Thermodynamics and kinetics of oxidation of Pr2O3 and Tb2O3 to non-stoicheiometric oxides

Isobaric gravimetric studies of PrOx and TbOx systems have shown the equilibrium existence of several non-stoicheiometric phases; the transformation temperatures found by differential thermal analysis agree well with the data from gravimetric studies. Oxidation reactions of Pr2O3 and Tb2O3 to non-stoicheiometric oxides generally show anomalously high entropy changes (ca.–25 e.u./mole of O2) compared with the value of ca.–44 e.u./mole of O2 generally found in oxidation of metals. The anomalous entropy changes are shown to arise from configurational factors. Cubic Pr2O3 with the defect structure is more readily oxidized than the hexagonal Pr2O3. Kinetic studies show that oxidation of Pr2O3 is a phase-boundary-controlled reaction.