Tuning the Pr Valence State To Design High Oxygen Mobility, Redox and Transport Properties in the CeO₂–ZrO₂–PrOₓ Phase Diagram

The preparation and characterization of pure fluorite-type phases allowed exploring the CeO₂–ZrO₂–PrOₓ (CZP) phase diagram. On the basis of magnetic susceptibility measurements, the Pr⁴⁺/Pr³⁺ molar ratio of several oxides annealed at T = 700 °C under air was determined; the higher the Zr content, the lower the Pr⁴⁺ concentration. Thermogravimetric analysis and temperature-programmed reduction measurements showed various Pr and Ce reduction steps. The Pr⁴⁺ reduction starts at T = 250 °C and is maximum around T = 400 °C. For the most reducible compositions, which exhibit the lowest Zr content and the highest Pr rate, the reduction phenomena strongly depend on the Pr/Ce molar ratio. As a remarkable result, Pr and Ce reductions can simultaneously take place at lower temperature (T > 430 °C) than for oxides of the CeO₂–ZrO₂ solid solution evidencing that the increase of Pr content also allows enhancing the reducibility of Ce⁴⁺ at low temperature. On the basis of a discussion taking into account the probability of oxygen surroundings in disordered fluorite networks and the rate of oxygen released in materials after the first reduction step performed at T < 500 °C, a mapping of the most probable labile oxygen sites in the CZP phase diagram is proposed. In particular, it is shown that for the oxides containing 10 atom % Zr, the most labile oxygen site should be systematically coordinated with one Zr atom, one Ce, and two Pr atoms. In the same series (10 atom % Zr), the electronic transport properties allowed showing semiconducting behavior with a strong increase of the total conductivity as the Pr content increases. On the basis of the thermal variation of the Seebeck coefficient, these phenomena are associated with hopping of electrons and holes, involving intra-atomic charge transfers, which depend on the reduction temperature of Pr⁴⁺ ions under air. Finally, the oxygen mobility strongly increases with the Pr content in this series. The oxygen tracer self-diffusion coefficient D* has been estimated by two independent measurements, and the best value is around 10–⁸ cm²/s at T = 400 °C for the Ce₀.₄₅Zr₀.₁Pr₀.₄₅O₂–ₓ composition, which is quite high in this temperature range. These fundamental properties of CZP phases design very promising new materials like automotive exhaust catalysts, gas sensors, electrolytes, or oxygen electrodes for solid oxide fuel cells.