Defect Chemistry of SrBi₂Ta₂O₉ and Ferroelectric Fatigue Endurance

Abstract

The defect chemistry and charge transport properties of doped and undoped SrBi₂Ta₂O₉ (SBT) were studied by making 4-point dc equilibrium electrical conductivity, thermopower, ionic transport number, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements. Results of high temperature equilibrium dc conductivity, thermoelectric power, and transport number measurements as a function of oxygen activity in the temperature range 650–725°C revealed that undoped SBT displays a broad centrally located plateau of ionic conductivity with an activation energy of mobility of 0.94 eV and a prominent upturn at high oxygen activity, caused by p-type conductivity. The effects of acceptor and donor dopants are consistent with a 1–2% net acceptor excess in the undoped compound. It has been observed that there is substantial (several percent) cation place exchange between the Sr²⁺ and Bi³⁺ in SBT. It is proposed that the net acceptor excess in undoped SBT consists of disordered Sr²⁺ substituting for Bi³⁺ in the fluorite-like bismuth oxide layers which are locally compensated by oxygen vacancies. The formation of the net donor excess by disordered Bi³⁺ substituting for Sr²⁺ in the perovskite-like layers does not manifest itself as n-type conductivity behavior, because the band gap is large and the mobility is highly thermally activated. The superior intrinsic ferroelectric fatigue endurance of SBT is attributed to the lack of mobile charged defects in the perovkite-like layers which create the ferroelectric response of the compound. The metallic bismuth presence on the surface of undoped SBT, as revealed by qualitative XPS measurements, is believed to result from long exposure to the highly reducing conditions in the XPS system.