High-pressure phase transformations in MgO-Y₂O₃ nanocomposites

Temperature and pressure dependence of phase evolution in 0.5 MgO-0.5 Y₂O₃ is studied using the diamond anvil method. At 300 K and 5.5 GPa, transformations in Y₂O₃ are observed, resulting in co-existence of cubic, hexagonal, and monoclinic phases. Heating to 1293 K results in increased crystallinity and increase in hexagonal and monoclinic phase content. Isothermal and isobaric hold at 1273 K and 5.5 GPa for 120 min results in yttrium dissolution in cubic MgO, causing 0.83 volumetric strain. Cooling to 300 K and 0 GPa yields a four phase co-existence among cubic MgO and cubic, hexagonal, and monoclinic Y₂O₃. The residual MgO unit cell volume expansion is 0.67 at 300 K, indicating solid solution formation. Aging of the nanocomposites for 240 h does not change the observed 4 phase co-existence.