Ceramics with eutectic microstructure in the ZrO2–PrOx system

Praseodymium oxides present redox properties analogous to those of Ce-based systems and have been proposed for catalytic applications in combination with CeO₂, ZrO₂, or both. However, uncertainties remain concerning the nature and redox behavior of Pr-rich mixtures, especially with ZrO₂. Here we study the eutectic composites of the ZrO₂–PrO_x system, focusing on the sensitivity of their microstructure, phase symmetry, and composition to variations of the processing atmosphere from oxidizing to reducing. Mixed oxides have been produced by a laser-assisted directional solidification technique in O₂, air, N₂, or 5%H₂(Ar) environment, and the resulting materials have been analyzed by scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, and magnetic susceptibility. In air, N₂, or 5%H₂(Ar) atmosphere, a lamellar, eutectic-like microstructure forms, the major phase being the one with less Pr content. Both the Pr concentration in each phase as the PrO_x molar percentage of the eutectic composites decrease as the atmosphere becomes more reducing. Both eutectic phases are fluorite-like when processing in air, whereas in N₂ or 5%H₂(Ar), the phase with high Pr content is of the A-R₂O₃ type, and the phase with low Pr content can be described as a fluorite phase containing C-R₂O₃-like short-range-ordered regions. The results obtained for samples processed in O₂ suggest that for high enough pO₂ no eutectic forms, in analogy with the ZrO₂–CeO₂ system. The evolution of the phase composition and symmetry is discussed in terms of the limited stability of the phases found in the ZrO₂–Pr₂O₃ system, namely, A- or C-R₂O₃-like, beyond a certain Pr oxidation degree and oxygen content.