The influence of Ni-doping concentration on multiferroic behaviors in Bi4NdTi3FeO15 ceramics

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 1087–1092 www.elsevier.com/locate/ceramint

The influence of Ni-doping concentration on multiferroic behaviors in Bi4NdTi3FeO15 ceramics Jun Xiaoa,b, Hengfeng Zhangb, Yun Xueb, Zhangwu Lub, Xiaoqin Chena,b,n, Peng Sub, Fujun Yanga,b,n, Xiangbin Zengc a

Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan, Hubei 430062, PR China b Faculty of Physics and Electronic Technology, Hubei University, Wuhan, Hubei 430062, PR China c School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China Received 22 August 2014; received in revised form 5 September 2014; accepted 7 September 2014 Available online 16 September 2014

Abstract Ceramics of Ni-doped Bi4NdTi3Fe1
xNixO15 (x ¼ 0.1, 0.3, 0.5 and 0.7) (Nix) were synthesized by a multicalcination procedure. The substitution of Ni for Fe does not change the layered perovskite structure of Bi4NdTi3FeO15 (BNTF) except that a small amount of Ni2O3 appears in the Ni0.7 sample. Plate-like morphology of the grains which is characteristic for layer-structure Aurivillius compounds was clearly observed for all the Nix samples. The εr, 2Pr and 2Mr values are not proportional to the Ni doping concentration but increase firstly and then decrease. Taking into account ferroelectric and magnetic properties together the optimum room temperature (RT) multiferroic property was acquired in the Ni0.3 sample within the range of experiment, which is preliminarily discussed from the radius difference of magnetic ions, their ratio and the location of different magnetic ions. The present work is meaningful for compositional design of RT multiferroic materials based on four-layer structured Aurivillius compounds. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Aurivillius phase; Multiferroic; Bi4NdTi3FeO15; Ni-doping

1. Introduction Single-phase (SP) multiferroic materials which exhibit ferroelectric and magnetic ordering simultaneously have attracted intensive attention due to their technological relevance and fundamental science challenges [1,2]. So far the most frequently investigated SP multiferroic materials were BiFeO3 (BFO) [3] and BiMnO3 [4,5], offering ferroelectric order and spin order at relatively high temperature and showing a weak ferromagnetism and ME coupling at low magnetic field. Nevertheless very few multiferroic materials exist in nature or have been synthesized in the laboratory because the transition metal d electrons, which are n Corresponding authors at: Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan, Hubei 430062, PR China. Tel.: þ 86 27 88663390. E-mail addresses: [email protected] (X. Chen), [email protected] (F. Yang).

http://dx.doi.org/10.1016/j.ceramint.2014.09.033 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

essential for magnetism, reduce the tendency for off-center ferroelectric distortion [6]. Theoretically the layer-structured material can potentially avoid this factor, and reach high reliability above RT [7]. The general formula of Aurivillius layer-structured compounds is expressed by (Bi2O2)2 þ (Am
1BmO3m þ 1)2
, where A represents Bi3 þ , Pb2 þ , Sr2 þ , Ca2 þ , Ba2 þ , etc., at the 12-coordinated site; B represents Fe3 þ , Mn3 þ , Ti4 þ , V5 þ , Nb5 þ , Ta5 þ , W6 þ , etc., at the 6-coordinated site; and m is an integer corresponding to the number of oxygen octahedra of the B ions in the pseudoperovskite block. Among them Bi5Ti3FeO15 (BTF), which can be viewed as inserting the well-known multiferroic BFO unit into the typical ferroelectric compound Bi4Ti3O12 (BTO), has been reconsidered as a multiferroics candidate because of its potential to present both ferroelectric and ferromagnetic transitions in a single phase and its possible ME coupling effect [8–12]. At the present stage, its structural, electric,and magnetic, as well as ME properties have been extensively investigated. Ferroelectric Curie temperature

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319

2218 1127

0218 313

Ni

0.7

2020

220 1119

1117

0020

0018 2010

*:Ni2O3(002) No.14-0481

028/208

*

206

200

119

The XRD patterns of the as-obtained BTO powders and the Nix samples are shown in Fig. 1. It is seen that after the multicalcination process the BTO phase has been formed. As for Nix samples, the Aurivillius structure containing four perovskite layers is identified by indexing all the diffraction peaks on the basis of an orthorhombic cell (Joint Committee for Powder Diffraction Standard (JCPDS) No. 38-1257), except that a small amount of Ni2O3 denoted with an asterisk appears in the Ni0.7 sample. And all the (119) peaks have the highest intensity which indicates that the samples are all randomly-oriented. Fig. 2 shows SEM images taken from fresh fracture surfaces of Nix samples. It is clearly seen that all of the Nix samples are dense and no preferential arrangement of grains is observed which is consistent with the random orientation of the samples indicated by Fig. 1. Furthermore, similar to the undoped BTF ceramics [23], the grains of the samples are all formed in the shape of thin platelets, which is typical for Aurivillius bismuth layered compounds due to the preferential growth of the ab crystalline plane, indicating that Ni-doping does not alter the layered microstructure shape. Fig. 3 exhibits the dielectric constant (εr) and loss tangent (tan δ) as a function of frequency ranging from 3 kHz to 1 MHz for Nix samples at RT. In consideration of the contribution of different polarizations such as electronic, ionic, dipolar, and space charge or interfacial polarization to the εr at low frequency [24], the intrinsic εr is most likely reflected at high frequencies because the defect-related/interfacial polarization reversals cannot keep up with the applied electric field [25]. At 1 MHz, the εr values are  301, 416, 289 and 249 for the Ni0.1, Ni0.3, Ni0.5 and Ni0.7 samples respectively, which are larger than those of the Cr doped BTF [20]. It can be seen that when the Ni doping concentration is 30%, the εr is the largest. For the dielectric loss shown in Fig. 3

117

Ni

0.5

Ni

0.3

20

30

40

50

60

1315

Ni0.1 2214 121

Polycrystalline samples with nominal composition Bi4Ti3 Fe1
xNixO15 (x¼ 0, 0.1, 0.3, 0.5 and 0.7) (Nix) were prepared by two steps. Firstly, Bi2O3 (analytic purity) and TiO2 (spectral purity) powders were used as starting materials. A 15 wt% excess Bi2O3 was added to compensate the volatilization loss. The raw materials were mixed by ball milling with an agate and ethanol for 12 h, and calcined at 760, 780, and 800 1C for 8, 16, and 24 h respectively to obtain BTO powders. Secondly, the obtained BTO powders were mixed with Fe2O3, Ni2O3, Nd2O3 and Bi2O3 (analytic purity) agents in stoichiometry. Final mixtures were ball milled for 12 h, sintered at 640 1C for 8 h, and then ball milled again for 12 h. The presintered powders were pressed into pellets with a diameter of 10 mm and a thickness of 1 mm. The discs were sintered at 900 1C for 6 h in air and then furnace cooled to RT. A Bruker D8 ADVANCE X-ray diffractometer (XRD) with Cu-Kα radiation was employed to characterize the phase constitution of the powders and ceramics, while a scanning electron microscope (SEM) (JEOL, JSM6510LV) was used to observe the morphology of its fresh cross section. For electrical measurement, the discs were lapped and polished to about 0.2 mm thickness. Silver was pasted on both surfaces as electrodes. A low-frequency impedance analyzer (WK-6420) was used to determine the dielectric properties of the ceramics. The ferroelectric properties were measured using a precision

3. Results and discussion

0010 111

2. Experimental procedure

workstation ferroelectric tester system (Radiant Technologies) at RT. A vibrating sample magnetometer (VSM) was used to measure RT magnetic hysteresis loops.

115

(Tc) was determined to be about 730 1C along with space group transition from A21am to I4/mmm [13] . And Li et al. even reported that except for a ferroelectric transition from the tetragonal phase to the polar orthorhombic phase thatoccurs at 1021 K, an abrupt lattice expansion of the tetragonal phase occurs at about 1110 K [11]. Ferromagnetic transition was also detected as occurring at about 620 K in BTF [14]. However, the pure BTF phase still exhibits weak magnetic behaviors discussed in [9]. Usually, element doping is a good choice to improve the performance of materials. It is demonstrated that the substitution of Nd for volatile Bi can not only improve the ferroelectric property of three-layered perovskite BTO [15,16] but also improve the multiferroic and electric transport properties of BFO [17]. Concerning the magnetic modification, Co is the most usually reported magnetic element substituted for Fe in BTF to improve the magnetic properties [18] . And Mn or Cr substitution is also reported while the magnetic modification effect is not obvious compared with Co substitution [19,20]. Apart from Fe, Co, Mn and Cr, Ni is an important ferromagnetic element as well. However, study on Ni substitution in BTF system is still lacking except for thereported multiferroic properties of Bi5Ti3Fe0.5Ni0.5O15 [21] and ME coupling effect of Bi4NdTi3Fe0.7Ni0.3O15 [22]. Thus, in this research, we prepared Ni-doped BNTF ceramics by a multicalcination procedure. The influences of Ni-doping concentration on the dielectric, ferroelectric and magnetic properties at RT were investigated, and the optimum RT multiferroic property was acquired when the Ni-doping concentration was 0.3 taking into account both ferroelectric and magnetic properties together.

014 008 111 113 115 0010 117 0012 200 024 206 0014 208 1113 0016 220 1115 226 2014 228 131 137

1088

70

BTO 80

2θ (degree) Fig. 1. X-ray θ–2θ diffraction patterns of as-obtained BTO powders and Nix samples.

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Fig. 2. Cross-sectional SEM micrographs of Nix samples: (a) x¼ 0.1, (b) x ¼0.3, (c) x¼ 0.5 and (d) x¼ 0.7.

500

0.7 0.6

Ni0.1

0.5

Ni0.3

Ni0.1

Ni0.3

Ni0.5

Ni0.7

tanδ

εr

400

300

0.4

Ni0.5

0.3

Ni0.7

0.2 0.1 0.0

200 4

10

5

10

6

10

f (Hz)

4

10

5

10

6

10

f (Hz)

Fig. 3. The frequency dependences of (a) dielectric constant and (b) loss tangent of Nix ceramic capacitors.

(b) the space charge created by oxygen vacancies would produce an absorption current in the boundaries of the grains, resulting in a loss factor (tan δ) in low frequencies and at higher frequencies they are not able to follow the external electric field and thereby tend to saturate to a low value [26]. For Nix samples, when the frequency is 1 MHz, the values of tan δ are about 0.01, comparable with those of Cr doped BTF ceramics. The polarization versus electric field (P–E) loops of Nix ceramic capacitors are shown in Fig. 4(a). The P–E loops exhibit leaky feature which is quite usual for SP multiferroic systems [9]. This leakage-related contribution was also confirmed in earlier reports [14,27,28] and seems to be intrinsic. As for undoped BTF [14] and Co-doped BTF [28] systems the positive-up-negativedown measurements confirmed that the ferroeletricity of BTF systems, although leaky, is intrinsic and natural and the leakage current does not play a big role in the ferroelectric behavior. Therefore, similarly, P–E loops shown in Fig. 4(a) reflected the

ferroelectricity of Nix samples, and it can be seen that with increasing Ni doping concentration from 0 to 0.5, the measured remnant polarization 2Pr values increased, while for the x¼ 0.7 sample, the 2Pr value decreased. Fig. 4(b) shows the result of the magnetic measurement at RT and the loops near the grid origin are enlarged in the inset. Very obviously all the samples exhibit typical ferromagnetic M–H loops, and it can be clearly noticed that the largest remnant magnetization 2Mr value appears in the Ni0.3 sample. According to the results of Fig. 4(a) and (b), Nix samples are all multiferroic at RT. Fig. 5(a) shows the variations of 2Pr and 2Mr with Ni doping concentration x. It can be clearly seen that the values of 2Pr and 2Mr increase firstly and then decrease with increasing content of Ni; however their maximum values are located at different x (for 2Pr, x¼ 0.5; 2Mr, x¼ 0.3). It has been known that the structural deviation of displacive ferroelectrics away from the underlying parent structure with a high symmetry is attributed to the origin

1090

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Ni0

10

Ni0.1

M (memu/g)

Ni0.5 Ni0.7

0 -5

Ni0.3

250

Ni0.5 Ni0.7

0

10

M (memu/g)

Ni0.3

5

P (μC/cm2)

Ni0

500

Ni0.1

-250

-10

-10 -1

-500 -100

-50

0

50

0

100

-8

0

0 H (kOe)

1

8

H (kOe)

E (kV/cm)

Fig. 4. RT (a) P–E hysteresis loops and (b) M–H loops of Nix samples; the curves near the grid origin in (b) are enlarged in the inset.

600

8

400 300

4

200

2

2M r (memu/g)

2P r (μC/cm2)

500 6

100 0

0 0.0

0.1

0.2 0.3

0.4 0.5

0.6

0.7

x Fig. 5. (a) The variations of 2Pr and 2Mr with Ni doping content x, (b) Schematic illustration of BTF crystal structure.

of ferroelectricity. For the Aurivillius phase, the contributions of atomic displacement of A-site ions [29] or B-site ions [14,30] to the ferroelectricity have been reported controversially. However, on the basis of Rietveld refinement results, Ahn proposed that the displacement of B-site ions is the main structural cause of ferroeletricity in Mn-doped BTF system [19]. In addition, the substitution of smaller Cr [20] or Mn [19] ions for larger Fe ions decreases the displacement of B-site ions, sequentially leads to a decrease in the distortion degree of the perovskite cell, so that the Curie temperature and the ferroelectric polarization decrease [31]. In the present study, larger Ni ions were substituted for smaller Fe ions. It might be inferred from the above that with increasing Ni content the displacement of B-site ions increases, and so do the structural distortion degree and the ferroelectric polarization. The reason for the decrease of 2Pr in Ni0.7 sample might be that the Ni-doping concentration is not as much as 70% due to the appearance of Ni2O3 shown in Fig. 1. Concerning the dependence of the Ni-doping concentration on magnetization, it is generally known that the superexchange of the Fe–O–Fe clusters possibly favors the local antiferromagnetic (AFM) interaction which results in the superparamagnetic

(SPM) state with local AFM interaction [9,32], or AFM ordering [33] and even weak ferromagnetism in paramagnetic background [8,34] in BTF. Therefore, researchers have adopted various doping elements to enhance the magnetic properties. It is suggested that a possible coupling over Fe–O–M (M represents magnetic element but not Fe) octahedral clusters might contribute to the improvement of ferromagnetic characteristic in M-doped BTF [10], and it can be inferred that the best magnetic modification effect would be expected when the ratio of Fe3 þ to M3 þ is 1:1 because chance of adjacently appearing Fe–O and M–O octahedra will be the biggest under the circumstance. In the present study, the largest 2Mr value appears in the Ni0.3 (Fe3 þ :Ni3 þ ¼ 7:3) but not Ni0.5 (Fe3 þ :Ni3 þ ¼ 1:1) sample shown in Fig. 5(a). This is not accidental because we got similar results in Mn-doped and Co-doped BNTF. Therefore some other factors might have to be taken into account. Sharma proposed that the filled or partially filled d-orbitals of transition metal ions could easily hybridize with the O 2p orbitals [35] and this hybridization could lead to high symmetrical octahedra or similar bond length between magnetic ion and oxygen ions at the opposite direction [36]. Fig. 5(b) illustrates the BTF crystal

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structure. It is reported that the occupancy of Fe ions is random with Ti ions in BTF [37]. Hence, Fe ions can occupy two nonequivalent positions between two Bi–O layers. One is at the center of the “inner” Ti/Fe(1)O6 octahedra, the other is at the center of “outer” Ti/Fe(2)O6 octahedra, as can be seen in Fig. 5 (b), and the outer octahedra are considerably more distorted than the inner octahedral [37]. Zulhadjri et al. considered that in Mndoped four-layer Aurivillius Pb1
xBi4 þ xTi4
xMnxO15, transition metal Mn ions mainly resided in the inner TiO6 octahedra with lower distortion because of the high symmetry of Mn octahedra which is induced by the hybridization between filled or partially filled d-orbitals of Mn ions and the O 2p orbitals [36]. Therefore, it might be inferred that Ni ions substituted for randomly distributed Fe ions are inclined to occupy the center of the inner octahedra in BNTF. Based on the above, let us make an estimation i.e., random distribution of Fe and Ti ions and the inclination of occupying the inner octahedral center of Ni ions. If the doping concentration of Ni ions is x the concentration of Fe ions is 1
x, and the concentration of Fe ions located at the inner octahedral center is (1
x)/2. Then combined with the opinion of “1:1 ratio”, namely, Fe:Ni¼ 1:1¼ (1
x)/2:x, we can obtain that when the Ni doping concentration x is 1/3 (0.3E1/ 3), the ratio of Fe ions to Ni ions located at the inner octahedral center is 1:1; consequently in the inner octahedra the strongest direct Fe–O–Ni interactions would take place, leading to the largest 2Mr. It should be noted that the above discussion is preliminary and within the range of the present experiment. More work is going on to understand the mechanism. 4. Conclusion In summary, Bi4NdTi3Fe1
xNixO15 (x¼ 0.1, 0.3, 0.5 and 0.7) ceramics were synthesized by a multicalcination procedure. The substitution of Ni for Fe ions does not change the layered perovskite structure of Bi4NdTi3FeO15 except that a small amount of Ni2O3 appears in the Ni0.7 sample. Cross-sectional scanning electron microscopy images indicate plate-like morphology of the grains. The εr, 2Pr and 2Mr values are not proportional to the Ni doping concentration but increase firstly and then decrease. Taking into account ferroelectric and magnetic properties, the optimum RT multiferroic property was acquired in the Ni0.3 sample within the range of experiment which is preliminarily discussed. The present work is meaningful for compositional design of RT multiferroic materials based on four-layer structured Aurivillius compounds. Acknowledgments The authors are grateful for financial support from the National Nature Science Foundations of China under Grant nos. 51002047, and 11274101 and from the Department of Education of Hubei Province (Q20081005). References [1] M. Fiebig, Revival of the magnetoelectric effect, J. Phys. D:Appl. Phys. 38 (2005) R123.

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