Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges

A structural phase transition between the cubic (space group, Fm3m) and tetragonal (space group, P42/nmc) phases in a zirconia–ceria solid solution (Zr1−xCexO2) has been observed by Raman spectroscopy. The cubic–tetragonal (c–t″) phase boundary in compositionally homogeneous samples exists at a composition X0 (0.8 < X0 < 0.9) at room temperature, where t″ is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition X′0 (0.65 < X′0 < 0.7) at room temperature. The sample with a composition between X0 and X′0 is t″ ZrO2. By Raman scattering measurements at high temperatures, the tetragonal (t″) → cubic and cubic → tetragonal phase transitions occur above 400°C in Zr0.2 Ce0.8O2 solid solution.

Raman Scattering Study of Cubic–Tetragonal Phase Transition in Zr1−xCexO2 Solid Solution

A stabilization mechanism of zirconia based on oxygen vacancies only

Metastable-stable phase diagrams in the zirconia-containing systems utilized in solid-oxide fuel cell application

Grain growth kinetics and grain-boundary segregation of 12Ce-TZP and 2Y-TZg containing divalent to pentavalent cationic dopants, were studied. In all cases, normal grain growth following the parabolic growth relation was observed at higher temperatures. The mobility of grain boundaries was suppressed by the addition of divalent and trivalent cations, unchanged or enhanced by the addition of tetravalent and pentavalent cations. Larger cations have a stronger effect in suppressing grain growth. From ESCA, AES, and STEM analysis of the near grain-boundary regions, it is further concluded that only divalent and trivalent cations segregate. These observations can be satisfactorily rationalized using the space charge concept and the model of impurity drag. [Key words: grain growth, tetragonal zircoda polycrystals, segregation, grain boundaries, dopants.]

Grain Size Control of Tetragonal Zirconia Polycrystals Using the Space Charge Concept

The high-temperature structure of zirconia was studied by powder neutron diffraction up to 2400/sup 0/C. The boundaries of the domain of the nonstoichiometric tetragonal form are defined. They are consistent with a tetragonal-cubic transition at 2350/sup 0/C for stoichiometric zirconia. The changes in the structural parameters of the tetragonal form (unit-cell, positional parameters and thermal B factors) with temperature give evidence of medium zirconium and oxygen mobilities. The oxygen ions are, however, always more mobile than the zirconium ions. An enhancement with temperature of the structural anisotropy tends to weaken the weaker of the two distinct Zr-O bonds of the tetragonal zirconia. It results in the transformation into the cubic form which is accompanied by a change in unit-cell volume; this change becomes sharper as the composition tends toward stoichiometry. This transition is probably followed by an increase of the ionic mobility.

Pierre Aldebert

Papers

Structure and Ionic Mobility of Zirconia at High Temperature

Structure and ionic mobility of zirconia at high temperature