| Summary: | Ceramics samples with the nominal composition [(ZrO<sub>2</sub>)<sub>0.95</sub>(Y<sub>2</sub>O<sub>3</sub>)<sub>0.05</sub>]<sub>1-<i>x</i></sub>[PrO<i><sub>y</sub></i>]<i><sub>x</sub></i> and praseodymia contents of <i>x</i> = 0.05–0.15 were prepared by the direct firing of compacted 5YSZ + PrO<i><sub>y</sub></i> mixtures at 1450–1550 °C for 1–9 h and characterized for prospective applicability in reversible solid oxide cells. XRD and SEM/EDS analysis revealed that the dissolution of praseodymium oxide in 5YSZ occurs via the formation of pyrochlore-type Pr<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> intermediate. Increasing PrO<i><sub>y</sub></i> additions results in a larger fraction of low-conducting pyrochlore phase and larger porosity, which limit the total electrical conductivity to 2.0–4.6 S/m at 900 °C and 0.28–0.68 S/m at 700 °C in air. A longer time and higher temperature of firing promotes the phase and microstructural homogenization of the ceramics but with comparatively low effect on density and conductivity. High-temperature processing leads to the prevailing 3+ oxidation state of praseodymium cations in fluorite and pyrochlore structures. The fraction of Pr<sup>4+</sup> at 600–1000 °C in air is ≤2% and is nearly independent of temperature. 5YSZ ceramics with praseodymia additions remain predominantly oxygen ionic conductors, with <i>p</i>-type electronic contribution increasing with Pr content but not exceeding 2% for <i>x</i> = 0.15 at 700–900 °C. The average thermal expansion coefficients of prepared ceramics are in the range of 10.4–10.7 ppm/K.
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