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, Na1+xZr2SixP3-xO12, has been measured in the temperature range 7001/21100 K using solid state galvanic cells: Pt|CO2+O2|Na2CO3 Na1+xZr2SixP3-xO12(Y2O3)ZrO2 In + In2O3|Ta, Pt for 1= 2.5, and PtCO2+O2 Na2CO3 AY-alumina Na1+xZr2SixP3-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,AY-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