Ba2Bi1.4Nb0.6O6: A Nonferroelectric, High Permittivity Oxide

Publication Date (Web): May 31, 2012. Copyright © 2012 American Chemical Society. *E-mail: [email protected]. Cite this:Chem. Mater. 24, 12, 22...
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Ba2Bi1.4Nb0.6O6: A Nonferroelectric, High Permittivity Oxide Chaou C. Tan,† Antonio Feteira,‡ and Derek C. Sinclair*,† †

Department of Materials Science and Engineering, University of Sheffield, United Kingdom, S1 3JD Materials and Engineering Research Institute, Sheffield Hallam University, United Kingdom, S1 1WB

‡

S Supporting Information *

KEYWORDS: Ba2Bi2O6, Ba2Bi1.4Nb0.6O6, ferroelectricity, permittivity higher than expected bulk permittivity (εr ∼80−100) but find no evidence for ferroelectricity. The RT crystal structures of BBO and BBNO were determined using XRD data and were consistent with previous reports in the literature;4−6,10 BBO adopts a body-centered monoclinic cell (I2/m) with lattice parameters a = 6.19043(2) Å, b = 6.14445(2) Å, c = 8.67684(2) Å, β = 90.042(2)°, V = 165.020(7) Å3 whereas BBNO adopts rhombohedral symmetry (R3̅), as shown in Figure A in the Supporting Information. The RT lattice parameters for BBNO were a = 6.09181 (3)Å, α = 60.3264 (1)°, V = 161.034 (2) Å3, consistent with previous reports in the literature.13 Variable temperature XRD results showed the rhombohedral (R3̅) to cubic symmetry (Fm3̅m) phase transition in BBNO to occur between ∼550 and 573 K, as shown in Figure B in the Supporting Information. IS data collected at RT showed BBO to be semiconducting with a total resistivity of ∼2.5 kΩ cm, whereas BBNO was too resistive (≫10 MΩ cm) to obtain meaningful data. To obtain information pertaining to the bulk (grain) response of the ceramics, we collected data over the range ∼10−300 K and ∼350−600 K for BBO and BBNO, respectively. Typical impedance complex plane (Z*) plots for BBO (300 K) and BBNO (473 K) are shown in panels a and b in Figure 1, respectively. The plot for BBO at 300 K shows two overlapping semicircular arcs with a high frequency, nonzero-intercept on the Z′ axis, Figure 1a. The nonzero intercept shown in the inset of Figure 1a is attributed to the semiconducting bulk response with resistivity R ∼48 Ω cm. The two overlapping arcs were hand fitted and have resistivity and capacitance values of R ∼1.8 kΩ cm, C ∼10 nF cm−1 and R ∼0.9 kΩ cm, C ∼630 nF cm−1, respectively. On the basis of the magnitude of the capacitance values, these correspond to grain boundary and electrode/ sample interface responses, respectively. This plot reveals that lower temperature data are required to obtain bulk capacitance (permittivity) values for BBO ceramics and that rf capacitance measurements at RT or above are dominated by nonbulk effects such as grain boundaries and electrode contact phenomena. The Z* plot for BBNO at 473 K shows two semicircular arcs of similar magnitude, Figure 1b. The higher frequency arc with resistivity R ∼7.5 kΩ cm and capacitance C ∼8.5 pF cm−1 is attributed to the bulk response, whereas the

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he study of bismuth-containing perovskites has become intense over recent years, in part to reduce or replace lead in piezoelectric materials1 but also to explore the high polarizability of the electron lone-pair associated with the Bi3+ ion (6s2,) which often induces ferroelectricity.2 The Bi3+ (1.03 Å, 6-coordination)2 ion usually occupies the larger A-site in the perovskite structure, such as in BiFeO3;3 however, BaBiO3 has Bi on the B-site and interestingly, Bi undergoes charge disproportionation into 3+ and 5+ valence states and is located at two different crystallographic sites.4−7 The Bi3+ and Bi5+ ions are 1:1 ordered, the unit cell is doubled, and the general formula is written as Ba2Bi2O6 (BBO) to indicate it is a double perovskite. At room temperature (RT), BBO adopts a body centered monoclinic structure (I2/m) with tilt system, a−a−c0. BBO undergoes three polymorphic phase transitions associated with BO6 octahedral tilting,8 i.e., primitive monoclinic (P21/n; a−a−c+) to body centered monoclinic (I2/m; a−a−c0) at 140 K, to rhombohedral (R3̅; a−a−a−) at 405 K and then to cubic (Fm3̅m; a0a0a0) at 750−800 K. Four probe electrical resistivity measurements9−12 show BBO to be semiconducting at RT. The conduction is attributed to mixed valency of the Bi3+ and Bi5+ ions on the B-sublattice. Ba2Bi1.4Nb0.6O6 (BBNO)13,14 has been recently reported to adopt a rhombohedral structure (R3̅) at RT and undergoes a phase transition from R3̅ to I2/m when placed under pressure, however P21/n was not observed.14 High-temperature X-ray Diffraction (XRD) results show BBNO to have cubic symmetry (space group Fm3̅m) when heated to 700 °C13 thus confirming a tilt-untilt transition from an a−a−a− to a0a0a0 system occurs >RT in this compound. In one study,13 BBNO is reported to be ferroelectric based on RT Polarization-Electric Field measurements and the existence of a giant peak in permittivity of ∼7 × 106 at ∼400 °C based on fixed frequency (100 Hz) capacitance measurements. This claim is questionable as the authors reported RT neutron diffraction data from which they concluded the space group R3̅ (a centrosymmetry space group) gave a more preferable fit to the data than R3 (a noncentrosymmetric space group). It is well-known that ferroelectricity is only exhibited by materials with noncentrosymmetric space groups and hence the crystal structure and electrical properties of BBNO are in contradiction. Here we use a combination of variable temperature X-ray diffraction (XRD) and impedance spectroscopy (IS) to resolve the structure−property relationships and show BBNO to have © XXXX American Chemical Society

Received: March 31, 2012 Revised: May 31, 2012

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Nb-based perovskites and, if correlated, can give rise to ferro- or antiferro-electricity, e.g., KNbO3 and NaNbO3,17 respectively. Further structural studies are required to confirm this suggestion for the higher than expected εr of BBNO ceramics. Bulk resistivity (Rb) values were extracted from the highest frequency arc with the lowest associated capacitance in the Z* plots and plotted in Arrhenius format as bulk conductivity values (σb = 1/Rb), Figure 2. Note that σb for BBO does not

Figure 1. Z* plot for (a) BBO and (b) BBNO, and modulus (M′′) vs log (frequency) plots for (c) BBO and (d) BBNO.

Figure 2. Arrhenius plot of σb for BBO and BBNO.

display Arrhenius-type behavior over the entire measured range and shows significant curvature at low temperature. An activation energy, Ea of ∼0.1 eV is extracted from the higher temperature regime where Arrhenius-type behavior is observed. σb data for BBNO, however, display Arrhenius-type behavior with Ea ∼0.7 eV. It is noteworthy that σb at 300 K is ∼7 orders of magnitude lower for BBNO compared to BBO at 300 K, and therefore, BBNO can be considered as a leaky dielectric at RT. The conductivity data and Ea values show a different conduction mechanism is present in BBNO and that semiconductivity in BBO can be suppressed by replacing Bi5+ with Nb5+, Figure 2. BBO is a p-type semiconductor12,18 and the conduction is related to the mixed valency of Bi3+/Bi5+ where Bi5+ serve as the “hole reservoir” (Bi5+ ≡ Bi3+ + 2h+). The replacement of Bi5+ with Nb5+ ions reduces the carrier concentration and also impedes the movement of the charge carriers. The combined effect results in reduced conductivity and a much higher Ea associated with the conduction mechanism. As noted in the introduction, BBNO has been reported to be a ferroelectric with a giant permittivity peak of ∼7 × 106 (at 100 Hz) at 400 °C;13 however, for fixed frequency data at >10 kHz, no peak was observed therefore casting doubt as to whether this low frequency peak is associated with a bulk related phenomenon. Figure 3a shows in-house fixed frequency capacitance measurements performed on our BBNO ceramics and two large peaks (ε > 1 × 106 at 1 kHz) are observed in the low frequency permittivity data. At higher frequency, no peaks are observed. Interestingly, the permittivity peaks occur at >600 K and this is above the tilt-untilt transition temperature observed by variable temperature XRD results, Figure B in the Supporting Information, and confirms they are not related to any structural transition. The high tan δ values (>10) in this temperature range, Figure 3b, shows the electrical response measured is not associated with the bulk but is associated with thin layer effects such as grain boundaries and/or electrode effects present in these samples. This is consistent with the IS data in Figure 1b for BBNO, which shows the low frequency data in this temperature range to be dominated by nonbulk effects. The giant permittivity values and peaks >600 K at low frequencies are therefore associated with grain boundary and/

lower frequency arc has an associated resistivity and capacitance of R ∼8.8 kΩ cm, C ∼5.9 nF cm−1 and is attributed to a grain boundary response. To characterize the temperature dependence of the bulk capacitance, we inspected IS data using spectroscopic plots of the imaginary component of the electric modulus M″ in the appropriate temperature range, ∼30−150 K for BBO and ∼373−500 K for BBNO, as such plots highlight small capacitance components and are therefore sensitive to the bulk response. Data for both sets of samples showed a single Debye peak at each temperature and the peak height increased slightly with increasing temperature, panels c and d in Figure 1. The capacitance value (C) was calculated from the peak maximum (Mmax″) using the equation Mmax″ = 1/2C and shows a slight decrease with increasing temperature for both sets of samples. The average C value for the temperature range measured was ∼3 and ∼9 pF cm−1 for BBO and BBNO, respectively. Such values confirm the data to be associated with the bulk response.15 They correspond to bulk relative permittivity, εr, values of ∼34 and ∼102, respectively. Note that εr values may be overestimated by ∼10% using this method due to broadening of the M″ Debye peaks; however, εr for BBO and BBNO is estimated to be ∼31 and ∼92, respectively, and both display very little temperature dependence over the range accessible to IS and/or radio-frequency (rf) capacitance measurements. εr for BBO and BBNO was calculated using the Clausius− Mossotti (CM) equation,16 see Clausius Mossotti calculation in the Supporting Information. The calculated εr is ∼28.3, which is close to the measured value, εr ∼31, for BBO ceramics. BBNO has a calculated εr value of ∼38.2, which is higher than BBO and the calculations reveal the higher εr is related to the higher polarizable Nb-ions (α ∼4.65 Å3) replacing the less polarizable Bi5+ ions (α ∼4.52 Å3) and a reduction in unit cell volume. However, the measured εr of ∼92 is more than twice the value calculated. This suggests that additional polarization exists in BBNO other than simple ionic polarization of the constituent ions and the most obvious suggestion is partial (noncorrelated) off-centring of the Nb-ions within the B-sites of the lattice. Off-centring of Nb-ions is commonly observed in B

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the EPSRC for funding (EP/G005001/1).

REFERENCES

(1) Sterianou, I.; Reaney, I. M.; Sinclair, D. C.; Woodward, D. I.; Hall, D. A.; Bell, A. J.; Comyn, T. P. Appl. Phys. Lett. 2005, 87, 242901. (2) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751. (3) Palewicz, A.; Przenioslo, R.; Sosnowska, I.; Hewat, A. W. Acta Crystallogr., Sect. B 2007, 63, 537. (4) Kennedy, B. J.; Howard, C. J.; Knight, K. S.; Zhang, Z.; Zhou, Q. Acta Crystallogr., Sect. B 2006, 62, 537. (5) Cox, D. E.; Sleight, A. W. Solid State Commun. 1976, 19, 969. (6) Thornton, G.; Jacobson, A. J. Acta Crystallogr. 1978, B34, 351. (7) Pei, S.; Jorgensen, J. D.; Hinks, D. G.; Lightfoot, P.; Zheng, Y.; Richards, D. R.; Dabrowski, B.; Mitchell, A. W. Mater. Res. Bull. 1990, 25, 1467. (8) Howard, C. J.; Stokes, H. T. Acta Crystallogr. 2005, A61, 93. (9) Sleight, A. W.; Gillson, J. L.; Bierstedt, P. E. Solid State Commun. 1975, 17, 27. (10) Cox, D. E.; Sleight, A. W. Acta Crystallogr. 1979, B35, 1. (11) Ghosh, A. Solid State Commun. 1999, 112, 45. (12) Lee, S.-H.; Jung, W.-H.; Sohn, J.-H.; Lee, J.-H.; Cho, S.-H. J. Appl. Phys. 1999, 86, 6351. (13) Mangalam, R. V. K.; Mandal, P.; Suard, E.; Sundaresan, A. Chem. Mater. 2007, 19, 4114. (14) Mangalam, R. V. K.; Narayana, C.; Sundaresan, A. High. Press. Res. 2009, 29, 272. (15) Sinclair, D. C.; Morrison, F. D.; West, A. R. Int. Ceram. 2000, 33. (16) Shannon, R. D. J. Appl. Phys. 1993, 73, 348. (17) Eglitis, R. I.; Kotomin, E. A.; Borstel, G. J. Phys.: Condens. Matter 2000, 12, L431. (18) Takagi, H.; Uchida, S.; Tajima, S.; Kitazawa, K.; Tanaka, S. Proceedings of the 18th International Conference on the Physics of Semiconductors; Stockholm, Sweden, Aug 11−15, 1986 ; Engström, O., Ed.; World Scientific: Singapore, 1986; p 1815 (19) Scott., J. F. J. Phys.: Condens. Matter 2008, 20, 021001.

Figure 3. (a) ε′ and (b) tan δ vs temperature plots for BBNO at various frequencies. Insert of (a) shows the ε′ vs temperature plot where ε′ were determined from M′′ spectroscopic plots.

or electrode relaxation effects and the bulk permittivity, on the basis of M″ spectroscopic plots, remains at ∼100 with little temperature dependence 500 K by rf electrical measurements due to the leaky nature of the samples in this temperature range and therefore no comment can be made on the influence of the tilt-untilt transition at ∼550−570 K. In addition, the reported P-E hysteresis loops at 300 K (for various voltages) are clearly nonsaturated in appearance and are indicative of a leaky dielectric as opposed to a ferroelectric material.19 The data presented in our study provides no evidence for ferroelectricity in BBNO ceramics, instead we propose BBNO to be a high permittivity, nonferroelectric oxide. Its high εr value and low processing temperature may make it an attractive material for applications in low temperature cofired ceramics (LTCCs) and further studies are in progress. In summary, bulk conductivity in BBO can be suppressed by Nb-doping. BBNO has high bulk permittivity (∼90−100) but based on the centrosymmetric space group adopted at RT it is not ferroelectric. Based on variable temperature XRD results it undergoes a tilt-untilt structural transition between ∼550−570 K. The giant permittivity response at >600 K reported in the literature and reproduced here is attributed to extrinsic effects such as grain boundary and/or sample−electrode interface effects and the nonsaturated P-E loops are indicative of a leaky (nonferroelectric) dielectric. Nonetheless, the measured bulk permittivity of ∼92 is higher than the expected permittivity based on a CM calculation. The disparity between the measured and calculated permittivity could be due to the tendency of Nb-ions to move off-center in the octahedral B-site in perovskite-type structures and is worthy of further investigation.

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ABBREVIATIONS BBO, Ba2Bi2O6 BBNO, Ba2Bi1.4Nb0.6O6

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on June 12, 2012, with an incorrect space group in the second paragraph. The correct version was published ASAP on June 12, 2012.

ASSOCIATED CONTENT

S Supporting Information *

Experimental section, Clausius Mossotti calculations, and variable temperature X-ray diffraction patterns for Ba2Bi1.4Nb0.6O6. This material is available free of charge via the Internet http://pubs.acs.org. C

dx.doi.org/10.1021/cm301013v | Chem. Mater. XXXX, XXX, XXX−XXX