Journal of the Korean Physical Society, Vol. 63, No. 3, August 2013, pp. 306∼309
Effects of Bismuth Donor Doping on the Phase Structure and the Magnetic and Ferroelectric Properties of Fe-doped BaTiO3 Widi Yansen, Kadek Juliana Parwanta, Hadiyawarman, Deokhyeon Kim, Younmi Gwan, Jaeyeong Kim, Chunli Liu, Chang Uk Jung and Bo Wha Lee∗ Department of Physics, Hankuk University of Foreign Studies, Yongin 449-791, Korea (Received 29 May 2012, in final form 4 January 2013) To compensate for the acceptor dopant Fe3+ in Fe-doped BaTiO3 ceramics, we introduced a donor dopant into these ceramics in order to investigate its effects on their phase structure and on the magnetic and ferroelectric properties. We prepared 5% Fe-doped BaTiO3 ceramic and used Bi3+ as the donor dopant. We found that the formation of the hexagonal phase in Fe-doped BaTiO3 was suppressed and that the magnetic and the ferroelectric properties of these ceramics were changed by the Bi3+ doping. These results indicate that the structural, magnetic and ferroelectric properties of BaTi0.95 Fe0.05 O3 ceramics can be controlled by Bi3+ doping and that a multiferroic property was observed in the Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 ceramic. PACS numbers: 61.50.-f, 77.80.-e, 75.50.Gg, 61.05.cp Keywords: Crystal structure, Ferroelectric, Ferrimagnetic, X-ray diffraction DOI: 10.3938/jkps.63.306
been reported yet. Bi3+ is believed to enhance the polarizability because of the particular orientation of lone pairs, which may create local dipoles ordered in a ferroelectric fashion [10]. Here, we chose an Fe doping concentration of 5% because the phase is still polymorphic, so we can easily observe the phase transformation after doping with Bi3+ . The effects of Bi3+ doping on the phase structure and on the magnetic and the ferroelectric properties of BaTi0.95 Fe0.05 O3 ceramics were investigated in this work.
I. INTRODUCTION Multiferroics are one of the hottest research topics in materials science because of their great potential for new device functions [1–3]. Introducing impurities having d electrons into ferroelectric materials yields multiferroic materials [2,3]. BaTiO3 is a well-known ferroelectric material with a Curie temperature (TC ) of around 120 ◦ C [4]. At temperatures below TC , BaTiO3 has a tetragonal perovskite structure with spontaneous polarization whereas at temperatures above TC , the tetragonal phase changes to cubic and hexagonal phases and its ferroelectricity is lost [4, 5]. Fe-doped BaTiO3 ceramics reportedly exhibit magnetic ordering due to the replacement of Ti4+ ions by Fe3+ ions [6–8]. In Fe-doped BaTiO3 ceramics, the tetragonal phase merges with the hexagonal phase (polymorphism) at a doping level of 10% whereas the pure hexagonal phase appears at higher Fe doping (more than 20%) [6]. In addition, the sintering temperature and time influence the phase structure of Fe-doped BaTiO3 ceramics [6]. Although Fe doping can induce ferromagnetism in BaTiO3 , the ferroelectricity is suppressed [6] by the appearance of the hexagonal phase [9]. Du et al. recently reported the effects of Nb5+ doping on the phase transition in and the magnetic properties of Fe-doped BaTiO3 ceramics [9]. However, to our knowledge, the effects of Bi3+ doping on the structural and the magnetic properties of Fe-doped BaTiO3 have not ∗ E-mail:
II. EXPERIMENTS AND DISCUSSION Ba1−x Bix Ti0.95 Fe0.05 O3 (0 ≤ x ≤ 0.05) ceramics were prepared using a solid-state reaction. The reagents, BaCO3 , TiO2 , Bi2 O3 , and Fe2 O3 in the forms of highpurity powders (99.99%), were thoroughly mixed and calcined in air at 1000 ◦ C for 24 h. Then, the ceramics were mixed again, pressed into pellets, and reheated at 1300 ◦ C for 12 h. X-ray diffraction (XRD) data were obtained at room temperature over scattering angles of 20 80◦ at a slow scanning rate by using Cu K radiation. The data were analyzed by using a Rietveld refinement with the General Structure Analysis System program with the EXPGUI front-end. The magnetic and the ferroelectric properties were measured using a vibration sample magnetometer (Lakeshore, model 7404) and a ferroelectric tester (RT66B, Radiant Technology), respectively.
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Effects of Bismuth Donor Doping on the Phase Structure and · · · – Widi Yansen et al.
Fig. 1. (Color online) Observed and calculated Rietveldrefined XRD patterns of Ba1−x Bix Ti0.95 Fe0.05 O3 (0 ≤ x ≤ 0.05) and the difference between them.
The XRD patterns for x = 0.05 were refined using a crystal system, i.e., a tetragonal phase with space group P 4 mm [11]. For x < 0.05, an additional crystal system, a hexagonal phase with space group P 63 /mmc, was required to reproduce all the observed peaks [12]. The initial fit was done using the scale factor, unit cell, and background parameters. Figure 1 shows the observed and the calculated Rietveld-refined XRD patterns of Ba1−x Bix Ti0.95 Fe0.05 O3 (0 ≤ x ≤ 0.05) ceramics and the differences between them. Good agreement between the observed and the calculated patterns was obtained with Rp ≤ 6.27%, Rwp ≤ 8.12%, and χ2 ≤ 1.55 for all the ceramics. The BaTi0.95 Fe0.05 O3 ceramics consisted of both tetragonal and hexagonal phases, as reported in the literature [6]. However, the tetragonal phase became dominant after Bi3+ doping was introduced. At x = 0.05, the hexagonal phase disappeared, and only the tetragonal phase was observed. The composition of the Ba1−x Bix Ti0.95 Fe0.05 O3 (0 ≤ x ≤ 0.05) ceramics with a tetragonal phase is shown in Fig. 2. We also tried to dope BaTi0.95 Fe0.05 O3 ceramic with a higher concentration of Bi3+ (x = 0.07) and observed that the structure was unchanged (data not shown here). These results indicate that Bi3+ doping suppresses and controls the formation of the hexagonal structure in Fe-doped BaTiO3 ceramics. The hexagonal phase in Fe-doped BaTiO3 ceramics at room temperature is reportedly due to oxygen vacancies [9]. Introducing Bi3+ doping into Fe-doped BaTiO3 ceramics suppressed the hexagonal phase, presumably by charge compensation in which Bi3+ compensated for the charge of Fe3+ and reduced the concentration of the oxygen vacancies in the BaTiO3 ceramics. Figure 3 shows the magnetization as a function of the external field (M − H plot) for the Ba1−x Bix Ti0.95 Fe0.05 O3 (0 ≤ x ≤ 0.05) ceramics at room temperature. Magnetic properties with small hysteresis
Fig. 2. (Color online) Phase Ba1−x Bix Ti0.95 Fe0.05 O3 (0 ≤ x ≤ 0.05).
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composition
of
Fig. 3. (Color online) Room-temperature M − H curves of Ba1−x Bix Ti0.95 Fe0.05 O3 (0 ≤ x ≤ 0.05). The inset shows the room-temperature hysteresis loop for x = 0.02.
loops were observed in the Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 ceramic, as shown in the inset of Fig. 3. Bi3+ doping at x = 0.02 in this ceramic induced ferrimagnetism. At higher concentrations of Bi3+ (up to x = 0.05), the magnetic properties changed from ferrimagnetic to paramagnetic. The origin of the paramagnetism can be understood as a structural transition from ferromagnetic hexagonal Fe-doped BaTiO3 to weakly magnetic or nonmagnetic tetragonal BaTiO3 , as shown in Fig. 1. To further study the magnetic properties of the ceramics at x = 0.02, we investigated the temperature dependence of the magnetization at temperatures between 300 and 600 K at an applied field of 0.5 T. Ferrimagnetic characteristics appear in the M − T plot in Fig. 4, with a TC of about 450 K. Before concluding that the observed ferrimagnetism in Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 is intrinsic, we need to rule out the contribution of possible magnetic impurities such as γ-Fe2 O3 and Fe3 O4 . According to the XRD data, iron oxides and secondary phases were
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Journal of the Korean Physical Society, Vol. 63, No. 3, August 2013
Figure 5 shows the polarization as a function of the electric field (P − E plot) for an applied voltage of 1 kV at room temperature for the Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 ceramic. A slightly distorted hysteresis, including a leakage current effect, appears at x = 0.02. However, a very high leakage current was observed for x = 0.05 (data not shown here). The saturation polarizations (Pr ) for x = 0.02 and 0.05 at room temperature are 1.12 µC/cm2 and 8.9 µC/cm2 , and the corresponding coercive field (EC ) values are 3.5 kV/cm and 7.87 kV/cm, respectively. These results indicate that the Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 ceramic has multiferroic properties.
Fig. 4. M − T curve of Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 at an applied field of 0.5 T.
III. CONCLUSION In conclusion, we studied the effects of Bi3+ donor doping in Fe-doped BaTiO3 ceramics on the phase structure and on the magnetic and the ferroelectric behavior at room temperature. A merging of the tetragonal phase merged with the hexagonal phase was observed in the Ba1−x Bix Ti0.95 Fe0.05 O3 (x < 0.05) ceramics whereas a pure tetragonal phase was observed for x ≥ 0.05. Ferrimagnetic and ferroelectric hysteresis loops at room temperature were obtained for x = 0.02 whereas for x = 0.05, paramagnetic properties appeared, and a very high leakage current was observed. The ferrimagnetism in the Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 ceramic is an intrinsic property with a TC of around 450 K.
Fig. 5. Room-temperature P − E hysteresis loops of Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 at an applied voltage of 1 kV.
not detectable in the ceramic sample. The TC of the Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 ceramic is around 450 K, which is much smaller than those of γ-Fe2 O3 (900 K) and Fe3 O4 (850 K) [13]. The coercive field (HC ) for our ceramic sample is about 59 Oe at room temperature, which differs from those of γ-Fe2 O3 (about 450 Oe) and Fe3 O4 (about 25 Oe) [12], indicating intrinsic ferrimagnetism in the Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 ceramic. This ferrimagnetism may originate in the magnetism in Fe-doped BaTiO3 [7,8]. In hexagonal Fe-doped BaTiO3 , Fe3+ ions replace Ti4+ ions and occupy the octahedral and the pentahedral sites [8, 14]. The super-exchange between octahedral Fe3+ has been known to be antiferromagnetic, but the super-exchange between pentahedral Fe3+ and between octahedral and pentahedral Fe3+ is reported to exhibit ferromagnetism [6,7]. Competition among these interactions induces the observed small magnetization, but further investigation is needed to confirm the origin of ferrimagnetism in the Ba0.98 Bi0.02 Ti0.95 Fe0.05 O3 ceramic.
ACKNOWLEDGMENTS B. W. Lee was supported by the Hankuk University of Foreign Studies Research Fund of 2012. C. U. Jung was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A22008595).
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