Metastable NaYF₄ fluorite at high pressures and high temperatures

International audienceHigh-pressure high-temperature behavior of metastable NaYF4 fluorite (Fm3- m, Z=4), with the Na and Y atoms randomly distributed in the cationic sublattice, is studied with synchrotron angle-dispersive powder X-ray diffraction in diamond anvil (DAC) and large-volume Paris–Edinburgh cells and synthesis in a multi-anvil apparatus. The onset of a pressure-induced phase transition at room temperature takes place above 10 GPa as observed in DACs loaded with different hydrostatic and non-hydrostatic pressure media (nitrogen, paraffin oil, or ethanol:methanol media). In situ powder X-ray diffraction measurements in the Paris–Edinburgh cell and syntheses using the multi-anvil apparatus at high pressures and high temperatures show that the new polymorph is of the gagarinite-type (P63/m, Z=1) with partially ordered cations, the formula being Na1.5Y1.5F6. This phase is structurally related to the Na1.5Y1.5F6 modification (P6- , Z=1) stable at ambient conditions. At higher temperatures, the new pressure-induced hexagonal variant of NaYF4 eventually decomposes into a non-stoichiometric gagarinite-like phase and yttrium fluoride YF3 (Pnma, Z=4)