I. Davoli

Structure of densified vitreous silica: Silicon and oxygen XANES spectra and multiple scattering calculations

The decrease of the mean Si-O-Si angle in vitreous silica upon densification from 2.20 to 2.36 gcm-3 has been followed by oxygen and silicon K-edge XANES spectroscopy. Multiple scattering calculations using clusters of two shells around the oxygen and silicon atoms, respectively, are in good agreement with experimental absorption spectra and confirm mean Si-O-Si angles between 130 and 144° for these samples, and a decrease of the mean angle with densification. The experimental spectra also exhibit features due to scattering at outer (>2) shells around the photoabsorbers.

Experimental evidence for the “shake-down” peak in LIII (and LII)-xanes of light rare earth intermetallics

Abstract LIII and LII x-ray absorption near edge structure, XANES spectra of light rare earths R = La, Ce, Pr, Nd and Sm in isomorph intermetallic compounds RPd3 have been measured at the Frascati Synchrotron Radiation Facility. We report evidence for break down of one-electron interpretation of XANES in light rare earth compounds where a localized valence band [4f] is hybridized with a delocalized valence band. A “shake-down” feature in the low energy side of the 2p→ 5, ed white line in the XANES of LaPd3 and CePd3 has been found. The intensity of the “shake-down” peak decreases going from LaPd3 to heavier isomorph compounds with decreasing 4f hybridization. In NdPd3 and SmPd3 the “shake-down” is quenched. The intensity of the “shake-down” is much weaker than the corresponding shake-down satellites in the 3d core level XPS spectra (in LaPd3 the shake-down is 12% of the main one-electron white line in XANES to be compared with a shake-down satellite 90% of the core direct photoemission line in XPS). Our results show that the multi-electron excitations play a minor role in XANES.

The local electronic structure of PdO crystal and PdO catalyst supported on SiO2 and γ-Al2O3 from L3 and L1 x-ray absorption Pd edge in XANES spectra

Abstract We have studied the local electronic structure of PdO catalysts supported on characteristic inert and quasi-inert substrate from experiment on X-ray absorption near edge structure XANES using Synchrotron radiation. From the joint analysis of L 3 absorption edge and XPS core data in Pd and PdO we find that the white line in PdO XANES is an excitonic state with 0.8 eV binding energy. From the joint analysis of L 3 and L 1 edges the p-like and the d-like local unoccupied electronic states have been determined. The local structure of PdO catalysts is different from that of PdO crystal. Evidence for structural disorder in PdO catalysts and PdO-substrate interaction is reported.

Internal friction in high Tc iron doped 1-2-3 yttrium ceramic superconductors

Abstract The internal friction, Youngs modulus and shielding effect in yttrium ceramics doped with various amounts of iron have been measured in the temperature range 20 – 160 K. There exist several internal friction peaks, some of them common for all specimens. Only one, around 100 K, might be inexplicitly associated with the superconducting properties. No anomaly in the internal friction has been found at the transition temperature.

Low-temperature mechanical energy dissipation phenomena in lanthanum superconductors

Abstract The anelastic properties of ceramics which belong to the lanthanum family of superconductors were studied. The internal friction and Youngs modulus measurements were carried out by the vibrating reed technique in the temperature range from 20 to 300 K. The measurement frequency was in the range of 90–760 Hz. A large internal friction, corresponding to a maximum of the relaxation, was observed at a temperature about 50–60 K. An interesting correlation of both the peak height and the relaxation strength with the dopant content x has been found. An electronic relaxation phenomenon was considered as a possible source of the low-temperature internal friction maximum. The well known HTT→LTO phase transition was observed through both a pronounced change in Youngs modulus and the internal friction maximum. Also the LTO→LTT phase transition was indirectly observed through an anomalous change in the Youngs modulus.

Xanes analysis on pyroxenes with different ca concentration in M2 site

X-ray Absorption Near-Edge Structure (XANES) analysis of the calcium K-edge of a series of natural pyroxenes is reported. The samples belong to the solid solution series diopside (CaMgSi2O6) — jadeite (NaAlSi2O6). In diopside, the M2 site is occupied by Ca only, but along the join Na substitutes Ca in this position. From XANES analysis of different samples we found a distortion of the polyhedron around the M2 site varying as a function of Ca content. This is probably due to compression of the site in a selected direction with an unchanged average distance, so that the coordination around the Ca atom changes from the 4-2-2 configuration typical of the diopside structure to the 6-2 configuration typical of Na in the jadeite structure. Intermediate pyroxenes exhibit both configurations, and acquire therefrom the structural order as already detected by X-ray diffraction techniques.

Partial density of unoccupied states and L2,3-x-ray absorption spectrum of bulk silicon and of the Si(1 1 1) 2 × 1 surface

Abstract The L2,3 surface absorption spectrum of the clean Si(1 1 1) 2 × 1 surface has been measured and compared with the bulk spectrum. The results are interpreted by comparison with tight binding calculations of the s density of states. The 2p core hole final state effect is shown to shift all absorption features in the bulk up to 3 eV above threshold by ≈ 0.3 eV. A surface feature 2.3 eV above the conduction band bottom has been identified.

Local electronic structures at selected sites of intermetallic perovskites Mn3MeX (Me=divalent metal, X=N, C)

SummaryWe have measured the XANES (X-ray absorption near edge structure) spectra on the manganese and metal MeK-edges of the intermetallic perowskites Mn3MeX (X=N, Me=Mn, Cu, Zn, Ga, Sn; X=C, Me=Ga, Sn) to investigate the distribution on the partial density of the empty states. The information on the local electronic structure at the two differents sites, obtained from the spectra in the first 10 eV above the onset, is discussed in terms of one-electron band model. The relative shape changes in the spectra between the different compounds are explained by the different magnitude of hybridization between the electronic states of different atoms and/or the filling of the empty band by the electrons supplied by the substituted metal. Moreover, information about the local geometrical structure has been obtained from the (10÷50) eV range of the spectra.RiassuntoLa distribuzione della densità parziale degli stati vuoti nelle perovskiti intermetalliche Mn3MeX (X=N, Me=Mn, Cu, Zn, Ga, Sn; X=C, Me=Ga, Sn) è stata studiata mediante spettroscopia XANES (X-ray absorption near edge structure) alle soglieK del Mn e del metallo Me. Le informazioni sulla struttura elettronica locale di entrambi i siti, ottenute dall’analisi dei primi 10 eV degli spettri XANES, sono discussed in termini di un modello proposto. Il cambiamento relativo della forma negli spetttri dei diversi composti è posto in relazione alla differente ibridizzazione tra stati di atomi diversi e/o dal riempimento degli stati della banda vuota con elettroni forniti dal metallo sostituente. Sono state anche ottenute informazioni sulla struttura geometrica locale analizzando gli spettri nell’intervallo da 10 a 50 eV.

Anelastic effects in CuO

Abstract The anelastic effects in CuO were measured at the frequency of a few hundred Hz in temperature range 20–300 K. The samples were prepared according to the procedure known for the yttrium ceramic superconductors. The spectra show distinct similarity to those of the yttrium ceramic superconductors. The only noticeable differences are visible in the range near the superconducting transition Tc where two peaks exist in the ceramic superconductor. The outgasing of the CuO sample at temperature of 200 °C reveals the appearance of two new peaks at temperatures 130 K and 155 K. The antiferromagnetic transition at 215 K and an unknown origin phenomena at 35 K were observed which do not depend on the outgasing procedure. The results show that at least some of the properties of the cupric oxide are preserved in the yttrium ceramic superconductors.

Sexafs study of the natural oxide on iron surface detected by total photoelectron yield

Abstract We have applied the total photoelectron yield method on ferritic-iron surface to measure the EXAFS of the natural oxide layer at the Frascati Synchrotron Radiation facility (PULS). We have found that the oxide contribution to the spectrum is relevant at low values of photoelectron wavevector k . The Fe O distance d = 2.04 A has been determined. The effective sampling depth of the total yield mode at ≅ 7 keV has been found. The energy dependence of the sampling depth of total yield in the X-ray range from 1.5 to 9 keV is found to increase with photoenergy.