R.C. Karnatak

Modifications near the surface owing to hydrogen cycling of the intermetallics LaNi5 and LaNi5−xAlx

Abstract The LaNi5-LaNi4.77Al0.22 pair of alloys has been selected to simulate a cycling procedure, as may be encountered in chemical thermal engines working with hydrides. After 1250 cycles the samples have been studied by yield photoelectron spectroscopy and extended X-ray absorption fine structure (EXAFS) technique. The results, recorded at room temperature near the lanthanum and nickel thresholds, are issued from samples which were cycled in a closed system. Little or no degradation is obtained depending on the intermetallic alloy. EXAFS measurements and UV spectra, using DCI and ACO rings (LURE, Orsay), were recorded while the samples were adjusted to work either in transmission or in total yield. Thus only particles of diameter less than 25 μm were used to investigate the structure changes due to cycling and the chemical shifts shown by the atoms in a surface layer some microns thick.

Correlation-effects, charge-transfer energies and covalency in nickel compounds as determined by x-ray absorption-spectroscopy

Abstract High resolution L2,3 near edge absorption spectra of Ni compounds are compared to an impurity model calculation including the d-d correlation effects. Covalent mixing in the final state gives a change in the observed structure which can be used as an analytical tool to study the ground state properties. From the analysis we obtain the d-d Coulomb interactions, the charge transfer energy and the covalency in the ground state.

X-ray absorption spectroscopic study of specific valence in CeO2, PrO2 and TbO2

Abstract The LIII and MIV–V spectra of flourite related CeO2, PrO2 and TbO2 were studied. In these spectra the presence of spectral contributions from localized and extended type of f states is revealed. This study yields a reasonable description of the valence state in these oxides.

X-ray absorption spectroscopy of rare earth intermediate oxides☆

Abstract The MIV–V and LIII absorption spectra (between 850 and 7500 eV) of intermediate rare earth oxides (Pr7O12 and Pr9O16) were studied. These oxides required careful preparation and handling in order to assure their composition. The spectra are characterized by multiplet features that are interpreted as having contributions from both trivalent and tetravalent sites. In the MIV–V spectra the appearance of distinct multiplet lines and additional weaker features demonstrate clearly the increasing ratio of the tetravalent sites as the oxygen content increases from Pr7O12 to PrO2. Similar behavior has been observed for the CeOx and TbOx systems. These observations show that in these oxides, the trivalent and tetravalent sites are inequivalent and that the evidence of valence transition is seen in the appearance of the complex spectral features originating from the tetravalent sites.