Dielectric relaxation and relaxor behavior in bilayered perovskites Y. González-Abreu, A. Peláiz-Barranco, E. B. Araújo, and A. Franco Júnior Citation: Appl. Phys. Lett. 94, 262903 (2009); doi: 10.1063/1.3168651 View online: http://dx.doi.org/10.1063/1.3168651 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v94/i26 Published by the AIP Publishing LLC. Additional information on Appl. Phys. Lett. Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 11 Jul 2013 to 186.217.234.138. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions http://apl.aip.org/?ver=pdfcov http://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/2074845429/x01/AIP-PT/APL_PDFCoverPg_061913/FreeContentHand_1640x440.jpg/6c527a6a7131454a5049734141754f37?x http://apl.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=Y. Gonz�lez-Abreu&possible1zone=author&alias=&displayid=AIP&ver=pdfcov http://apl.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=A. Pel�iz-Barranco&possible1zone=author&alias=&displayid=AIP&ver=pdfcov http://apl.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=E. B. Ara�jo&possible1zone=author&alias=&displayid=AIP&ver=pdfcov http://apl.aip.org/search?sortby=newestdate&q=&searchzone=2&searchtype=searchin&faceted=faceted&key=AIP_ALL&possible1=A. Franco J�nior&possible1zone=author&alias=&displayid=AIP&ver=pdfcov http://apl.aip.org/?ver=pdfcov http://link.aip.org/link/doi/10.1063/1.3168651?ver=pdfcov http://apl.aip.org/resource/1/APPLAB/v94/i26?ver=pdfcov http://www.aip.org/?ver=pdfcov http://apl.aip.org/?ver=pdfcov http://apl.aip.org/about/about_the_journal?ver=pdfcov http://apl.aip.org/features/most_downloaded?ver=pdfcov http://apl.aip.org/authors?ver=pdfcov Dielectric relaxation and relaxor behavior in bilayered perovskites Y. González-Abreu,1 A. Peláiz-Barranco,1,a� E. B. Araújo,2 and A. Franco Júnior3 1Facultad de Física-Instituto de Ciencia y Tecnología de Materiales, Universidad de La Habana, San Lázaro y L, Vedado, La Habana 10400, Cuba 2Departamento de Física e Química, Universidade Estadual Paulista (UNESP), 15385-000 Ilha Solteira, São Paulo, Brazil 3Instituto de Física, Universidade Federal de Goiás, 74001-970 Goiânia, Goiás, Brazil �Received 3 May 2009; accepted 15 June 2009; published online 30 June 2009� Sr0.5Ba0.5Bi2Nb2O9 ferroelectric ceramics exhibit a complex dielectric behavior, showing typical relaxor behavior. The relaxation processes are described by the Cole–Cole model �K. S. Cole and R. H. Cole, J. Chem. Phys. 9, 341 �1941��. At temperatures below 490 K, the dielectric relaxation is associated to the relaxorlike ferroelectric behavior, resulting from the inhomogeneous distribution of barium due to its preference for the bismuth site. Above that, the interaction between the dipoles, which form the microdomains above the relaxor ferroelectric peak and the electrons, which are due to the ionization of the oxygen vacancies are discussed as the probable origin of the relaxation. © 2009 American Institute of Physics. �DOI: 10.1063/1.3168651� SrBi2Nb2O9 is a member of the family of the bismuth layer-structured materials,1–6 which are interesting ferroelec- trics for technical devices. The crystal structure is composed of �Bi2O2�2+ layers interleaved with perovskite-type blocks �Am−1BmO3m+1�2−. It has been suggested that the main contri- bution to its spontaneous polarization is due to displacement of the A cation in the perovskite block, which is quite differ- ent for the perovskite structure. An interesting feature of the- ses materials is that some of them allow cation site mixing among atom positions,7 especially between the bismuth and the A-site of the perovskite block.2 Barium doping has showed its preference for the bismuth site, which occur to equilibrate the lattice dimensions between the �Bi2O2�2+ lay- ers and the perovskite blocks.1 On the other hand, for SrBi2Nb2O9 the incorporation of barium to the structure pro- motes a complex dielectric spectrum showing a transition from a normal to a relaxor ferroelectric.3 For these ceramics the analysis of the dielectric relaxation mechanisms could be complex but very interesting. The present paper shows the study, which have been made on the dielectric relaxation mechanisms in Sr0.5Ba0.5Bi2Nb2O9 ferroelectric ceramics. Sr0.5Ba0.5Bi2Nb2O9 ceramics were prepared by solid- state reaction method. The powders of the starting materials were mixed with approximately 4.5% excess weight of Bi2O3 due to its high vapor pressure. The oxide mixtures were calcined in air atmosphere at 900 °C for 2 h; ground and again milled for 24 h. Then, these powders were pressed uniaxially by using 200 MPa and sintered in a closed alu- mina crucible at 1100 °C for 1 h. The relative density for the final ceramic was �98%. The x-ray diffraction analysis showed a pure orthorhombic structure. Gold painted elec- trodes were applied to the opposite faces of sintered samples to make the dielectric analysis. The dielectric measurements were carried out by using a computer controlled Agilent 4284A LCR Meter over a wide frequency range �102–106 Hz� from room temperature up to 550 °C ap- proximately, applying 200 mV ac to the samples. Figure 1 shows the temperature dependence of the real ���� and imaginary ���� parts of the dielectric permittivity, at various frequencies, as example of the observed behavior in the studied frequency range. For the real part �� it could be observed that its maximum value decreases with the increase in frequency and its corresponding temperature �Tm� shifts up to higher temperatures, showing high frequency disper- sion. The value of Tm is �466 and �529 K at 100 Hz and 1 MHz, respectively, showing a shift of temperature of ap- proximately 63 K, which is in good agreement with previous reports,3 but lower than that of other relaxor ceramics.8 For �� the maximum values are observed at lower temperatures than that of Tm and the corresponding temperature again shows significant dispersion with the frequency. These fea- tures are typical of materials showing a relaxor behavior of a�Author to whom correspondence should be addressed. Electronic mail: pelaiz@fisica.uh.cu. FIG. 1. Temperature dependence of the real ���� and imaginary ���� parts of the dielectric permittivity at various frequencies. APPLIED PHYSICS LETTERS 94, 262903 �2009� 0003-6951/2009/94�26�/262903/3/$25.00 © 2009 American Institute of Physics94, 262903-1 Downloaded 11 Jul 2013 to 186.217.234.138. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions http://dx.doi.org/10.1063/1.3168651 http://dx.doi.org/10.1063/1.3168651 the ferroelectric-paraelectric transition. At the higher tem- perature zone an abrupt increase of �� is observed, especially in the low frequency range, which could be associated to the conductivity losses. The origin of the relaxor behavior for these ceramics can be explained from a positional disorder of cations on A or B sites of the perovskite blocks that delay the evolution of long-rage polar ordering.8 It has been reported8 that the in- corporation of barium to the SrBi2Nb2O9 system provides a higher frequency dependence of the dielectric parameters, showing a relaxorlike ferroelectric. This behavior could be explained by considering the incorporation of a bigger ion into the A site of the perovskite block.9 The barium ions not only substitute the strontium ions in the A site perovskite block but enter the �Bi2O2�2+ layers leading to an inhomoge- neous distribution of barium and local charge imbalance in the layered structure.1 From Fig. 1 it is evident that the dielectric parameters do not reflect the relaxation behavior described by the Debye model. The Cole–Cole model, which considers a distribution of the relaxation time,10 is used to analyze the deviation from the ideal Debye model. In this model, �� and �� can be written as ����� = �� + ���� 2 ��1 − sinh��z� cosh��z� + cos�� � 2 � , �1� ����� = ���� 2 � sin�� � 2 � cosh��z� + cos�� � 2 � , �2� where z=ln����, ���=�s-��, �� is the permittivity at high frequency, �s is the static permittivity, � is the angular fre- quency, � is the mean relaxation time, and �= �1-��, where � reflects the distribution width of the relaxation time. For the studied ceramics the frequency dependence of �� and �� can be described by using Eqs. �1� and �2�, respec- tively. Figure 2 shows the experimental data �solid points� and the corresponding theoretical results �solid lines� at a few representative temperatures, showing a good agreement between experimental and theoretical values. The analysis was made from 423 to 523 K approximately. For tempera- tures above 523 K the dielectric spectrum could not be de- scribed by using the Cole-Cole equations considering the behavior of �� �Fig. 1�. Figure 3 shows the temperature dependence for the � parameter, calculated from the � values, which were ob- tained from the fitting in Fig. 2. From � values two charac- teristic regions could be considered from the break around 490 K, which were named as first �TR-1� and second �TR-2� temperature region, respectively. Below 490 K, � decreases, being 0.86 and 0.67 for 433 and 488 K, respectively; show- ing a high dispersive relaxation. Above 490 K, � is almost constant �between 0.57 and 0.53�, showing a lower disper- sive relaxation. The results for TR-1 have been obtained in the temperature range where a higher dispersion of the di- electric parameters is observed �Fig. 1�, which is related to the relaxor ferroelectric behavior. Then, it could be consid- ered that the main origin of the dielectric relaxation process below 490 K could be associated to the relaxorlike ferroelec- tric behavior of the material. It is known that the relaxation time ��� of a relaxor be- havior follows the empirical Vogel–Fulcher law, which could be written as �=�o exp�U /kB�T-TVF��, where �o, U, kB, and TVF are, respectively, the pre-exponential term, the activation energy, the Boltzmann�s constant and the characteristic tem- perature. It is found that the temperature dependence of the relaxation time for TR-1 can be well fitted to the Vogel– FIG. 2. Frequency dependence of the real �� ��� and imaginary �� ��� parts of the dielectric permittivity at a few representatives temperatures. Solid lines represent the fitting by using Eqs. �1� and �2�. FIG. 3. Temperature dependence for the � parameter, calculated from the � values, which were obtained from the fitting in Fig. 2. 262903-2 González-Abreu et al. Appl. Phys. Lett. 94, 262903 �2009� Downloaded 11 Jul 2013 to 186.217.234.138. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions Fulcher law, as shown in Fig. 4. On the other hand, it is found that the relaxation time for TR-2 follows the Arrhenius relation. Notice the temperature dependence of the mean re- laxation time for TR-2 in the inset of Fig. 4. The correspond- ing activation energy value is quite different than that of the activation energy value obtained for TR-1. It is clear that the relaxorlike ferroelectric behavior cannot be the main cause of the second relaxation process. However, from Fig. 1 it could be assumed an important influence of the conductivity pro- cesses on the relaxation process above 490 K. It is known that barium ions have preference for the bismuth site in the barium doped SrBi2Nb2O9 ceramics.1 The electrical charge unbalance caused by the trivalent Bi3+ ion substitution for the divalent Ba2+ ions is compensated by the creation of oxygen vacancies. The activation energy value obtained for TR-2 is in the same order than those previous reports, which have been associated to ionized oxygen vacancies.11,12 Hence, it is reasonable to assume that the hop- ping of the electrons, which appears due to the ionization of the oxygen vacancies, could contribute to the dielectric re- laxation and its long-distance movement contributes to the electrical conduction. On the other hand, the second relaxation process �above 490 K� occurs at higher-temperature side of the relaxor peak. For a relaxor ferroelectric, microdomains can be observed even at higher temperature far from the temperature of the relaxor behavior peak.13 From this point of view, the authors suggest that the second relaxation process could result from the contribution of the interaction between the dipoles, which forms the microdomains existing at higher-temperature side of the relaxor peak, and the electrons that are due to the ionization of the oxygen vacancies. As summary, two dielectric relaxation processes have been analyzed in Sr0.5Ba0.5Bi2Nb2O9 ferroelectric ceramics. The first one has been associated to the relaxorlike ferroelec- tric behavior as the main cause. The other one has been associated to the interaction between the dipoles, which form the microdomains that exist above the relaxor ferroelectric peak, and the electrons, which are due to the ionization of the oxygen vacancies. Further analysis is required concerning the electrical conductivity behavior to archive a better under- standing. The authors wish to thank to the ICTP for financial sup- port of Latin-American Network of Ferroelectric Materials. A.P.-B. wishes to thank to CNPq Brazilian agency �Grant No. 450070/2008-5�. E.B.A. would like to thank to Brazilian agencies FAPESP �Grant Nos. 2007/05302-4 and 2007/ 00183-7� and CNPq �Grant No. 301382/2006-9�. 1M. S. Haluska and S. T. Misture, J. Solid State Chem. 177, 1965 �2004�. 2S. M. Blake, M. J. Falconer, M. McCreedy, and P. Lightfoot, J. Mater. Chem. 7, 1609 �1997�. 3S. Huang, Ch. Feng, L. Chen, and Q. Wang, J. Am. Ceram. Soc. 89, 328 �2006�. 4B. Wachsmuth, E. Zschech, N. W. Thomas, S. G. Brodie, S. J. Gurman, S. Baker, and S. C. Bayliss, Phys. Status Solidi A 135, 59 �1993�. 5D. Nelis, D. Mondelaers, G. Vanhoyland, A. Hardy, K. Van Werde, H. Van den Rul, M. K. Van Bael, J. Mullens, L. C. Van Poucke, and J. D’Haen, Thermochim. Acta 426, 39 �2005�. 6B. J. Kennedy, and Ismunandar, J. Mater. Chem. 9, 541 �1999�. 7M. Mahesh Kumar and Z.-G. Ye, J. Appl. Phys. 90, 934 �2001�. 8C. Miranda, M. E. V. Costa, M. Avdeev, A. L. Kholkin, and J. L. Baptista, J. Eur. Ceram. Soc. 21, 1303 �2001�. 9Y. Wu, M. J. Forbess, S. Seraji, S. J. Limmer, T. P. Chou, C. Nguyen, and G. Z. Cao, J. Appl. Phys. 90, 5296 �2001�. 10K. S. Cole and R. H. Cole, J. Chem. Phys. 9, 341 �1941�. 11S. A. Long and R. N. Blumenthal, J. Am. Ceram. Soc. 54, 577 �1971�. 12R. Waser, J. Am. Ceram. Soc. 74, 1934 �1991�. 13G. Burns, Phase Transitions 5, 261 �1985�. FIG. 4. Temperature dependence of the mean relaxation time ��� for both characteristic temperature regions. The first one �TR-1� can be well fitted to the Vogel–Fulcher law and the second one �TR-2� follows the Arrhenius relation �inset of the figure�, respectively. 262903-3 González-Abreu et al. Appl. Phys. Lett. 94, 262903 �2009� Downloaded 11 Jul 2013 to 186.217.234.138. This article is copyrighted as indicated in the abstract. 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