Raman spectroscopy and power diffraction study of synthetic Coffinite (USiO4) at high pressures

Coffinite, USiO4, can form under reducing conditions from UO2 in contact with silica-rich waters (Langmuir’s criterion) [1]. Spent nuclear fuel (SNF) consists to > 90% of UO2, so coffinite needs to be taken into account in the safety assessment as a potential secondary phase. While high pressures are not of specific relevance for a possible final repository for SNF, its structural behaviour at high pressures is of general interest to understand the phase stabilities and to benchmark model calculations. The high pressure behaviour of coffinite has been studied before on natural samples [2,3]. A pressure-induced irreversible phase transformation from the zircon- to the scheelite-type structure was found at about 15 GPa using an alcohol-water mixtures as a pressure medium [3].
Here, synthetic coffinite was studied under high pressure conditions in the diamond anvil cell with neon as quasi-hydrostatic pressure medium up to pressures of 35 GPa. The samples are free of impurities of UO2, as characterized by XRD and HRTEM. Powder diffraction experiments with synchrotron radiation indicate a pressure-induced phase transformation at 18-20 GPa. In contrast to the earlier high pressure study [3], this transformation is reversible on pressure release and no UO2 is formed during the process. A detailed data analysis is currently in progress.
Raman spectra were obtained up to a pressure of 18 GPa. The study of the Raman spectra at higher pressures is on-going.

[1] Langmuir (1978), Geochim. Cosmochim. Acta 42, 547-569
[2] Liu (1982), Earth Plan. Sci. Lett. 57, 110-116
[3] Zhang et al. (2009), Am. Min. 94, 916-920