High-Pressure Phase Transition in Yttrium Iron Garnet
Wednesday, 17 December 2014
High-pressure investigations of rare earth transition metal oxide garnets have revealed structural and magnetic phase transitions, including pressure-induced amorphization, magnetic collapse, spin crossover transitions and metallization, as well as transformation to highly-incompressible oxides. Yttrium iron garnet (YIG), Y3Fe5O12, exhibits interesting magnetic properties, behaving as a ferrimagnet with a Néel temperature TN ~ 559 K at ambient pressure and transforming to a paramagnetic material at high temperatures. At high temperatures and pressures, it converts to a GdFeO3-type perovskite structure. However, the composition of the perovskite is debated, with two proposed reaction pathways, leading to either (Y0.75Fe0.25)FeO3–perovskite or a mixture of YFeO3-perovskite and Fe2O3. Here, we synthesized YIG-perovskite in the diamond anvil cell above 18 GPa and 1500 K at GeoSoilEnviroCARS beamline 13-ID-D of the Advanced Photon Source. NaCl was used as a quasihydrostatic pressure medium, and Au, with the Fei equation of state, was used as the pressure calibrant and laser absorber. Our measurements of the perovskite structure extend to 70 GPa with annealing at each pressure step. We do not find any iron oxide diffraction peaks after heating, indicating that the perovskite structure is likely (Y0.75Fe0.25)FeO3. We observe a previously unknown ~5% volume discontinuity in the perovskite between 44 and 50 GPa. This discontinuity is not accompanied by a change in the diffraction pattern, suggesting that the Fe in this structure undergoes a high-spin to low-spin transition. When compressing without laser heating, we confirm the amorphization of YIG starting material at 50 GPa that was previously observed by Gavriliuk et al. (2006). This amorphous material is easily converted to the low-spin perovskite upon heating above 1400 K.