STRUCTURAL EVOLUTION OF A URANYL PEROXIDIE NANO-CAGE FULLERENE: U60, AT ELEVATED PRESSURES

U60 is a uranyl peroxide nano-cage with a highly symmetric fullerene topology; its isomer is identical to C60. Several studies on the aqueous-phase of U60: [UO₂(O₂)(OH)]₆₀^60-, have shown its persistence in complex solutions and over lengthy time scales. U60 (Fm-3) crystallizes with approximate formula: Li₆₈K₁₂(OH)₂₀[UO₂(O₂)(OH)]₆₀(H₂O)₃₁₀. We have used the diamond anvil cell (DAC) to examine U60 to understand the stability of this cluster at high pressures. We used a symmetric DAC with 300 μm culet diamonds and two different pressure-transmitting media: a 4:1 mixture of methanol+ethanol and silicone oil. By using in situ Raman spectroscopy and synchrotron powder x-ray diffraction (XRD), and electrospray ionization mass spectroscopy (ESI-MS) ex situ, we have determined the pressure-induced evolution of U60. Crystalline U60 undergoes an irreversible cubic to tetragonal phase transition at 4.1 GPa, and irreversibly amorphizes between 11-14 GPa. This amorphous phase likely consists of clusters of U60. Above 15 GPa, the U60 cage is irreversibly destroyed—ESI-MS shows that this phase consists of species that likely have 10-20 uranium atoms. Raman spectroscopy of U60 shows two modes: a symmetric stretch of the uranyl U-O bond at 810 cm^-1, and a symmetric stretch of the U-O₂-U peroxide bond at 830 cm^-1. As pressure is increased, these two modes shift to higher wavenumbers, and eventually overlap at 4 GPa. At 15 GPa, their intensity is below detection. These vibrational results complement the synchrotron XRD measurements. U60 degrades quickly in methanol and ethanol mixtures, as evidenced by broad, diffuse diffraction patterns even at low pressures, however, in silicone oil, crystalline U60 remains intact for several hours. In both studies, the phase transition and amorphization were irreversible. These experiments reveal several novel behaviors: U60 undergoes irreversible phase transitions, it has a tetragonal high-pressure phase, and the amorphization of U60 occurs before the collapse of the cluster. Notably, this is different from the behavior of solvated C60, which maintains a hcp structure up to 30 GPa, while the clusters become disordered. These results suggest that uranyl nano-cage clusters may be very persistent once formed, regardless of the state of the cluster—crystalline, amorphous, or even in solution.