Raman and X‐ray Studies of Uranium–Lanthanum‐Mixed Oxides Before and After Air Oxidation

Abstract

UO 2 samples doped with 6, 11, 22 mol% lanthanum were examined before and after air oxidation. To verify the formation of uranium-lanthanum-mixed oxide solid solutions, powder X-ray diffraction (XRD) analyses of the crystalline phases in the materials were carried out. The presence of oxygen vacancies in the La-doped UO 2 samples was identified by Raman spectrometry. It was evidenced by changes induced in the Raman spectra by air oxidation. This latter was carried out either by increasing the Raman laser power or by thermally treating the samples at 500 K for 370 h. In addition, oxidation behavior differences of pure and La-doped UO 2 samples were reported by comparing XRD and Raman results of the samples before and after air oxidation. It was shown that the concentration of the M 4 O 9 (M: U, La) phase increased with increasing content of La, whereas inhibition for the formation of M 3 O 8 phase was observed.

Chemically stabilized δ-Bi2O3 phase: Raman scattering and X-ray diffraction studies

“…We suggest that this mode at 719 cm -1 is related to oxygen defects in the structure. Similar behavior has been observed in fluorite structure having punctual defects [29][30][31] , i.e. interstitial oxygens as in case of UO 2+x 31 or oxygen vacancies as in UO 2 doped trivalent metals 29,30 .…”

Melting behaviour of americium-doped uranium dioxide

“…Nonetheless, it can be deduced from the present results that a similar ideal solution behaviour is most likely followed by the (U, Am)O 2 mixture at temperatures close to melting to an even larger extent than assumed by Kato et al Apparently, this ideal solution behaviour seems even to be compatible with a complete reduction of Am to Am +III , at least for the current low Am-doping levels. Such a behaviour is approximately followed at lower temperatures by similar solid state solutions between UO 2 and trivalent cation oxides such as Bi 2 O 3 and La 2 O 3 [48,49]. In reality, the fact that high-temperature ideal solution behaviour is observed brings along no certainty about the system behaviour at lower temperatures, where the cation valence states were measured by XAS analysis.…”

The dissolution of helium in La-doped UO2 as a surrogate of hypo-stoichiometric UO2

“…The oxygen diffusion coefficients for hypo-stoichiometric UO 2 were reported almost two orders of magnitude higher than those of stoichiometric UO 2 [40]. Differences between the three He-release profiles could thus be due to the acceleration of oxygen diffusion rather than grain size effect and its effect on helium diffusion when the content of dopant increases, hence when the content of oxygen vacancies is higher [33].…”

“…Powders were pressed at 110 MPa into disks that were subsequently sintered at 1923 K under reducing atmosphere (6% H 2 , 94% Ar atmosphere) for 6 hours. In order to verify the formation of U 1-y La y O 2-y/2 solid solutions and the presence of oxygen vacancies in the structure, X-ray diffraction (XRD) and Raman spectroscopy analyses of the samples were performed and discussed elsewhere [33]. The existence of oxygen vacancies in La-doped UO 2 samples was demonstrated by the presence of a band at~540 cm −1 in their Raman spectra.…”

“…In the present study, after infusion (under the same conditions), dissolved He quantities in (U 0.94 La 0.06 )O 1.97 , (U 0.89 La 0.11 )O 1.95 , and (U 0.78 La 0.22 )O 1.89 samples were measured as 4.05 × 10 −7 , 4.32 × 10 − 7 , and 5.13 × 10 −7 mol g −1 , respectively with ±2.4% relative standard deviation. It should be realized that the higher La-concentration in UO 2 introduces higher concentration of oxygen vacancies in the lattice [33] which means dissolved helium quantities were slightly increased by increasing oxygen vacancy concentration in the samples. Therefore, our experimental results support the conclusion of Yakub's theoretical study.…”