A thermochemical study of the phase reaction ( 1 7)Pr₇O₁₂ + ( 1 7 - x 2)O₂ = PrO_2-x

The heat of reaction and equilibrium pressure for both the oxidation and reduction reactions, ( 1 7)Pr₇O₁₂ + ( 1 7 - x 2)O₂ = PrO_2-x, have been measured by means of a Tian-Calvet-type calorimeter and thermal balance. The results of equilibrium pressure measurements under isothermal conditions show a reproducible and unsymmetrical hysteresis loop. The unsymmetrical hysteresis loop is interpreted as due to a different pattern of intergrowth formed during oxidation and reduction. The ordered intermediate phases (Pr_nO_2n-2) are intergrown coherently at the unit cell level with each other but not with the disordered α phase (PrO_2-x). The role of coherent intergrowth in both symmetric and unsymmetric hysteresis loops has been discussed. The partial molar enthalpy (-ΔH_O2) is about 58 kcal/mole of O₂ in the Pr₇O₁₂ phase; it increases slightly as O/Pr increases, then increases sharply to about 85 kcal/mole of O₂ around PrO_1.78. No difference in partial molar enthalpy is observed between the oxidation and reduction paths within the experimental error, even though different equilibrium pressures are observed depending upon the path. The slight increase in the partial molar enthalpy in the "two-phase" region is attributed to the interaction term of the different domains using the same regular solution model used to account for the hysteresis observed previously. The sharp change in -ΔH_O2 around PrO_1.78 is believed associated with the phase change from the coherently intergrown ordered phases to the disordered α phase. The difference between the partial molar entropy on oxidation and that on reduction is explained by both the regular solution model and the usual thermodynamic treatment using the data of isothermal hysteresis.