High-Pressure Phase Transition of Coffinite, USiO₄

Abstract

Synchrotron powder diffraction patterns and Raman spectra of synthetic coffinite, USiO4, were collected for pressures up to 35 GPa and are complemented with DFT+U-based calculations. USiO4 undergoes a first-order phase transition from a zircon-type (space group I41/amd) to a scheelite-type structure (space group I41/a) at ≈15 GPa and ambient temperature. Contrary to earlier reports, the data indicate that this transition is completely reversible upon pressure release. Bulk moduli were obtained from the p–V data for the zircon-type and scheelite-type USiO4 phases. For zircon-type USiO4, the value for B = 186(5) GPa, whereas, for the scheelite-type phase, B = 204(9) GPa, where the latter is significantly lower than a value proposed earlier (B = 274(16) GPa, [Zhang, F. X.; et al. Am. Mineral. 2009, 94, 916]). Lattice dynamical calculations point toward a Γ-point soft mode triggering the pressure-induced phase transition.