Thermodynamic partial properties of Na2O in Nasicon solid solution, Na1+xZr2SixP3−xO12

Abstract

The thermodynamic activity of sodium oxide (Na2O) in the Nasicon solid solution series, Na1+xZr2SixO12, has been measured in the temperature range 700–1100 K using solid state galvanic cells: Pt|CO2 + O2|Na2CO3∥Na1+xZr2SixP3−xO12∥(Y2O3)ZrO2∥In + In2O3|Ta, Pt for 1 ≤ ⊠ ≤ 2.5, and Pt∥CO2 + O2∥Na2CO3∥β-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, 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.