New Oxide-Ion Electrolytes

Abstract

AbstractStrategic considerations for the design of new O2--ion solid electrolytes are presented, and preliminary measurements of the oxide-ion conductivity σo = (B/T) exp (-EA/kT) and transport number tO = σO/σ are reported for several oxides with perovsldte-related structures. Ba2In2O5 exhibits a first-order transition at Tt ∼ 930°C from the brownmillerite structure to a cubic perovskite with disordered oxygen vacancies and fast O2-ion conduction. Ba3In2MO8 (M = Ce, Zr, or Hf) contains oxygen vacancies ordered into every second BO2 (001) plane. An octahedralsite preference of the disordered M(IV) cations makes incomplete the oxygen-vacancy ordering, so a high extrinsic σo with EA ∼ 0.6 eV is found for T ≤ 400 °C. The transport properties of Ba3In2ZrO8 at 400 °C are equivalent to those of the stabilized zirconias at 800 °C. BagIn6O17 consists of an intergrowth of (BaO)2 rocksalt and Ba3In3O7.5 oxygen-deficient perovskite layers. Although there is no evidence of long-range ordering of the oxygen vacancies, nevertheless an EA = 1.06 eV and transport properties equivalent to stabilized zirconia indicate retention of short-range correlations to at least 950 °C. Attempts to prepare Bag Y6O17 yielded a mixture of hexagonal Ba3Y4O9 and unidentified phases. An EA = 0.45 eV above a Tt=370 °C gives a remarkable O2--ion conductivity at temperatures T > Tt in both the mixed-phase and pure Ba3Y4O9 at temperatures T > 370 °C, but the samples are extremely hygroscopic.