G. Samoggia

An XPS study of yttria-stabilised zirconia single crystals

Abstract An X-ray photoelectron spectroscopy study of yttria-stabilised zirconia (YSZ) single crystals is reported. It is shown that the shape and the binding energy positions of zirconium and yttrium core levels are independent of the yttria stoichiometry when the content of this oxide ranges from 12 to 24 mol.% of Y2O3. These results could indicate that the local crystallographic order around the Zr4+ and Y3+ ions does not depend on the Zr/Y ratio.

Optical properties of cubic stabilized zirconia

Optical absorption across the fundamental edge and the associated photoluminescence, have been studied in single crystals of yttrium stabilized zirconia (YSZ) of varying yttrium concentration. The results allow discrimination of a region of intrinsic transitions (above 5.2 eV) from a low energy region, where disorder-induced transitions are dominating. The size of extra absorption in this range shows an inverse correlation to the stoichiometric ratio, to be interpreted in terms of progressive ordering of the vacancy system.

Experimental Evidence of a Fluctuating Charge State in Cupric Oxide

This letter reports on a X-ray photoelectron spectroscopy study of cupric oxide which demonstrates that the split observed in the O-1s core line arises from the fluctuating valence character of the Cu-O bond. This state is also confirmed by the valence band spectrum where the lowest binding-energy transition in the 3d9 → 3d10 L manifold is at the Fermi level allowing valence charge fluctuations at zero energy cost. Since valence fluctuation phenomena lead to a strong electron-phonon interaction, the observation of this effect also in HTSCs (high-temperature superconductors) would be a noteworthy point for an understanding of the mechanisms that originate high-Tc in cupric-oxide-based compounds.

Site-selective spectroscopy of Eu3+ doped lead germanate glasses

Abstract A series of lead germanate glasses of molar compositions (x−1)PbO–(100−x)GeO2–1Eu2O3 with x=30, 40 and 50 were prepared and studied by laser-induced fluorescence line narrowing (FLN) techniques. The energies of the Stark levels of the 7 F 1 and 7 F 2 multiplets were obtained from experimental FLN spectra and a crystal field (CF) analysis was carried out for each excitation energy. The dependence of the CF Bkq parameters on the excitation energy is qualitatively similar to the behaviour observed for Eu3+ in other oxide glasses. The CF calculated ratios B22/B20 and B44/B40 agree reasonably well with the trend predicted by the Brecher and Riseberg geometric model. The CF strength parameter S decreases with increasing lead content in the glass host. This behaviour is explained on the basis of the tendency of lead to form strongly directional Pb–O bonds. On the basis of the present CF analysis, we find no evidence of changes of the local structure at the Eu3+ sites on increasing the amount of lead and also of 6-fold co-ordinated germanium.

X-ray photoelectron study of the relaxor lead magnesium niobate

Abstract A high resolution X-ray photoemission, XPS, study of PbMg 1 3 Nb 2 3 O 3 is reported. The binding energy of the Pb core lines are consistent with a formal valence of 2+, while a valence of 5+ is found for Nb. In addition, on the basis of present data, the existence of different coordination sites for Pb, Nb and Mg is discarded. Finally, within the limit of the present experimental conditions, domains with non-homogeneous electrostatic charges, as proposed by some structural models, are not observed. Instead, the XPS results are consistent with a random distribution of Nb and Mg ions on the cationic site.

Structural properties of plasma-sprayed zirconia-based electrolytes

Zirconia-based electrolytes, stabilized either with yttrium oxide or calcium oxide, were prepared in 100 to 200 μm thick layers by plasma spraying and densified by high-temperature vacuum sintering. The structure and the microstructure were investigated by optical microscopy, scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The results are compared with the data we obtained on the powders used for plasma spraying and on single crystals. In the zirconia-yttria system, dense and fully stabilized zirconia films with structural properties similar to the corresponding single crystals were obtained. On the other hand, cracks and deformations were observed on calcia-stabilized films. This phenomenon is explained by the dramatic increase of monoclinic phase content due to the preferential evaporation of calcium oxide which occurs during high-temperature vacuum sintering.

Fluorescence line narrowing spectroscopy of Eu3+ doped sodium germanate glasses

Sodium germanate glasses of compositions (x − 0.5)Na2O–(100 − x)GeO2–0.5Eu2O3 with x = 10, 16, and 22 were studied using laser-induced fluorescence line narrowing (FLN) techniques. From the site selective luminescence spectra of Eu3+ at 20 K, the energies of the Stark components of the 7F1 and 7F2 states were obtained, and a crystal field (CF) analysis was carried out assuming a C2v site symmetry. The dependence of the CF parameters Bkq on the excitation wavenumber was similar to that observed in other oxide glasses. The behavior of the CF ratios B22/B20 and B44/B40 appears to be consistent with that calculated using the geometric model proposed by Brecher and Riseberg. The behavior of the CF strength parameter S suggests that the amount of network modifier in the glass host does not influence the CF at the Eu3+ sites. No evidence was found in the FLN spectra of the influence of 6-coordinated germanium.

Study of defects introduced by electroreduction in YSZ

Abstract The nature of lattice disorder in electroreduced Zirconia is considered in the light of electrical and optical observations. Discussion about the relative role of metallic precipitates vs. point defects and electronic centres shows new points in favour of the latter.

Fluorescence line narrowing spectroscopy of a lead germanate glass doped with Eu3

Abstract Site selective luminescence spectra of Eu 3+ in a lead germanate glass were measured at 150 K as a function of the excitation wavelength by using laser-induced fluorescence line narrowing (FLN) techniques. The energies of the Stark components of the 7 F 1 and 7 F 2 states were obtained by Gaussian fitting of the spectra. Assuming a C 2 v site symmetry, a crystal field (CF) analysis was carried out. The behaviour of the major CF ratios B 44 /B 40 versus −B 22 /B 20 seems to agree with the trend predicted by the model proposed by Brecher and Riseberg for the co-ordination of Eu 3+ in oxide glasses. No effect due to the occurrence of an incipient amorphous phase separation is observed.

Cooperative phenomena induced by Nb admixture in K1-xLixTaO3

Raman scattering observations were performed on samples of K1-xLixTa1-yNbyO3, with x in the range 0.6-1.0 mol% and y in the range 0.2-0.3 mol%. Concentrations of Li and Nb, determined by analytical methods, were below the known thresholds at which these substituents can separately induce a ferroelectric transition in the quantum paraelectric KTaO3. The temperature evolution of TO1 and TO4 spectra was monitored. The results show a critical behaviour below 50 K, characterized by an abrupt increase of first-order scattering strengths and by spontaneous splitting of the non-softening TO1 mode. From our data, we infer the occurrence of a ferroelectric phase transition of order-disorder character. This proves that small additions of Nb enhance the tendency of the Li subsystem towards cooperative ordering in the KTaO3 matrix.