Phase transitions in reduced ceria (CeO2−x) due to solid-state reactions yield a complex phase diagram. Associated with these transformations is a change in molar volume of the sample. For certain mixtures of coexisting phases internal stress develops and the sample is not in true equilibrium. As a result, the molar volume of the phase under pressure is reduced. Experimental and theoretical evaluations confirm the phase diagram determined before by specific heat measurements. New data collected concerning the local pressure effect enabled the development of a more detailed explanation of this deviation from equilibrium.

Nonstoichiometric phases and their transformation temperatures were determined by thermal expansion measurements. The phase diagram of CeO122≥y≥1.77, was determined from these measurements. This phase diagram is in agreement with the one determined earlier by Cp measurements. It is different from that determined in the past by Bevan and Kordis.

Specific heat measurements on non-stoichiometric cerium oxide, CeOy, were carried out in the temperature range 320 ⩽ T ⩽ 1200K for the composition range 1.700 ⩽ y ⩽ 2.000. The main results and their analysis is presented.

Ceria (CeO2) can readily be reduced to form a wide range of binary compounds CeOy, 2 ≥y ≥ 1.5. Specific heat measurements at constanty were carried out for the composition range 2≥y > 1.714 and for the temperature range 300 K <T < 1200 K. In thisy,T region the specific heat exhibits a complex form reflecting various transformations. The results and theoretical evaluations of the specific heat are presented as the temperature is varied from low values (T ≈ 400K) where two phases coexist, through several phase transformations to a high temperature α phase. Special features of the specific heat due apparently to increased internal local pressure appearing for small deviations from stoichiometry are also discussed.

The phase diagram for nonstoichiometric ceria, CeO2−x, was determined from specific heat measurements in the temperature range 320–1200 K and composition range CeO2CeO1.72. Coexistence temperatures of three phases are found at 722, 736, 766, 913, and 1084 K. There is some indication for the existence of two other coexistence temperatures at 850 and at 880 K. The maximum of the miscibility gap occurs at T = 910 K and 2 − x = 1.93. The phase diagram exhibits some phases in the homologous series CenO2n−2 with n = 7, 10, 11, and two phases at 2 − x = 1.79 and 2 − x = 1.808 not belonging to this series.