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This paper reports a detailed structural study on the nucleation of t- HfO 2 nanocrystals in thin films of 70 SiO 2 – 30 HfO 2 prepared by sol–gel route on v- SiO 2 substrates. Thermal treatment was performed at different temperatures ranging from 900 to 1200 °C for short (30 min) or long (24 h) time periods. Crystallisation and microstructure evolutions were traced by X-ray diffraction (XRD). The local structure around hafnium ions was determined from Hf L 3 -edge extended X-ray absorption fine structure (EXAFS) measurements carried out at the BM08-GILDA Beamline of ESRF (France). XRD shows the nucleation of HfO 2 nanocrystals in the tetragonal phase after heat treatment at 1000 °C for 30 min, and a partial phase transformation to the monoclinic phase (m- HfO 2 ) starts after heat treatment at 1200 °C for 30 min. The lattice parameters as well as the average crystallites size and their distributions were determined as a function of the heat treatment. EXAFS results are in agreement with the XRD ones, with hafnium ions in the film heat treated at 1100 °C for 24 h are present in mixed phases.

The role of metal impurities (Cr and Ni) in the crystallization of a-Si thin films has been studied using x-ray absorption fine structure spectroscopy (XAFS). The growth process involves two geometries (1) the deposition of 50 nm Cr/Ni films on fused silica substrates followed by deposition of 400 nm amorphous Si (a-Si) films at different substrate temperatures and (2) 400 nm a-Si deposition followed by 50 nm Cr/Ni deposition. X-ray Absorption studies were carried out using the BM08 GILDA Beamline of the European Synchrotron Research Facility (ESRF, Grenoble, F) in fluorescence mode. To understand the evolution of the local structure of Cr/Ni diffusing from bottom to top and from top to bottom, in metal on top and metal bottom geometries respectively, total reflection (to study top layers) and higher incidence angles (to study bottom layers) were employed. The resulting metal environment is quite complex: we applied independent XANES and EXAFS analysis to validate a method of quantifying the relative content of metal, metal oxide and metal silicides compounds in the different films. Significant differences have been found when comparing Cr- and Ni-doped films, due to the different diffusivity of metals and to the strong influence of deposition and annealing temperature. In general, from this work, we conclude that metal diffuses into the a-Si matrix and reacts with it at sufficiently high temperature to form metal silicides, in particular (Cr/Ni)Si2. Later these silicate nuclei will act as crystallizing seeds for the a-Si crystallization.

Magnetism of w-ZnO doped by transition metal atoms is an intriguing phenomenon which evades a consistent explanation. Reliable and consistent information on the local structure and local magnetic properties around the dopant is therefore very desirable. EXAFS, XANES and XMCD analysis done at the same edge is expected to be a very useful tool in this respect. Our study focuses on polycrystalline Cu-doped ZnO pellets, synthesized with Cu concentration varying from 2 to 10 weight percents by solid state reaction. Structural characterizations were carried out by x-ray diffraction and by EXAFS and XANES spectroscopies at the Cu K-edge. Magnetic properties were measured globally using a SQUID magnetometer and locally by relying on the Cu K-edge XMCD. At low Cu content, most of the Cu atoms substitute Zn in the ZnO wurtzite lattice while, for the higher Cu concentrations, Cu starts to segregate as CuO. XMCD measurements show that local moments at the Cu atoms behave paramagnetically. The calculations demonstrate that the measured XMCD signal indeed corresponds to magnetic Cu atoms in substitutional sites. We also investigated theoretically the effect of oxygen vacancies on the XANES and on the XMCD and find that the effect on the XANES is not very large while the XMCD signal changes considerably. Calculation also predicts that the replacement of Zn atoms by Cu atoms cannot be seen in the XAS, which is consistent with our experimental data.

Er3+ doped glasses are important for basic as well as applied science. Since the spectroscopic properties of Er3+ in the glasses are environment dependent, it is desirable to probe the local structure around Er3+ to understand the effect of different surroundings on such properties. In this work, we report results on EXAFS investigations about changes in local structure of Er3+ in two different systems: tellurite bulk glasses with different relative Zinc/Lead content and silica hafnia glass ceramics waveguides having different thermal history. Er-doped tellurite glasses (TZPE) [(70TeO2-(30-x)ZnO-xPbO)0.99-(Er2O3)0.01] with x=5, 10, 15, 20 were produced by melt quenching. SiO2-HfO2 glassy thin films were prepared following the sol-gel (SG) route and successive thermal treatments. Hafnia nanocrystals were grown inside Er-doped waveguides produced in two different ways: in the standard one, the glassy film 80SiO2-20HfO2 was heated at 900 °C for 30 min or at 1000 °C for 30 min and 24 hr; in an alternative way, HfO2 nanocrystals were mixed in the starting sol-gel composition and successively heat treated at 900 °C for 30 min, 24 hr and 48 hr. EXAFS measurements were performed at the BM08-Gilda Beamline of ESRF (Grenoble, F) in fluorescence detection configuration. In tellurite glasses, independently from Zn/Pb content, the structural information for the first shell of oxygen atoms around Er3+ give a constant distance of 2.34 Å, while coordination number (N) and Debye Waller factor (DW) were found to be ~6.5 and ~0.015 Å2 respectively, slightly increasing for x >= 10. Next nearest shells are difficult to quantify, due to high static disorder, indicating the absence of preferential bonds. In the silica-hafnia waveguides, the first shell coordination number of Erbium is reduced to about 5, but the most important difference is the presence of temperature dependent next nearest shell contributions, due to the progressive ordering of the HfO2 nanocrystals where Erbium ions are located. The paper will present in details the evolution of Er environment in different waveguides as a function of thermal history.

The role of nickel impurity in the crystallization of a-Si thin films has been studied using x-ray absorption fine structure spectroscopy (XAFS). The growth process involves two geometries (1) the deposition of 50 nm Ni films on fused silica substrates followed by deposition of 400 nm amorphous Si (a-Si) films at different substrate temperatures and (2) 400 nm a-Si deposition followed by 50 nm Ni deposition. X-ray Absorption studies were carried out at Ni K-edge using the BM08 GILDA Beamline of the European Synchrotron Research Facility (ESRF, Grenoble) in fluorescence mode at two different angles of incidence. To understand the evolution of the local structure of Ni diffusing from bottom to top and from top to bottom, in Ni on top and Ni bottom geometries respectively, total reflection (to study top layers) and the 2 degree incidence angle (to study bottom layers) were employed. In case of Ni bottom geometry, a clear sign of pure metallic Ni is observed in the bottom layers of the film for the deposition carried up to 300 °C. However the top layers have combined features of metallic Ni and nickel rich silicide. XAFS analysis of the Ni indicates that the film deposited up to 350 °C contains mostly unreacted metallic Ni in both top and bottom layers. When Ni is on top, no traces of metal oxide or silicide are observed in the near edge (XANES) as well as extended x-ray absorption (EXAFS) parts of the XAFS spectra. However, a sudden change to high metal oxidation is observed in case of the film deposited at 400 °C. Some traces of metallic Ni and silicides are also observed in the EXAFS part of the spectra. The quantity of metal oxidation is found to be higher in the top layers than the bottom layers, indicating that the top layers are highly oxidized. For the film deposited beyond 500 °C, Ni shows pure NiSi2 phase in both geometries. The appearance of metallic Ni on the top of 400 nm a-Si strongly supports the argument of metal diffusion mechanism in the crystallization process of a-Si thin films. At further higher deposition temperatures, the bottom layers have the combined features of metallic Ni and NiSi2, and the top layers have the combined contribution of both NiSi2 and NiO.

Most attempts to explain the difference between the properties of borate and silicate glasses, {i.e. 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. The present paper discusses the origin of this co-ordination number change at a fundamental level, together with the important role played, both by superstructural units and by network-modifying cations, in determining borate glass properties. In addition, the Krogh-Moe-Griscom superstructural unit model for the structure of borate glasses is updated in line with more modern experimental data, and used to account for the way in which the fraction of 4-fold co-ordinated boron atoms in lithium, sodium and silver borate glasses varies with composition. It is concluded that the "anomalous" properties of vitreous B2O3 and binary M2O-B2O3 glasses are entirely consistent with respect to the electron configuration and bonding of the boron atoms and to the presence of superstructural units. Extrema in the properties of the binary glasses, when plotted as a function of composition, arise due to the interplay between two (or more) opposing factors. The major remaining question concerns the origin and relative magnitudes of the stabilisation energies for the various superstructural unit species.

This paper describes the structure of Er3+-doped SiO2-HfO2 waveguides containing nanocrystals of HfO2. Pure and 1 mol% Er3+-doped 70SiO(2)-30HfO(2) 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 degrees C, for either short (30 min) or long (24 h) durations. Crystallization and microstructure were studied by x-ray diffraction (XRD). The local environments of hafnium and erbium ions were determined, respectively, from Hf and Er L-3-edges extended x-ray absorption fine structure (EXAFS) experiments. Both XRD and EXAFS results demonstrate the substitution of Hf4+ by Er3+ ions in the crystalline structure. XRD shows the nucleation of tetragonal HfO2 nanocrystals after heat treatment at 1000 degrees C for 30 min in the pure waveguide, and at 900 degrees C for 24 h in the waveguide doped with Er3+. In both series, partial transformation from tetragonal to monoclinic HfO2 nanocrystals starts after heat treatment at 1100 degrees C for 24 h. The average crystallite size and size distribution can be controlled by thermal annealing temperature and duration, respectively, with brief treatment yielding a more homogeneous nanocrystal size.

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 between average atomic positions measured by diffraction; the inadequacy of the riding correction to diffraction data has been evidenced. The weak temperature dependence of the parallel mean-square relative displacement (MSRD) indicates a rather stiff Cu-O bond, while the large perpendicular MSRD measures an intense relative motion of copper with respect to oxygen atoms perpendicular to the c axis. The comparison between the two compounds enlightens the relevance of the correlation of perpendicular motion to quantitatively explain the apparent NTE of the Cu-O bond.

Er3+-doped SiO2-HfO2 glassy waveguides with HfO2 concentrations ranging from 10 to 50 mol % were prepared using the sol-gel route and deposited on v-SiO2 substrates using the dip-coating technique. The local environment around Er3+ ions was determined from Er L3-edge extended x-ray-absorption fine-structure (EXAFS) measurements. The first coordination shell around Er3+ ions is composed of oxygen atoms. Hafnium is the main constituent of the second coordination shell of Er3+, differing from the cases of pure SiO2 and SiO2-TiO2 glassy hosts, in which silicon is the main atomic species. The local structure around Er3+ ions has been found to be independent on HfO2 concentration within the studied composition range. This fact implies that Er3+ ions are preferentially dispersed in HfO2-rich regions of the glassy waveguide, even at the lowest HfO2 concentration. For all samples, no Er3+-Er3+ coordination shell has been detected by EXAFS. The presented structural results allow us to understand some spectroscopic properties typical of Er3+-doped SiO2 glassy waveguides co-doped with HfO2.

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 in a crystalline phase. Losses measurements at different wavelength highlight a very low attenuation coefficient indicating that this nanostructured material is suitable for a single band waveguide amplifier in the C band of telecommunication.

Silica-hafnia glass-ceramics waveguides activated by Er 3+ 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 in a crystalline phase. Losses measurements at different wavelength highlight a very low attenuation coefficient indicating that this nanostructured material is suitable for a single band waveguide amplifier in the C band of telecommunication.

The non-standard cumulant analysis has been used to disentangle the various phonon displacement cumulants up to 4th order and 3rd neighbors. Ab initio phonon dynamics has been employed to compute the correlation functions up to 8th neighbors and Debye-Waller factors in the harmonic approximation. Excellent agreement between experimental and theoretical data is obtained.