Determination of the sodium ion transference number of the Dion-Jacobson-type layered provskite NaCa₂Nb₃O₁₀ using ac impedance and dc methods

The electronic conductivity of the Dion-Jacobson-type layered perovskite NaCa₂Nb₃O₁₀ was determined by employing ac impedance measurements using ion-blocking electrodes, electron-blocking electrodes, and dc Hebb-Wagner polarization measurements using NaCoO₂ as the reversible reference electrode. Oxygen partial-pressure-dependent electrical conductivity studies revealed that the conductivity is nearly independent of the oxygen partial pressure in the range from 0.21 to 10^-6 atm. Open-circuit voltage measurements of a galvanic cell employing air and 2 ppm of O₂ in argon as electrodes revealed that layered perovskites appear not to be oxide ion conductors. The transference number for Na⁺ ions in NaCa₂Nb₃O₁₀ at 550 °C is found to be 0.95 at the sodium activity of NaCoO₂. The ac measurements using Na-β-alumina with electron-blocking electrodes and Pt-nonblocking electrodes produced similar impedance data at 550 °C suggesting a high ionic nature, which is consistent with the polarization study. However, the ionic and electronic conductivities varied depending on the experimental conditions, such as temperature and partial pressures.