Electrochemical determination of activities in Cr₂O₃ Al₂O₃ solid solutions

Abstract

The activity of Cr₂O₃ in Cr₂O₃Al₂O₃ solid solution has been determined in the temperature range 800°–1320°C from electromotive force measurements on the solid oxide galvanic cell Pt,Cr+CrO₃/Y₂O₃ThO₂/Cr+Cr₂Al₂O₃,Pt The activities of Formula and Formula in the solid solution show both positive and negative deviations from Raoult's law. The heat and entropy of mixing of the solid solution obtained from the temperature dependence of the emf can be expressed as ΔH=X_Cr2O3X_Al2O3[31.700X_Cr2O3+37.470X_Al2O3]Jmole^-1 ΔS=-1.8R[X_Al2O3InX_Al2O3+_Al2O3In_Al2O3] The entropy of mixing is 10% lower than that predicted by the Temkin model. The large positive heat of mixing in the Cr₂O₃Al₂O₃ solid solution, however, suggests that this apparent entropy discrepancy originates with the clustering of positive ions on the cation sublattice. The asymmetric miscibility gap exhibited in the Cr₂O₃Al₂O₃ system below 900°C is consistent with the thermodynamic data trends recorded at the more elevated temperatures.