Metastable NaYF_4 fluorite at high pressures and high temperatures

摘要

High-pressure high-temperature behavior of metastable NaYF_4 fluorite (Fm3m, 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 (P6_3/m, Z = 1) with partially ordered cations, the formula being Na_(1.5)Y_(1.5)F_6. This phase is structurally related to the Na_(1.5)Y_(1.5)F_6 modification (P6, Z = 1) stable at ambient conditions. At higher temperatures, the new pressure-induced hexagonal variant of NaYF_4 eventually decomposes into a non-stoichiometric gagarinite-like phase and yttrium fluoride YF_3 (Pnma, Z = 4).