Raman study of P–T phase diagram for U₃O₈

Abstract

Raman spectroscopy is very sensitive to the U‐O bonding strength changes under high pressure, and it could reveal important information about the bonding and stoichiometry of the U‐O systems. The pressure and temperature (P–T) phase diagram of U3O8 was investigated by Raman spectroscopy via four independent P–T experimental paths. The phase boundaries between different phases have been determined by the changes in Raman spectra. A new phase (α′‐U3O8) was observed below 233 K at 0.6 GPa or 198 K at zero pressure accompanied by the discontinuities in Raman wavenumber and the new shoulder peak near the B2(6) Raman modes. Upon compressing at low temperature (123 K), the high‐pressure irreversible phase transition from orthorhombic to cubic structure (α′‐δ) was observed at around 11.3 GPa, evidenced by the merging of B2(6) and B′2(6) Raman modes.