| Summary: | Phase stability and transport properties of (ZrO<sub>2</sub>)<sub>0.91−x</sub>(Sc<sub>2</sub>O<sub>3</sub>)<sub>0.09</sub>(Yb<sub>2</sub>O<sub>3</sub>)<sub>x</sub> crystals (x = 0–0.01) have been studied before and after air annealing at 1000 °C for 400 h. The crystals have been grown by radio frequency (RF) heating in a cold crucible. The microstructure, phase composition, and electrical conductivity of the crystals have been studied using optical microscopy, X-ray diffraction, Raman spectroscopy, and impedance spectroscopy. Phase stability and degradation of ionic conductivity of the crystals upon long-term high-temperature heat treatment have been discussed. We show that the stabilization of ZrO<sub>2</sub> co-doped with 9 mol.% Sc<sub>2</sub>O<sub>3</sub> and 1 mol.% Yb<sub>2</sub>O<sub>3</sub> provides transparent uniform crystals with the pseudocubic t″ phase structure having high phase stability. Crystals of this composition had the highest conductivity in the entire temperature range. Long-term high-temperature annealing of these crystals did not lead to conductivity degradation.
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