Raman study of P–T phase diagram for U3O8

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.