Formation of β-U₃O₈ from UCl₃ Salt Compositions under Oxygen Exposure

Complementary X-ray absorption fine structure (XAFS) and Raman spectroscopy studies were conducted on various UCl₃ concentrations in alkali chloride salt compositions. The samples were 5 mol % UCl₃ in LiCl (S1), 5 mol % UCl₃ in KCl (S2), 5 mol % UCl₃ in LiCl-KCl eutectic (S4), 50 mol % UCl₃ in KCl (S5), and 20 mol % UCl₃ in KCl (S6) molar concentrations. Samples were heated to 800 °C and allowed to cool to room temperature with measurements performed at selected temperatures; the highest temperatures showed the most stability and will be primarily referenced for conclusions. The processing and interpretation of the Raman and extended X-ray absorption fine structure (EXAFS) peaks revealed several uranium-oxygen bond lengths and symmetries in the samples before, during, and after heating. Based on published thermodynamic data of similar systems, X-ray absorption fine structure spectroscopy, and identification of Raman peaks, a β variation of α-U₃O₈, typical at room temperature, is the suspected dominant phase of all samples at high temperatures (800 °C). In the existing literature, this β structure of U₃O₈ was synthesized by slow cooling of uranium oxides from 1350 °C. This paper suggests the rapid formation of the compound due to the decomposition of the uranium chlorides or oxychlorides at increasing temperatures and O₂ reaction kinetics.