The high-temperature stability of zirconias co-doped with Y and Gd and synthesized as metastable single phases within the equilibrium tetragonal + cubic field is investigated. The motivation arises from strategies to enhance the insulating efficiency of thermal barrier systems by co-doping the conventional ZrO₂-7.6 % YO_1.5 composition with trivalent rare-earth cations. The issue is the partitioning of the metastable tetragonal (t') or cubic (c') phases into the equilibrium t + c phases dictated by the phase diagram, whereupon the tetragonal phase becomes susceptible to the disruptive monoclinic transformation. The experiments are based on compositions synthesized by precursor pyrolysis, all of which yield initially supersaturated single-phase solid solutions. It is found that t' stabilized by Gd alone is significantly less resistant to partitioning at high temperature than its Y counterpart with the same amount of stabilizer. However, modest substitution of Gd for Y does not degrade the stability, and may improve it in some cases. Increasing the total amount of stabilizer generally enhances phase stability. The roles of thermodynamics and kinetics on the relative stability of the system are discussed.

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