Chemically induced volume change of CeO2 − δ and nonstoichiometric phases

Deeper understanding of thermal and chemical volume change in non-stoichiometric CeO2 − δ is very important for the development of reactive ceria ceramics applied for thermochemical cycles. Dilatometric studies at 1538 K-1690 K under reduced and air atmosphere were performed using polycrystalline ceria rods, which were produced from powder synthesized via Pechini method. Based on chemical expansion and weight loss during reduction, the oxygen-deficiency parameter δ ranging between 0.02 and 0.09 was calculated. Above 1650 K, however, δ cannot be calculated from weight loss data due to initiating ceria evaporation. Log δ- vs. log pO2 curves reveal a discontinuous development, which is interpreted in terms of defect complex formation. Cooling down CeO2 − δ from high temperatures under vacuum conditions results in unmixing and phase transformations according to the phase diagram going along with faint discontinuities in dilatometric curves. Reduced ceria ceramics (δ = 0.02-0.09) consist of α-CeO1.985, coexisting with structurally related CeO1.778 and CeO1.714, while the expected β-phase (CeO1.818) was not observed. The combination of dilatometric studies, XRD and thermal analyses on CeO2 − δ reoxidation shows a multi-step mechanism. CeO1.778 and CeO1.714 re-oxidize at temperatures of 373 K while the transformation from CeO1.985 to CeO2 requires temperatures > 900 K.