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

Abstract

Isobaric gravimetric studies of PrO_x and TbO_x 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.