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The title compounds are characterized by powder neutron and X-ray diffraction, magnetic susceptibility and dielectric measurements, and 57Fe Moessbauer spectroscopy.

The crystal structure of the title system is determined at 295 and 125 K by single crystal XRD.

The title compound is characterized by variable temperature powder XRD and dielectric and magnetic measurements.

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 P2(1)am) up to at least 900 degrees 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 mu S cm(-1) at 250 degrees 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.