High Pressure Phase Transition of Coffinite, USiO4

Synchrotron powder diffraction patterns and Raman spectra of synthetic coffinite, USiO4, were collected for pressures up to 35 GPa and are complemented with DFT+Ubased calculations. USiO4 undergoes a first order phase transition from zircon-type (space group I41/amd) to scheelite-type structure (space group I41/a) at 15GPa 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 phase. For zircon-type USiO4 the value for B = 186(5)GPa, while for the scheelite-type phase B = 204(9)GPa, where the latter is significantly lower than a value proposed earlier (B = 274(16) GPa1). Lattice dynamical calculations point towards a Γ-point soft mode triggering the pressure-induced phase transition.