Defect CHEMISTRY and Charge TRANSPORT in SrBi₂Nb₂O₉

Abstract

Equilibrium dc conductivity and thermopower measurements at 650–800°C on undoped and 1% acceptor-doped SrBi₂Nb₂O₉, SBN, indicate that the n-type conductivity is similar to that of a simple transition metal oxide that contains 1–2% donor excess. The donor content is attributed to the presence of Bi⁺³ on Sr⁺² sites in the perovskite-like layers of the structure. These centers arise from cation place exchange between these ions in the alternating layers of the crystal. This exchange is apparently not completely self-compensating, and there is local charge compensation in each layer. While the equilibrium conductivity of SrBi₂Ta₂O₉, SBT, is dominated by ionic conduction in the Bi layers, in SBN conduction by electrons in the perovskite-like layers prevails. The difference in behavior is attributed to the expected smaller band gap of the niobate. The electron mobility in SBN is extremely small, of the order of 10⁻⁵ cm²/v · sec at 750°C, and is highly activated with an activation energy of about 1.6 eV. The resulting low mobility at ambient temperatures is proposed as the basis for the observed resistance to ferroelectric fatigue. Reports of metallic Bi on the surface of SBT and SBN by XPS analysis are shown to result from the highly reducing atmosphere of the XPS apparatus.