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

E. K. Akdoǧan, I. Şavklyldz, B. Berke, Z. Zhong, L. Wang, D. Weidner, M. C. Croft, T. Tsakalakos

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Abstract

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.

Original languageEnglish (US)
Article number053506
JournalJournal of Applied Physics
Volume111
Issue number5
DOIs
StatePublished - Mar 1 2012

All Science Journal Classification (ASJC) codes

  • General Physics and Astronomy

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