Potentiometric determination of activities in the two-phase fields of the system Na₂O-(α)Al₂O₃

Abstract

Three compounds have been found to be stable in the pseudobinary system Na₂O-(α)Al₂O₃ between 825 and 1400 K; two nonstoichiometric phases, β-alumina and β″-alumina, and NaAlO₂. The homogeneity of β-alumina ranges from 9.5 to 11 mol% Na₂O, while that of β″-alumina from 13.3 to 15.9 mol% Na₂O at 1173 K. The activity of Na2O in the two-phase fields has been determined by a solid-state potentiometric technique. Since both β- and β″-alumina are fast sodium ion conductors, biphasic solid electrolyte tubes were used in these electrochemical measurements. The open circuit emf of the following cells were measured from 790 to 980 K: Pt, Ir, Pb-Na/α- + β″-alumina/Fe+″Feo″,Pt (I) Pt, Ir, Pb-Na/β- + β″-alumina/Fe+″Feo″,Pt (II) Pt, Ir, Pb-Na/β″-alumina+NaAlO₂/Fe+″Feo″,Pt (III) The partial molar Gibbs' energy of Na₂O relative to γ-Na₂O in the two-phase regions can be represented as: ΔḠNa₂O(α- + β-alumina) = −270,900 + 24.03 T, ΔḠNa₂O(β- + β″-alumina) = −232,700 + 56.19 T, and ΔḠNa₂O(β″-alumina + NaAlO₂) = −13,100−4.51 T J mol⁻¹. Similar galvanic cells using a Au-Na alloy and a mixture of Co + CoAl_2+2xO_4+3x+(α)Al₂O₃ as electrodes were used at 1400 K. Thermodynamic data obtained in these studies are used to evaluate phase relations and partial pressure of sodium in the Na₂O-(α)Al₂O₃ system as a function of oxygen partial pressure, composition and temperature.