Leakage and Proton Conductivity in the Predicted Ferroelectric CsBiNb₂O₇

Abstract

The layered perovskite CsBiNb 2 O 7 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 °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 polarizationselectric 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 CsBiNb 2 O 7 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.

‘Ferroelectric’ metals reexamined: fundamental mechanisms and design considerations for new materials

“…The product of R 1 and R 2 establishes the polar space group symmetry, P ∼ R 1 ⊕ R 2 . This is a rather general mechanism that can induce ferroelectricity in many families of layered oxides; 7,10,[126][127][128][129][130][131][132] see Ref. 133 for a recent review.…”

Octahedral tilting and ferroelectricity in RbANb2O7(A = Bi, Nd) from first principles

“…As reported in early experimental and theoretical work on the Dion-Jacobson CsBiNb 2 O 7 compound, one of the most important ingredients is octahedral tilting [17][18][19][20]. The room temperature structure of CsBiNb 2 O 7 is a Pmc2 1 orthorhombic structure [21] with a − a − c + tilting of the Glazer notation [21][22][23][24].…”

“…Their calculation clearly showed that the coupling of the zone-center ferroelectric mode and zone-boundary octahedral tilting mode is the key origin for ferroelectric realization of a CsBiNb 2 O 7 material. Experimentally, the ferroelectric nature of CsBiNb 2 O 7 is unclear because of the large ionic conductivity of this material [19].…”

Phase Transition Behavior of the Layered Perovskite CsBi0.6La0.4Nb2O7: A Hybrid Improper Ferroelectric

“…Later, Fennie and Rabe predicted ferroelectricity in CsBiNb 2 O 7 from a first-principles DFT study [15]. Our follow-up experimental studies [16] on CsBiNb 2 O 7 suggested that ferroelectricity was hindered by moisture uptake, leading to proton conductivity and leakage. Nevertheless, a more recent work by Chen et al [17] did provide experimental evidence for ferroelectricity in this compound.…”

“…Nevertheless, a more recent work by Chen et al [17] did provide experimental evidence for ferroelectricity in this compound. Goff et al [16], attempted to determine any high temperature phase transitions in CsBiNb 2 O 7 using powder neutron diffraction: no evidence for phase transitions was identified up to 900 • C, above which sample deterioration precluded further study. By analogy with CsNdNb 2 O 7 , we speculated that La-doping of CsBiNb 2 O 7 would reduce the orthorhombic distortion such that any phase transition to higher symmetry might be brought down in temperature relative to that of CsBiNb 2 O 7 , and make this transition more amenable to experimental study.…”