Phase formation and thermal stability of fcc (fluorite) Ce_1-xTb_xO_2-δ solid solutions

Ce_1−xTb_xO_2−δ solid solutions (x = 0.3, 0.4, and 0.5) were synthesized by a coprecipitation method, using ammonia. The formation process of the solid solutions was studied as a function of temperature up to 1200°C by X-ray diffraction, thermogravimetric analysis, and differential scanning calorimetry. The results showed that chemically homogeneous, oxidic fcc structures were formed. The large increase (∼1.5%) of the fcc unit-cell parameter on increase of temperature in ambients of oxygen or nitrogen could be correlated with changes in the oxygen stoichiometry (≈3.5%) that have led to a reversible change in the valence state from (IV) to (III) of a fraction of the Ce and Tb ions. The fcc structure was chemically and structurally stable to at least 1200°C.