substoichiometric SiO x films, deposited by PECVD (plasma enhanced chemical vapour deposition) technique with different amount of Si concentrations (42 and 46 at.%). Both nucleation and evolution of Si-nc together with the changes of the amorphous...

The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO 2 has been studied by x-ray absorption spectroscopy (XAS) and by ab-initio total energy calculations. Si-nc have been formed by PECVD deposition of SiO x with different Si content (from 35 to 42 at.%) and thermal annealing at high temperature (1250 °C). T...

A combined approach has been used to study thermal effects on the extended x-ray absorption fine-structure (EXAFS) of copper between 4 and 500 K. A phenomenological data analysis shows that the thermal expansions measured from the first and third cumulants significantly differ between each other and from the crystallographic thermal expansion. Path...

Report: In the last years, the XAFS experimental techniques have undergo remarkable developments: experiments with unprecedented accuracy under extreme conditions of high pressure and temperature, that were not even conceivable just a few years ago, can nowadays be performed. New applications, stimulated by accurate experimental temperature-depende...

Light-emitting silicon nanocrystals embedded in SiO 2 have been investigated by x-ray absorption measure-ments in total electron and photoluminescence yields, by energy filtered transmission electron microscopy and by ab initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocryst...

The local coordination around Ag cations in silver borate glasses has been studied by comparing their experimental x-ray absorption near edge spectra (XANES) at the Ag K edge with results of theoretical simulations. Different trial structural models have been tested using known crystallographic environments as starting points for further modificati...

X-ray absorption spectroscopy (XAS) at the Si–K absorption edge has been applied to study the influence of size and chemical-physical surface effects on the optical properties of porous silicon (PS). The comparison between XAS spectra obtained by monitoring both total electron yield (TEY) and photoluminescence yield (PLY) has allowed us to discrimi...

EXAFS temperature dependent measurements have been carried out on the borate glass doped with silver iodide, (AgI)0.55(Ag2O.4B2O3)0.45, at the K-edges of Ag and I, in the range 39-500 K. Local negative thermal expansion has been measured in the explored temperature range; in the same range, crystalline AgI shows a positive trend. This behaviour, al...

X-ray absorption spectra obtained by total electron yield (TEY) at the Si absorption K-edge have been measured to have chemical and structural information about Si nanocrystals (Si-nc) produced by plasma-enhanced chemical vapour deposition (PECVD). The TEY technique has been employed to investigate the formation of Si-nc and the modiÿcation of the...

X-ray absorption measurements in total electron yield mode have been carried out on Si nanodots embedded in amorphous silica produced by plasma enhanced chemical vapor deposition (PECVD). The amount of Si atoms composing the Si nanodots and the chemical composition of the amorphous host matrix has been determined thanks to the comparison with Ruthe...

Temperature dependent extended X-ray absorption fine structure (EXAFS) measurements give original information on local structure and dynamics of materials with framework structure, exhibiting negative thermal expansion. By proper cumulant data analyses, the temperature dependence of the nearest neighbours distance can be directly mea-sured and the...

Original information on local thermal expansion can be obtained through a cumulant analysis of EXAFS. The difference between first and third EXAFS cumulants, and the comparison with Bragg diffraction results, can help in disentangling the contributions to thermal expansion of potential anharmonicity and geometrical effects. In germanium, the perpen...

Si nanocrystals (Si-nc) embedded in amorphous silica matrix have been obtained by thermal annealing of substoichiometric SiOx films, deposited by PECVD (plasma enhanced chemical vapour deposition) technique with different amount of Si concentrations (42 and 46 at.%). Both nucleation and evolution of Si-nc together with the changes of the amorphous...

The local environment of light emitting silicon nanocrystals (Si-nc) embedded in amorphous SiO2 has been studied by x-ray absorption spectroscopy (XAS) and by ab-initio total energy calculations. Si-nc have been formed by PECVD deposition of SiOx with different Si content (from 35 to 42 at.%) and thermal annealing at high temperature (1250 °C). The...

Rare-earth (Pr3+, Tb3+, Er3+) doped silica xerogels were studied by x-ray absorption spectroscopy and x-ray diffraction. A change of the local environment around rare-earth ions upon xerogel densification at 900–950°C and co-doping with aluminum ions was determined from the rare-earths L3-edge EXAFS signals. The densification process induces a decr...