Thermodynamics and Kinetics of Oxidation of Pr₂O₃ and Tb₂O₃ to Nonstoicheiometric Oxides

Isobaric gravimetric studies of PrO_z and TbO_z systems have shown the equilibrium existence of several nonstoicheiometric 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.