Interface Excess and Polymorphic Stability of Nanosized Zirconia-Magnesia

Controlling the phase stability of ZrO 2 nanoparticles is of major importance in the development of new ZrO 2 -based nanotechnologies. Because of the fact that in nanoparticles the surface accounts for a larger fraction of the total atoms, the relative phase stability can be controlled throughout the surface composition, which can be tuned by surface excess of one of the components of the system. The objective of this work is to delineate a relationship between surface excess (or solid solution) of MgO relative to ZrO 2 and the polymorphic stability of (ZrO 2 ) 1− x −(MgO) x nanopowders, where 0.0 ≤ x ≤ 0.6. The nanopowders were prepared by a liquid precursor method at 500 °C and characterized by N 2 adsorption (BET), X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), and Raman spectroscopy. For pure ZrO 2 samples, both tetragonal and monoclinic polymorphs were detected, as expected considering the literature. For MgO molar fractions varying from 0.05 to 0.10, extensive solid solution could not be detected, and a ZrO 2 surface energy reduction, caused by Mg surface excess detected by XPS, promoted tetragonal polymorph thermodynamic stabilization with relation to monoclinic. For MgO molar fractions higher than 0.10 and up to 0.40, Mg solid solution could be detected and induced cubic phase stabilization. MgO periclase was observed only at x = 0.6. A discussion based on the relationship between the surface excess, surface energy, and polymorph stability is presented.