Pressure effects on phase equilibria and solid solubility in MgO-Y ₂O ₃ nanocomposites

We study the temperature and pressure dependence of phase evolution in the 0.5MgO-0.5Y ₂O ₃ nanocomposite system using a diamond anvil apparatus in conjunction with in situ synchrotron energy dispersive x-ray diffraction at 7 GPa hydrostatic pressure. At (298 K, 7.0 GPa), structural transformations in the Y ₂O ₃ phase are observed, giving rise to the co-existence of its cubic, hexagonal, and monoclinic polymorphs together with cubic MgO. An increase in temperature to 1273 K causes the crystallinity of the Y ₂O ₃ hexagonal and monoclinic phases to increase. Isothermal and isobaric hold at (1273 K, 7.0 GPa) for 60 min results in yttrium dissolution in cubic MgO, causing ∼1.0 expansive volumetric lattice strain despite the large differences in the ionic radii of the cations. Cooling the nanocomposite to (298 K, 0 GPa) after a 60 min soak yields four phase co-existence among cubic MgO and cubic, hexagonal, and monoclinic Y ₂O ₃. The residual MgO unit cell volume expansion is 0.69 at 298 K, indicating solid solution formation at room temperature despite large differences in the ionic radii of Mg ²⁺ and Y ³⁺. The macroscopic shrinkage due to densification is 3 by volume. Thermodynamic considerations suggest that the relative molar partial volume of Y ³⁺ in MgO is a negative quantity, indicating that the partial molar volume of Y ³⁺ in the solid solution is smaller than its molar volume in the pure state. Aging of the nanocomposites for 240 h does not change the observed 4 phase co-existence. We propose a crystallographic model in which the observed volumetric expansion of the MgO unit cell is primarily attributed to two hydrostatic expansive strain components accompanying solid solution formation: (i) Coulomb repulsion among O ^2- ions in the immediate vicinity of Mg ²⁺ vacancies, and (ii) misfit strain due to differences in ionic radii upon Y ³⁺ substitution on Mg ²⁺ sites.