Ferroelectricity in Dion–Jacobson ABiNb₂O₇ (A = Rb, Cs) compounds

Abstract

The ferroelectric properties of 2-layer Dion–Jacobson compounds ABiNb₂O₇ (A = Rb and Cs) were studied. Ferroelectricity and piezoelectricity of CsBiNb₂O₇ were demonstrated for the first time. The ferroelectric domain structure of Dion–Jacobson compounds were imaged using PFM. The Curie points of RbBiNb₂O₇ and CsBiNb₂O₇ are 1098 ± 5 and 1033 ± 5 °C, respectively. The piezoelectric constant of RbBiNb₂O₇ and CsBiNb₂O₇ are approximately 5 and 8 pC N⁻¹. Thermal depoling was also studied to confirm the Curie temperature and the stability of the piezoelectricity.

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Perovskite-like layered structured (PLS) compounds display a range of interesting physical and chemical properties, including photocatalysis, photoluminescence, ion conductivity, electrochemical stability, magnetic properties, ferroelectricity and piezoelectricity.^1–7 Generally, the perovskite layers of PLS materials are formed by corner-sharing BO₆ octahedra separated by oxygen rich layers. There are mainly three homologous series of PLS materials according to their different BO₆ octahedra orientation: the Dion–Jacobson type phases (A′A_n−1BnO_3n+1); the A_nB_nO_3n+2 type phases; and the hexagonal phases (A_nB_n−1O_3n).¹ The crystal structure of Dion–Jacobson phase can be regarded as a result of cutting the idealized perovskite structure across the (1 0 0)_perovskite plane, and the crystal structure of A_nB_nO_3n+2 phase and the hexagonal phase can be regarded as a result of cutting the perovskite structure across the (1 1 0)_perovskite and (1 1 1)_perovskite planes, respectively.^1,8 The A_nB_nO_3n+2 type PLS materials have been shown to be ferroelectrics with super high Curie point, especially the 4-layer La₂Ti₂O₇ and Sr₂Nb₂O₇ phases, which have Curie point above 1300 °C.^9–11 However, materials with Dion–Jacobson structure and Hexagonal structure have rarely been reported to present ferroelectricity.

The crystal structure of the Dion–Jacobson compound CsBiNb₂O₇ was studied by Snedden, et al. and found to have similar structural distortions to that of ferroelectric Aurivillius phase SrBi₂Ta₂O₉, but they concluded that CsBiNb₂O₇ does not display ferroelectricity according to dielectric measurements.^12,13 Recently, Fennie et al. demonstrated the polar nature of CsBiNb₂O₇ using first principles and group theoretical analysis and estimated that CsBiNb₂O₇ has a spontaneous polarization of 40 μC cm⁻².¹⁴ More recently, Goff et al. reported that the ferroelectricity of CsBiNb₂O₇ cannot be detected due to its large leakage current and significant proton conductivity.¹⁵ Recently, the ferroelectricity and piezoelectricity of RbBiNb₂O₇, which has similar crystal structure to CsBiNb₂O₇, was reported by Li et al.,¹⁶ which made us to reconsider the possibility of discovering ferroelectricity for CsBiNb₂O₇.

In this work, we present the ferroelectricity and piezoelectricity of ABiNb₂O₇ (A = Rb, Cs) ceramics by direct evidence of ferroelectric domain switching and piezoelectric activity. The ferroelectric domain structures of CsBiNb₂O₇ and RbBiNb₂O₇ were investigated using PFM images. The Curie point for CsBiNb₂O₇ and RbBiNb₂O₇ were found to be 1033 ± 5 and 1098 ± 5 °C by studying the temperature dependence of permittivity and thermal depoling.

Fig. 1 shows the XRD patterns of ABiNb₂O₇ powders measured at room temperature. Both the powders of CsBiNb₂O₇ and RbBiNb₂O₇ are single phase, and no impurity can be observed in the XRD patterns. CsBiNb₂O₇ and RbBiNb₂O₇ have the same 2-layer Dion–Jacobson structure at room temperature, which is orthorhombic and belongs to space group P2₁am (26).^13,14,16 The lattice parameters are a = 5.4964, b = 5.4223 and c = 11.3704 Å for CsBiNb₂O₇ and a = 5.4193, b = 5.3589 and c = 11.2099 Å for RbBiNb₂O₇. Their spontaneous polarization directions are along the a-axis. The insets in Fig. 1 show the SEM images of the plate-like powders. Due to the plate-like grain shape, preferred orientation of the (0 0 l) plane can be observed in the powders. After SPS sintering, greater preferred (0 0 l) orientation was found in the ceramics (Fig. S1†). The orientation factors for CsBiNb₂O₇ and RbBiNb₂O₇ ceramic were 0.9 and 0.7, respectively. The benefit of orientation is to decrease the coercive field for polarization switching.

Fig. 1 X-ray diffraction patterns of ABiNb₂O₇ powders.

Fig. 2 shows the HRTEM images of ABiNb₂O₇ ceramics. Unfaulted lattice planes were observed for both CsBiNb₂O₇ (Fig. 2(a)) and RbBiNb₂O₇ (Fig. 2(b)). The d-pacing of the lattice planes was measured to be 1.12 ± 0.02 nm for CsBiNb₂O₇ and 1.08 ± 0.02 nm for RbBiNb₂O₇. Selected area electron diffraction patterns are shown in insets in Fig. 2(a) and (b). The linear reflections were indexed to be (0 0 l) lattice planes. The d-spacing of the (0 0 l) plane, which corresponds to the length of c axis of the unit cell, was measured to be 1.13 ± 0.02 nm for CsBiNb₂O₇ and 1.10 ± 0.02 nm for RbBiNb₂O₇.

Fig. 2 HRTEM images of ABiNb₂O₇ ceramics: (a) CsBiNb₂O₇; (b) RbBiNb₂O₇.

Fig. 3 shows vertical-mode PFM images of CsBiNb₂O₇ ceramic. Because the SPS sintered ceramics were highly textured on the (0 0 l) plane and the polar axis is in the a-direction, samples with surfaces perpendicular to SPS pressing direction were prepared to investigate the ferroelectric domain structure with PFM. Several grains with clear grain boundaries can be observed in the topography image shown in Fig. 3(a). Ferroelectric domain morphology is clearly observed in the PFM amplitude and phase images (Fig. 3(b) and (c)). Bright areas in the amplitude image reveal the stripe domains with vertical piezo-response signal. Some of the dark areas reveal regions with in-plane polarization. The bright and dark areas in the phase image reveal the up and down out-of-plane polarization, respectively. Comparing the PFM amplitude and phase images with the topography image, clear ferroelectric domains can be observed in several grains (A to E), and stripe domains with different orientations show strong contrast in each grain. Fig. 3(d) shows the schematic of the out-of-plane and in-plane polarization in grain A. The black, yellow and orange stripes represent the dark, bright and grey areas. The polarization direction can be indexed through comparing the phase and amplitude images. The width of the domains was typically from 10 to 600 nm. Similar ferroelectric domain structure was also observed for RbBiNb₂O₇ ceramic (Fig. S2†).

Fig. 3 Vertical-mode PFM images of CsBiNb₂O₇ ceramic: (a) topography; (b) amplitude; (c) phase and (d) schematic of the selected area from (c).

The temperature dependence of dielectric constants of CsBiNb₂O₇ and RbBiNb₂O₇ are shown in Fig. 4(a). The Curie point, T_c, of CsBiNb₂O₇ and RbBiNb₂O₇ are 1033 ± 5 and 1098 ± 5 °C, respectively. The ferroelectric-to-paraelectric phase transition temperature for CsBiNb₂O₇ is demonstrated for the first time. Goff et al. reported that the orthorhombic phase of CsBiNb₂O₇ was stable from room temperature to 900 °C and no ferroelectric orthorhombic to paraelectric tetragonal phase transition was observed using high-temperature XRD,¹⁵ which is in agreement with our result that the T_c of CsBiNb₂O₇ is above 900 °C (∼1033 °C). The T_c of RbBiNb₂O₇ was first reported to be 943 °C by Li et al.,¹⁶ which is about 155 °C lower than our result. To calibrate our equipment, the T_c of LiNbO₃ single crystal was measured with the same equipment. The T_c of LiNbO₃ was measured as ∼1138 °C. According to the literature,¹⁷ the T_c of single crystal LiNbO₃ is 1140 °C, which means that the T_c values we measured are relatively accurate. The insets of Fig. 4(a) show the current–electric field (I–E) and electric displacement–electric field (D–E) hysteresis loops of CsBiNb₂O₇ and RbBiNb₂O₇. The I–E and D–E loops were measured at 110 °C and 10 Hz for CsBiNb₂O₇. A typical, unsaturated, ferroelectric D–E loop obtained for CsBiNb₂O₇ is shown in the inset of Fig. 4(a). In its I–E loop, a current peak (marked by arrow) produced by ferroelectric domain switching is observed. For RbBiNb₂O₇, I–E and D–E loops, measured at 200 °C and 100 Hz, are shown in the inset of Fig. 4(a). A typical vesica piscis-shaped D–E loop due to leakage current was observed, but the ferroelectric domain switching is demonstrated by the peak observed in the I–E loop.

Fig. 4 (a) Temperature dependence of dielectric constant; insets: I–E and D–E hysteresis loops; (b) thermal depoling results.

CsBiNb₂O₇ and RbBiNb₂O₇ have similar polar orthorhombic structure P2₁am. Due to the off-centre displacements of A-site Bi ions and octahedral tilting of NbO₆, a spontaneous polarization develops along their a-axis. Recently Benedek reported that the origin of ferroelectricity in polar oxides with Dion–Jacobson phases is induced by a combination of octahedral distortions and cation ordering.¹⁸ The macroscopic polarizations of CsBiNb₂O₇ and RbBiNb₂O₇ were both reported to be 48 μC cm⁻² using symmetry principles, crystal chemical models, and first-principles calculations. Here the spontaneous polarization was calculated to be 43.8 μC cm⁻² for CsBiNb₂O₇ and 47 μC cm⁻² for RbBiNb₂O₇ according to Shimakawa's model,^19,20 which is in good agreement with Benedek's report. The measured piezoelectric constant d₃₃ at room temperature was 8 ± 0.5 pC N⁻¹ for CsBiNb₂O₇ and 5 ± 0.5 pC N⁻¹ for RbBiNb₂O₇. Fig. 4(b) shows the thermal depoling results for poled CsBiNb₂O₇ and RbBiNb₂O₇. All the samples poled at room temperature were annealed at different temperatures for 2 hours. Then their d₃₃ values were measured at room temperature. For both CsBiNb₂O₇ and RbBiNb₂O₇, their d₃₃ values are very stable with increasing depoling temperature. The d₃₃ starts to drop when the depoling temperature is close to T_c and tends to zero above T_c.

In summary, highly textured 2-layer Dion–Jacobson ceramics ABiNb₂O₇ (A = Rb and Cs) were prepared by one-step spark plasma sintering with pressure due to their layered crystal structure, which was demonstrated using XRD. High resolution TEM showed well ordered (0 0 1) lattice planes. Striped ferroelectric domains were observed using PFM. The ferroelectricity and piezoelectricity of CsBiNb₂O₇ has been demonstrated for the first time. The T_c of RbBiNb₂O₇ and CsBiNb₂O₇ are 1098 ± 5 and 1033 ± 5 °C, respectively. The piezoelectric constant of RbBiNb₂O₇ and CsBiNb₂O₇ were approximately 5 and 8 pC N⁻¹. Thermal depoling studies confirmed the T_c measurements and the stability of the piezoelectricity.

Acknowledgements

The Innovation Research and Development Programme of the Department of Business, Innovation and Skills, UK is acknowledged. Chen thanks China Scholarship Council (CSC) for supporting his PhD studies.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, XRD results of sintered ceramics, PFM of RbBiNb₂O₇. See DOI: 10.1039/c4tc02136c

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