Solid State Communications 151 (2011) 708–711

Contents lists available at ScienceDirect

Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Evolution from relaxor-like dielectric to ferroelectric in Ba[(Fe0.5Nb0.5)1−xTix]O3solid solutionsZhuo Wang a,b, Xiang Ming Chen a,∗

a Laboratory of Dielectric Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, Chinab School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China

a r t i c l e i n f o

Article history:Received 14 October 2010Received in revised form14 February 2011Accepted 15 February 2011by M. WangAvailable online 2 March 2011

Keywords:A. FerroelectricsC. Crystal structure and symmetryD. Dielectric responseD. Phase transitions

a b s t r a c t

Ba[(Fe0.5Nb0.5)1−xTix]O3 (x = 0.2, 0.4, 0.6, 0.8, 0.85, 0.9 and 0.95) solid solutions were synthesizedby a standard solid-state reaction technique. X-ray diffraction at room temperature and dielectriccharacteristics over a broad temperature and frequency range were evaluated systematically. Thestructure of Ba[(Fe0.5Nb0.5)1−xTix]O3 solid solutions changed from cubic to tetragonal with increasingx. A Debye-like dielectric relaxation following the Arrhenius law similar to that in Ba(Fe0.5Nb0.5)O3 wasobserved at lower temperature in the composition range 0.2 ≤ x ≤ 0.8, while the relaxor ferroelectric,diffused ferroelectric and normal ferroelectric behavior were observed for x = 0.85, 0.9 and 0.95,respectively. The process of the evolution of relaxor-like dielectric to ferroelectric suggested the changingfrom dilute polar micro-domains to polar micro-domains, polar micro/macro-domains and then polarmacro-domains in the present ceramics.

© 2011 Elsevier Ltd. All rights reserved.

1. Introduction

It is well known that BaTiO3 has a paraelectric-to-ferroelectrictransition at about 120 °Cwith a very high dielectric constant. Sub-stitution of other ions for host cations at the A- or B-site in BaTiO3leads to remarkable changes in various characteristics in order tobroaden out its permittivity maximum and to increase the tem-perature stability of the high permittivity state [1,2]. Fe-dopingin many perovskite systems changes strongly their structural andelectric characteristics. Recently, a number of Fe-containing com-plex perovskites A(Fe0.5B0.5)O3(A = Ba, Sr, Ca; B = Nb, Ta, Sb)[3–5] have attracted much scientific attention because of their gi-ant dielectric response and the unique dielectric relaxation behav-ior similar to that in CaCu3Ti4O12 [6,7]. These materials generallyexhibit a giant dielectric constant step (in the order of 103–105)over a broad temperature and frequency interval, and the dielec-tric constant drops rapidly into a low value less than 100 as thema-terials are cooled through a critical temperature where no phasetransition is detected. In our previous work, the dielectric relax-ations in Ba(Fe0.5B0.5)O3(B = Nb, Ta) ceramics are investigatedsystematically, and the strong frequency dispersionwith a high di-electric loss near the relaxation temperature was detected [8,9].These relaxor-like dielectrics attracted attention towards investi-gating the origin of dielectric relaxation.

∗ Corresponding author. Tel.: +86 571 8795 2112; fax: +86 571 8795 2112.E-mail address: xmchen59@zju.edu.cn (X.M. Chen).

0038-1098/$ – see front matter© 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2011.02.015

Both BaTiO3 and A(Fe0.5B0.5)O3(A = Ba, Sr, Ca; B = Nb, Ta, Sb)are described as typical perovskite-type compounds and could beexpected to form the solid solutions. Li et al. have shown that thetemperature of the dielectric constant peak shifts towards roomtemperature with increasing Fe content in BaTiO3–Ba(Fe1/2Ta1/2)O3 system [10]. All these samples are normal ferroelectrics witha dielectric constant relatively higher about 9000 at 10 kHz forBaTi0.96(Fe0.5Ta0.5)0.04O3 specimen. On the other hand, Abdelkafiet al. reported the structure and dielectric characteristics ofBaTi1−x(Fe0.5Nb0.5)xO3 solid solution [11–13]. This system exhibitsmany interesting features, such as shift in transition temperature,diffuse phase transition and strong increase in real permittivity.The evolution from a normal ferroelectric to a ferroelectric relaxorwas observed with increasing Fe and Nb concentrations upto composition x = 0.15. It should be noted that these studieswere based on the data in the narrow range of composition. In thepresent letter, we report the structural and electric characteristicsof Ba[(Fe0.5Nb0.5)1−xTix]O3 (x = 0.2, 0.4, 0.6, 0.8, 0.85, 0.9 and 0.95)solid solutions comprehensively, and try to understand the processof evolution from relaxor-like dielectric to relaxor ferroelectric,diffuse ferroelectric and then normal ferroelectric.

2. Experimental

Ba[(Fe0.5Nb0.5)1−xTix]O3 (x = 0.2, 0.4, 0.6, 0.8, 0.85, 0.9 and0.95) solid solutions were prepared by a standard solid statereaction process at 1573 K in air for 3 h. The crystal phasewas confirmed by powder X-ray diffraction (XRD) analysis with

Z. Wang, X.M. Chen / Solid State Communications 151 (2011) 708–711 709

Fig. 1. (Color online) (a) X-ray diffraction patterns, (b) an expanded view and(c) lattice parameter as a function of Ti substitution for Ba[(Fe0.5Nb0.5)1−xTix]O3ceramics.

Cu Kα radiation (RIGAKU D/max 2550/PC, Rigaku Co., Tokyo,Japan). The dielectric characteristics of the dense ceramics weremeasured with a broadband dielectric spectrometer (TurkeyConcept 50, Novocontrol Technologies, Germany) in a broad rangeof temperatures (133–573 K) and frequencies (40–2000000 Hz).Ferroelectric P–E hysteresis loop measurements were performedat various temperatures by a precision materials analyzer (RTPremier II, Radiant Technologies, Inc., NM, USA).

3. Results and discussions

Fig. 1(a) gives the XRD patterns of dense Ba[(Fe0.5Nb0.5)1−xTix]O3 (x = 0.2, 0.4, 0.6, 0.8, 0.85, 0.9 and 0.95) ceramics. Two kindsof solid solutions exist in the Ba[(Fe0.5Nb0.5)1−xTix]O3 system atroom temperature. The first one is cubic in space group Pm3m inthe composition range 0.2 ≤ x ≤ 0.9, while another kind ofsolid solution is tetragonal for x = 0.95 and it is in space groupP4mm (see Fig. 1(b)). Fig. 1(c) shows the lattice parameter (a) as afunction of Ti substitution for Ba[(Fe0.5Nb0.5)1−xTix]O3 ceramics. Itis clear that a linear relationship exists in the composition range0.2 ≤ x ≤ 0.9, where an increase in BaTiO3 substituting ofBa(Fe0.5Nb0.5)O3 leads to a decrease in lattice parameter (a). Thisdecrease should be related to the values of ionic radii: the radius ofTi4+(0.605 Å) is quite smaller than both the radii of Fe3+(0.64 Å)and Nb5+(0.7 Å) [14].

The dielectric relaxations similar to those in Ba(Fe0.5Nb0.5)O3[8] are observed in Ba[(Fe0.5Nb0.5)1−xTix]O3 solid solutions inthe composition range 0.2 ≤ x ≤ 0.8. The temperature de-pendence of dielectric constant (ε′) and dielectric loss (tan δ) forBa[(Fe0.5Nb0.5)0.8Ti0.2]O3 ceramics are given as an example inFig. 2. To understand the variation tendency of dielectric naturein the present ceramics, let us discuss the low temperature di-electric relaxation. The low temperature dielectric anomaly withstrong frequency dispersion followed by a giant dielectric con-stant step (∼30000), is distinctly detected in the temperature

Fig. 2. (Color online) Temperature dependence of (a) dielectric constant ε′ and(b) dielectric loss tan δ of Ba[(Fe0.5Nb0.5)0.8Ti0.2]O3 ceramics at different frequen-cies. The inset shows the Arrhenius plot for the low-temperature relaxation. Sym-bols are experimental points and the solid line is the least squares fitting.

range of 150–400 K in the ε′-T curve. There is an obvious peakin the tan δ-T curve corresponding to the rapid drop of dielectricconstant at low temperature, and it shifts towards a higher tem-perature with increasing frequency. However, the frequency dis-persion in Ba[(Fe0.5Nb0.5)1−xTix]O3 (0.2 ≤ x ≤ 0.8) solid solutionsis much stronger than that in the typical relaxor ferroelectrics suchas Pb(Mg1/3Nb2/3)O3. 1Tm (the difference between the dielectricconstant peak temperatures at 1 MHz and 1 kHz) for the presentceramics is greater than 200 K, while that for Pb(Mg1/3Nb2/3)O3 isjust 12 K [15].

In order to get a deep insight into the low temperature dielectricrelaxation in the Ba[(Fe0.5Nb0.5)1−xTix]O3 (0.2 ≤ x ≤ 0.8) solidsolutions, the frequency dependence of the tan δ peak temperaturehas been studied. The variation relationship obeys the Arrheniuslaw:

f = f0 exp(−Ea/kT ) (1)

where, f0 is the pre-exponential factor, Ea is the activation energy,and k is the Boltzmann constant. Fitting the present experimentaldata for Ba[(Fe0.5Nb0.5)0.8Ti0.2]O3 solid solution as an example (theinset in Fig. 2(b)) with the Arrhenius law yields the parametersEa = 0.202 eV and f0 = 6.99 × 108 Hz. The activation energiesfor Ba[(Fe0.5Nb0.5)1−xTix]O3 (0.2 ≤ x ≤ 0.8) are 0.202, 0.184,0.186 and 0.188 eV, respectively, and these values are comparablewith 0.174 eV in the un-substituted Ba(Fe0.5Nb0.5)O3 ceramics [8].A similar mechanism of dielectric relaxation originating from themixed-valent structure of Fe2+/Fe3+ and the hopping of chargecarriers between them is expected.

The dielectric constant ε′ as a function of temperature at variousfrequencies (40 Hz–2 MHz) of Ba[(Fe0.5Nb0.5)1−xTix]O3 (0.85 ≤

x ≤ 0.95) solid solutions are shown in Fig. 2. For the composition ofx = 0.85 (see Fig. 3(a)), ε′(T ) shows a maximum which decreaseswith rising frequency in the low temperature range from 133 to∼200 K. The value of the dielectric constant peak temperature

710 Z. Wang, X.M. Chen / Solid State Communications 151 (2011) 708–711

Fig. 3. (Color online) Temperature dependence of dielectric constant ε′ of Ba[(Fe0.5Nb0.5)1−xTix]O3 ((a) x = 0.85, (b) x = 0.9 and (c) x = 0.95) ceramics at differentfrequencies between 40 Hz and 2 MHz. (d) The inverse of dielectric constant peak temperature as a function of frequency for Ba[(Fe0.5Nb0.5)0.15Ti0.85]O3 ceramics, wherethe solid squares are the experimental data and the curve is the Vogel–Fulcher fitting.

Tm shifts to higher temperature with increasing frequency. 1Tmfor Ba[(Fe0.5Nb0.5)0.15Ti0.85]O3 solid solution is 11.98 K matchedwell with that for typical relaxor ferroelectrics. Such behavior isa relaxor-type similar to the earlier reports [12]. Fig. 3(b) showsthe temperature dependence of dielectric constant ε′ for the solidsolution with composition x = 0.9. A broadened dielectric peakis observed and the dielectric constant peak temperature Tmincreases to about 250 K, and it is almost not dependent onfrequency. All these dielectric results for Ba[(Fe0.5Nb0.5)0.1Ti0.9]O3are in agreement with a diffuse ferroelectric behavior [16]. For thecomposition close to BaTiO3 (x = 0.95), three dielectric peaksare observed, which originate from the phase transitions fromthe cubic paraelectric to tetragonal ferroelectric at 330 K, andthen to an orthorhombic ferroelectric at 275 K, and finally to arhombohedral ferroelectric at 230 K similar to those of BaTiO3 (seeFig. 3(c)).

Fig. 3(d) presents the variation of dielectric constant peak tem-perature with frequency for Ba[(Fe0.5Nb0.5)0.15Ti0.85]O3 solid solu-tion, and the data well follow the Vogel–Fulcher relationship [15]:f = f0 exp[−Ea/k(Tm − Tf )], (2)where Tf is the static freezing temperature, Ea is the activationenergy for polarization fluctuation of an isolated micro polarregion, f0 is the pre-exponential factor, k is the Boltzmann constantand Tm is the dielectric constant peak temperature. The fittingparameters are obtained as: Ea = 0.01 eV, Tf = 129.3 K andf0 = 3.99 × 106 Hz. It is worth noticing that the value of thecalculated Tf matched well with the observed peak in dielectricconstant ε′ at lower frequency. This analysis just indicates that therelaxor ferroelectric behavior in Ba[(Fe0.5Nb0.5)0.15Ti0.85]O3 solidsolution is a typical spin glass-like thermally activated process.

The diffuse degree of the ferroelectric phase transition wasevaluated using an empirical relation [17]:

1ε′

−1

ε′max

=(T − Tm)γ

C(3)

Fig. 4. Plots of ln(1/ε′−1/ε′

max) vs. ln(T−Tm) at 1MHz for Ba[(Fe0.5Nb0.5)1−xTix]O3ceramics with x = 0.85, 0.9 and 0.95.

where, ε′max is the peak value of ε′, C is a constant and γ is the

diffuseness exponent and it is between 1 and 2 for diffused fer-roelectrics. The larger γ indicates the higher degree of diffuseness,and γ = 2 corresponds to the typical relaxor transition and fornormal ferroelectrics γ equals 1. According to Fig. 4, the values ofγ or Ba[(Fe0.5Nb0.5)1−xTix]O3 (x = 0.85, 0.9 and 0.95) solid so-lutions at 1 MHz are 1.94, 1.90 and 1.27, respectively. The resultsindicate that the relaxor behaviors in the Ba[(Fe0.5Nb0.5)1−xTix]O3(x = 0.85, 0.9 and 0.95) system decrease gradually with Ti contentincreasing.

Fig. 5 shows the P–E hysteresis curves of Ba[(Fe0.5Nb0.5)1−xTix]O3 ((a) x = 0.85, (b) x = 0.9 and (c) x = 0.95) solid solutionsbelow the dielectric constant peak temperature Tm or the Curietemperature TC . For samples with x = 0.85 and 0.9, the sponta-neous polarizations (Pr ) are 4.4 and 6.7 µC/cm2, and the coercivefields (Ec) are 3.3 and 6.4 kV/cmat 123K, respectively. On the otherhand, Pr and Ec for Ba[(Fe0.5Nb0.5)0.05Ti0.95]O3 ceramics are about7.9 µC/cm2 and 2.7 kV/cm at room temperature, respectively. Itshould be noted that Pr is enhanced with increasing Ti content.

Z. Wang, X.M. Chen / Solid State Communications 151 (2011) 708–711 711

Fig. 5. P–E curves of Ba[(Fe0.5Nb0.5)1−xTix]O3 ((a) x = 0.85, (b) x = 0.9 and (c) x = 0.95) solid solutions at different temperatures.

In the present solid solutions, the enhanced Pr with increasingcontent of Ti should be related to the electric ordered nature ofBaTiO3. These results combined with the dielectric characteristicsdisplay that the samples with x = 0.85, 0.9 and 0.95 belong torelaxor ferroelectric, diffuse ferroelectric and normal ferroelectricclass with the presence of polar nano regions (PNRs), micro/macromixed polar regions and macro polar regions, respectively [16].Therefore, it can be imagined that the relaxor ferroelectric-like be-havior in Ba[(Fe0.5Nb0.5)1−xTix]O3 (0.2 ≤ x ≤ 0.8) solid solutionsis attributed to the more diluted polar clusters due to the mixed-valent structure, which grows in size with decreasing temperaturebut is never frozen. The microstructure evidence for this evolutionneeds to be investigated further.

4. Conclusions

In conclusion, single-phase Ba[(Fe0.5Nb0.5)1−xTix]O3 (x = 0.2,0.4, 0.6, 0.8, 0.85, 0.9 and 0.95) solid solutions can be synthe-sized via a standard solid state reaction. The phase structuresof Ba[(Fe0.5Nb0.5)1−xTix]O3 change from cubic to tetragonal withincreasing Ti4+ content. A Debye-like dielectric relaxation follow-ing the Arrhenius law similar to that in Ba(Fe0.5Nb0.5)O3 is ob-served at lower temperature in the composition range 0.2 ≤

x ≤ 0.8, while the relaxor ferroelectric, diffused ferroelectric andnormal ferroelectric behavior is observed for x = 0.85, 0.9 and0.95, respectively. The Ba[(Fe0.5Nb0.5)1−xTix]O3 (x = 0.85, 0.9 and0.95) solid solutions exhibit typical ferroelectric P–E hysteresisbehavior below the transition temperatures. The process of evolu-tion of relaxor-like dielectric to ferroelectric suggests the chang-ing from dilute polar micro-domains to polar micro-domain,

polar micro/macro-domains and then polar macro-domains in thepresent ceramics.

Acknowledgements

The present work was supported by National Science Founda-tion of China under Grant No. 50832005, and National Basic Re-search Program of China under Grant No. 2009CB929503.

References

[1] F. Bahri, H. Khemakhem, M. Gargouri, A. Simon, R.V.D. Mühll, J. Ravez, SolidState Sci. 5 (2003) 1235.

[2] J. Ravez, C. Broustera, A. Simon, J. Mater. Chem. 9 (1999) 1609.[3] S. Saha, T.P. Sinha, J. Phys.: Condens. Matter. 14 (2002) 249.[4] I.P. Raevski, S.A. Prosandeev, A.S. Bogatin, M.A. Malitskaya, L. Jastrabik, J. Appl.

Phys. 93 (2003) 4130.[5] C.-Y. Chung, Y.-H. Chang, G.J. Chen, J. Appl. Phys. 96 (2004) 6624.[6] C.C. Homes, T. Vogt, S.M. Shapiro, S. Wakimoton, A.P. Ramirez, Science 293

(2001) 673.[7] L. Ni, X.M. Chen, Appl. Phys. Lett. 91 (2007) 122905.[8] Z. Wang, X.M. Chen, L. Ni, X.Q. Liu, Appl. Phys. Lett. 90 (2007) 022904.[9] Z. Wang, X.M. Chen, L. Ni, Y.Y. Liu, X.Q. Liu, Appl. Phys. Lett. 90 (2007) 102905.

[10] G.B. Li, S.X. Liu, F.H. Liao, S.J. Tian, X.P. Jing, J.H. Lin, Y. Uesu, K. Kohn, K. Saitoh,M. Terauchi, N.L. Di, Z.H. Cheng, J. Solid State Chem. 177 (2004) 1695.

[11] Z. Abdelkafi, N. Abdelmoula, H. Khemakhem, R.V.D. Mühll, L. Bih, J. AlloysCompd. 427 (2007) 260.

[12] Z. Abdelkafi, N. Abdelmoula, H. Khemakhem, O. Bidault, M. Maglione, J. Appl.Phys. 100 (2006) 114111.

[13] Z. Abdelkafi, N. Abdelmoula, H. Khemakhem, A. Simon, M. Maglione, Phys.Status Solidi 205 (2008) 2948.

[14] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. Sect. A 32 (1976) 751.[15] D. Viehland, S.J. Jang, L.E. Cross, J. Appl. Phys. 68 (1990) 2916.[16] I. Rivera, A. Kumar, N. Ortega, R.S. Katiyar, S. Lushnikov, Solid State Commun.

149 (2009) 172.[17] S.M. Pilgrim, A.E. Sutherland, S.R. Winzer, J. Am. Ceram. Soc. 73 (1990) 3122.