Thermochemical study of the phase reaction (1/7)Pr7O12 + (1/7-x/2)02 = PrO/sub 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/sub 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/sub n/O/sub 2n-2/) are intergrown coherently at the unit cell level with each other but not with the disordered α phase (PrO/sub 2-x/). The role of coherent intergrowth in both symmetric and unsymmetric hysteresis loops has been discussed. The partial molar enthalpy (-Δ anti H 0/sub 2/) 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 PrO178. 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 -Δ anti H 0/sub 2/ around PrO178 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