Thermodynamic investigation of the redox properties for ceria-hafnia, ceria-terbia, and ceria-praseodymia solid solutions

Ceria-hafnia, ceria-terbia, and ceria-praseodymia solid solutions were prepared by the citric-acid (Pechini) method and characterized using X-ray diffraction (XRD) for structure and coulometric titration for redox thermodynamics. Following calcination at 973 K, XRD showed that both Ce0.5Hf0.5O2 and Ce0.8Hf0.2O2 exist as single-phase solid solutions and that the lattice parameters of the cubic fluorite structures shifted linearly with Hf content. Following calcination at 1323 K, Ce0.8Hf0.2O2 remained single phase but Ce0.5Hf0.5O2 separated into two phases with compositions of Ce0.81Hf0.19O2 and Ce0.15Hf0.85O2. At 973 K, oxidation isotherms for Ce0.8Hf0.2O2 and Ce0.5Hf0.5O2 were almost identical to oxidation isotherms for Ce0.8Zr0.2O2 and Ce0.5Zr0.5O2. Mixed oxides of ceria with terbia and praseodymia also formed single-phase fluorite structures, but each of the solid solutions was phase stable to calcination at 1323 K. The oxidation isotherms for the solid ceria-terbia and ceria-praseodymia solutions were similar to what would be expected for physical mixtures of these oxides.