IN-SITU INVESTIGATION OF U3O8 PHASE SPACE VIA POWDER X-RAY DIFFRACTION AND RAMAN SPECTROSCOPY

The high-temperature calcination product (T > 600°C) of numerous uranium oxides and oxysalts, U₃O₈, is an important intermediate material in the production of nuclear fuel and may also be present as an alteration product thereof. In addition, the sheet structure of uranium polyhedra in U₃O₈ is found in several uranyl minerals. Previous studies have observed a phase transition (C2mm or Amm2 to P-62m) upon heating, and the synthetic conditions used to prepare U₃O₈ may determine the temperature at which this transition occurs. In the low temperature structural arrangement, uranium occupies two distinct crystallographic sites. However, only one uranium site is present in the high-temperature hexagonal phase. The implications of this increased symmetry, particularly for charge-balancing mechanisms in hexagonal U₃O_8, remain unclear. To provide clarity, the orthorhombic to hexagonal phase transformation has been explored in detail for U₃O₈ samples resulting from a range of synthetic preparation conditions. In-situ temperature dependent high-resolution powder X-ray diffraction (XRD) and Raman spectroscopy were employed to describe the structural attributes and lattice dynamics of these phase transitions as related to the initial calcination temperature of U₃O₈. Powder XRD data suggest a gradual transition to the hexagonal phase upon heating as evidenced by convergence of reflections associated with the low-symmetry modification. Numerous convergence temperatures were observed, which implies a multi-step phase change. Furthermore, the structural transition temperatures appear to be sensitive to calcination conditions used to produce U₃O₈, which is reflected in structural and vibrational observables.