G. Dalba

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.

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 systems affected by negative thermal expansion. Recent results on copper, germanium and Ag2O are presented and critically compare

X-ray absorption spectroscopy study of crystalline ReO3 was performed at the Re L3-edge in the temperature interval from 77 to 670 K. The analysis of both XANES and EXAFS regions shows that in ReO3, a progressive localization of the 5d states following the temperature increase is accompanied at T > 350 K by a distortion of the ReO6 octahedra. The distortion increases continuously up to the ReO3 decomposition temperature Td≈ 673 K and leads to the lowering of the rheniurn site symmetry from Oh(T≤350K) →D4h(T≈513K)→C4v(T≈668K). The observed distortion is explained by (i) the strong electron-phonon coupling of the 5d electron with the M3 and M'z3 phonons and (ii) the Jahn-Teller-like effect caused by the rhenium 5d1 electron which becomes localized at high temperature

The circular bulk photovoltaic effect in the piezoelectric cystal La₃Ga₅SiO₁₄:Pr (Syimetry group D₃) is discovered. The circular tensor components β₃₃ and G₃₃ ^C are measured.

EXAFS spectroscopy has been used to study and compare the local structure in systems of various kinds and degrees of disorder: c-Ge, a-Ge, AgI and AgI-doped superionic glasses. The non-standard cumulant analysis of EXAFS has allowed an accurate characterization of local disorder of both thermal and static origin. Non-Gaussian distributions of interatomic distances and the corresponding anharmonic pair potentials have been reconstructed in a model-independent way from experimental results. Harmonic and anharmonic contributions to thermal disorder have been singled out, leading to refined comparisons with harmonic vibrational models. A strong correlation of vibrational motion of silver and iodine nearest-neighbour ions has been found in both β-and α-AgI. In amorphous systems the cumulant method allows the accurate characterization of local static disorder; as for a-Ge, interatomic distance and coordination number are in very good agreement with neutron diffraction results.