Leakage and Proton Conductivity in the Predicted Ferroelectric CsBiNb₂O₇

The layered perovskite CsBiNb₂O₇ has been reinvestigated in the light of recent predictions of ferroelectric behavior. Variable temperature powder neutron diffraction experiments show that this material retains polar symmetry (space group P2₁am) up to at least 900 °C, with only a slight decrease in net polarization up to this temperature, calculated using a simple ionic model. However, ac impedance measurements show only a modest dielectric constant (19−27) throughout the temperature range studied. Moreover, convincing evidence for ferroelectric behavior in polarizationelectric field loops could not be obtained because of overriding electrical conductivity. Further measurements of electrical behavior, at different temperatures and under both “wet” and “dry” atmospheres, provide clear evidence that CsBiNb₂O₇ is hygroscopic, with water uptake resulting in significant protonic conductivity. Even in the “dehydrated” state the material is a semiconductor with a modest conductivity of 3.8 μS cm⁻¹ at 250 °C. Although ferroelectric behavior is not observed in the samples studied here, we cannot rule out the possibility that ferroelectricity might be obtained in samples processed under appropriate conditions.