Further insights into the chemistry of the Bi–U–O system

Abstract

Cubic fluorite-type phases have been reported in the U(IV)O2-Bi2O3 system for the entire compositional range, but an unusual non-linear variation of the lattice parameter with uranium substitution has been observed. In the current extensive investigation of the uranium(iv) oxide-bismuth(iii) oxide system, this behaviour of the lattice parameter evolution with composition has been confirmed and its origin identified. Even under inert atmosphere at 800 °C, U(IV) oxidises to U(V)/U(VI) as a function of the substitution degree. Thus, using a combination of three methods (XRD, XANES and Raman) we have identified the formation of the BiU(V)O4 and Bi2U(VI)O6 compounds, within this series. Moreover, we present here the Rietveld refinement of BiU(V)O4 at room temperature and we report the thermal expansion of both BiU(V)O4 and Bi2U(VI)O6 compounds.

New insight in the uranium valence state determination in U Nd1−O2±

“…Hexavalent uranium U 6þ is not observed in our case below y ¼ 0.2, confirming that U 6þ formation in a fluorite structure is probably energetically too expensive compared to the formation of two U 5þ cations. This is in good agreement with the absence of U 6þ in the early oxidation stages of UO 2 where U 4þ /U 5þ mixtures are reported in U 4 O 9 and U 3 O 7 [32], or in the cases of U 1Ày Bi y O 2±x and U 1Ày La y O 2±x where only U 5þ formation is reported up to y ¼ 0.5 [15] and y ¼ 0.4, respectively [35]. These upper limits are about two times the value found for Nd doped UO 2 and suggest that the dominating mechanism depends on the dopant nature.…”

“…Finally, when the Nd concentration increases, the probability to find two Nd atoms as first neighbors increases as well, leading to the competitive compensation mechanisms of forming one V O , two U 5þ cations or one U 6þ cation. Then, when the concentration of Nd is well above y ¼ 0.5, U 5þ is converted to U 6þ , with a possible formation of V O , as suggested by comparison to U 0.40 Bi 0.60 O 1.95 [15]. However, both experimental and theoretical assessments are needed, especially in the y ¼ 0.2e0.7 concentration range, to confirm or deny these assumptions and give a complete understanding of the charge compensation mechanisms in the U 1Ày Nd y O 2±x system.…”

“…This approach, high energy resolution fluorescence detected-XAS (HERFD-XAS), allows highly precise determination of electronic and local structures of actinides, including direct evaluation of valence state mixtures [14]. For example, this approach has been successfully applied to (U,Bi)O 2 mixed oxides [15].…”

Charge compensation mechanisms in Nd-doped UO2 samples for stoichiometric and hypo-stoichiometric conditions: Lack of miscibility gap

“…XANES: comparison with other ternary systems Other ternary systems have been also studied through XANES to analyse the local structure surrounding uranium atoms by measuring the L 3 -edge or the M 4 -edge spectra. Popa et al studied the U-Bi-O system also through HERFD-XANES by analysing the U M 4 -edge [38]. The study used samples varying the concentration of Bi from 15% until 79%.…”

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study

“…Experimentally, High Energy Resolution X-ray Absorption Near Edge Structure (HR-XANES) and Resonant Inelastic X-ray Scattering (RIXS) have recently emerged as very sensitive tools for characterisation of oxidation states and electronic structures of actinide elements. [28][29][30][31][32][33][34][35][36][37][38] In particular, since they directly probe the An 5f valence unoccupied orbitals, they can distinguish small variations in the chemical bonding at the metal for uranyl(VI) and uranyl(V) compounds. These experimental spectroscopic techniques are most readily interpreted in combination with computational analyses.…”