Thermodynamic partial properties of Na₂O in Nasicon solid solution, Na_1+xZr₂Si_xP_3−xO₁₂

The thermodynamic activity of sodium oxide (Na₂O) in the Nasicon solid solution series, Na_1+xZr₂Si_xO₁₂, has been measured in the temperature range 700–1100 K using solid state galvanic cells: Pt|CO₂ + O₂|Na₂CO₃∥Na_1+xZr₂Si_xP_3−xO₁₂∥(Y₂O₃)ZrO₂∥In + In₂O₃|Ta, Pt for 1 ≤ ⊠ ≤ 2.5, and Pt∥CO₂ + O₂∥Na₂CO₃∥β-alumina∥Na_1+xZr₂Si_xP_3−xO₁₂∥Ar + O₂∥Pt for x = 0, 0.5, 2.5, and 3. The former cell, where the Nasicon solid solution is used as an electrolyte along with yttria-stabilized zirconia, is well suited for Nasicon compositions with high ionic conductivity. In the latter cell, β-alumina is used as an electrolyte and the Nasicon solid solution forms an electrode. The chemical potential of Na₂O is found to increase monotonically with x at constant temperature. The partial entropy of Na₂O decreases continuously with x. However, the partial enthalpy exhibits a maximum at x = 2. This suggests that the binding energy is minimum at the composition where ionic conductivity and cell volume have maximum values.