Pierre Bouvier

Metastable NaYF4 fluorite at high pressures and high temperatures

Abstract High-pressure high-temperature behavior of metastable NaYF4 fluorite (Fm 3 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 (P 6 , 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).

High-pressure phase transitions in BiFeO3: hydrostatic versus non-hydrostatic conditions

We report high pressure X-ray diffraction experiments on BiFeO3 (BFO) single crystals in diamond-anvil cells up to 14 GPa. Two data sets are compared, one in hydrostatic conditions, with helium used as a pressure-transmitting medium (PTM), and the other in non-hydrostatic conditions, with silicon oil as a PTM. It is shown that the crystal undergoes different phase transitions in the two cases, highlighting the high sensitivity of BFO to non-hydrostatic stress. Consequences for the interpretation of high pressure structural studies are discussed.

Pressure-induced phase transition(s) in KMnF3 and the importance of the excess volume for phase transitions in perovskite structures

We report a pressure-dependent investigation of KMnF(3) by x-ray diffraction up to 30 GPa. The results are discussed in the framework of Landau theory and in relation to the isostructural phase transition in SrTiO(3). The phase transition temperature near 186 K in KMnF(3) shifts to room temperature at a critical pressure of P(c) = 3.4 GPa; the pressure dependence of the transition point follows ΔP(c)/ΔT(c) = 0.0315 GPa K(-1). The transition becomes second order under high pressure, close to the tricritical point. The phase transition is determined by the rotation of MnF(6) octahedra with their simultaneous expansion along the rotation axis. The rotation angle was found to increase to 10.5° at 24 GPa. An additional anomaly was observed at higher pressure around 25 GPa, suggesting a further phase transition.