Raman spectroscopic studies of defect structures and phase transition in hyper-stoichiometric UO2+x

Abstract

A method to determine the defect structures in hyper-stoichiometric UO(2+x) using a combination of XRD and Raman spectroscopy has been developed. A sequence of phase transitions, from cubic to tetragonal symmetry, occurs with increasing degree of non-stoichiometry. This sequence proceeds from a cubic phase through an intermediate t''-type tetragonal (axial ratio c/a = 1) phase to a final t-type tetragonal (c/a not = 1) phase. Four distinct structural defect regions can be identified in the stoichiometry range, UO(2) to U(3)O(7): (i) a random point defect structure (x (in UO(2+x)) < or = 0.05); (ii) a non-stoichiometry region (0.05 < or = x < or = 0.15) over which point defects are gradually eliminated and replaced by the Willis 2:2:2 cluster; (iii) a mixture of Willis and cuboctahedral clusters (0.15 < or = x < or = 0.23); (iv) the cuboctahedral cluster (x > or = 0.23). The geometry and steric arrangement of these defects is primarily determined by the concentration of the excess-oxygen interstitials.

Uranium Oxide Nanocrystals by Microwave‐Assisted Thermal Decomposition: Electronic and Structural Properties

“…To synthesize UO 2 nanoparticles without an additional reduction step of uranium(VI) starting materials, a microwaveassisted decomposition using the uranium(IV) compound 4 ] was performed. This compound exhibits preformed U-O bonds and predetermined breaking points, which enables a single-step decomposition of the precursor and nucleation of UO 2 nanocrystals.…”

“…Chem. 2018, 12-18 www.zaac.wiley-vch.de wave assisted decomposition of the uranium(IV) complex [U(DMOTFP) 4 ] without the need of additional reducing agent or co-catalyst. The particles show a narrow size distribution of 6.4 Ϯ 1.2 nm.…”

“…To investigate the role of the microwave method compared to pure thermolysis, the following experiments were performed: (a) thermal decomposition of [U(DMOTFP) 4 ] at 330°C in air, (b) heating up the precursor in DEG to 330°C and (c) hot-injection of a solution of [U(DMOTFP) 4 ] in oleylamine into preheated (350°C) trioctyl phosphine (TOP). [30] Whereas simple heating of the precursor only yielded a black, amorphous powder, both thermolysis in air and hot-injection resulted in the formation of crystalline UO 2 as shown by XRD analysis ( Figure S4, Supporting Information).…”

“…[4] There are a several examples in the literature for the synthesis of uranium oxide nanoparticles via solution-based methods. [5] A brief review of selected uranium oxide nanoparticle syntheses relevant to this report follows.…”

Radiolytic Corrosion of Uranium Dioxide: Role of Molecular Species

“…It presents two characteristic bands around 445 cm -1 and 560 cm -1 . The former is affected to the triply degenerated Raman active mode (T 2g ) of U-O stretch 13,[40][41]37 whereas the latter characterizes the defects present initially in the UO 2 fluorite structure 37 .…”