Structure, dielectric and multiferroic properties of three-layered aurivillius SrBi3Nb2FeO12 ceramics

Author’s Accepted Manuscript Structure, Dielectric and Multiferroic Properties of Three-layered Aurivillius SrBi 3Nb2FeO12 Ceramics Yu Shi, Yongping Pu, Jingwei Li, Ruike Shi, Wen Wang, Qianwen Zhang, Linghua Guo www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(19)30146-4 https://doi.org/10.1016/j.ceramint.2019.01.129 CERI20584

To appear in: Ceramics International Received date: 24 October 2018 Revised date: 11 January 2019 Accepted date: 16 January 2019 Cite this article as: Yu Shi, Yongping Pu, Jingwei Li, Ruike Shi, Wen Wang, Qianwen Zhang and Linghua Guo, Structure, Dielectric and Multiferroic Properties of Three-layered Aurivillius SrBi 3Nb2FeO12 Ceramics, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.01.129 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structure, Dielectric and Multiferroic Properties of Three-layered Aurivillius SrBi3Nb2FeO12 Ceramics Yu shia, Yongping Pua,* Jingwei Lia, Ruike Shia, Wen Wanga, Qianwen Zhanga, Linghua Guob,* a

School of Materials Science and Engineering, Shaanxi University of Science & Technology, 710021, Xi’an, China

b

College of Bioresources chemical and Materials Engineering, Shaanxi University of Science & Technology, 710021, Xi’an, China

[email protected] [email protected] *

Corresponding author: Tel: +86-15109282563;

*

Corresponding author: Tel: 029-86132609

Abstract Single-Phase Aurivillius SrBi3Nb2FeO12 ceramics were prepared by conventional solid-state method. BiFeO3 was successfully introduced into SrBi2Nb2O9 which was confirmed by XRD and Raman results. Plate-like morphology grains which are 400~600 nm in thickness and 2~4 μm in length were observed for SrBi3Nb2FeO12 ceramics. Their permittivity and dielectric loss at 1 MHz are about 135 and 0.018, respectively. Two permittivity peaks were observed in the temperature dependent permittivity curves. Ferroelectric and weak antiferromagnetic properties were obtained simultaneously at room temperature. X-ray photoelectron spectroscopy (XPS) was carried out to investigate the chemical state of SBNF and Fe ion was found in

mixed state. These results indicate that three-layered SrBi3Nb2FeO12 ceramics can be used as a kind of multiferroics in further research.

Graphical abstract

Keywords: Multiferroics; single-phase; Aurivillius; ceramics 1. Introduction Multiferroic materials, which possess ferroelectric and magnetic orders simultaneously, have attracted considerable attention due to their potential applications in magnetic sensors, data storage, spintronics[1, 2]. Nowdays, there are very few single-phase multiferroics known mainly because of the mutual exclusion between ferroelectric and magnetic electronic structures (ferroelectric materials often require d-orbital electrons to be vacant while magnetic materials require non-filled

d-orbitals), the coexistence of ferromagnetic (antiferromagnetic) and ferroelectric compounds is very limited[3]. Therefore, it is necessary to search for new materials as multiferroics which show good ferroelectric and magnetic properties. The typical single-phase multiferroics BiFeO3 material that simultaneously exhibits good spontaneous magnetic and ferroelectric orders above room temperature (TC~830 °C, TN~370 °C) has been extensively investigated[1], however, BiFeO3 suffers from high leakage current, unexpected second phase and its weak magnetization[4]. The structure of SrBi2Nb2O9 is characterized by two perovskite layers sandwiched by two fluorite-type (Bi2O2)2+ layers (shown in Fig. 1). Compared with BiFeO3, SrBi2Nb2O9 shows reasonable spontaneous polarization[5] and fairly low leakage currents where the high resistance (Bi2O2)2+ layers are playing a pivotal role in space-charge compensation of the ceramic materials[6]. To overcome the drawbacks, BiFeO3 was introduced into SrBi2Nb2O9 to form a new three-layered Aurivillius multiferroic materials [7, 8]. Schematic diagram of SrBi3Nb2FeO12 (SBNF) prepared by introducing [FeO6] octahedral layer (of BiFeO3) into perovskite-like layer (of SrBi2Nb2O9), known as inserting method, is shown in Fig. 1. As-prepared SBNF is comprised of a stacked, two layers of fluorite-like (Bi2O2)2+ sandwiching three-layered perovskite-like (SrBiNb2FeO10)2- in the middle along the c-axis. All [FeO6] octahedral distributes in perovskite layer randomly. In this work, three-layered SBNF composed of BiFeO3 and SrBi2Nb2O9 was synthesized by conventional solid-state method. The microstructure, dielectric and multiferroic properties of SBNF were discussed systematically.

2. Experimental procedure The SBNF ceramics were synthesized by the conventional solid-state method. SrCO3, Bi2O3, Nb2O5 and Fe2O3 were used as raw materials. These powders were thoroughly milled for 6 h by ball-milling. Afterwards, the mixed powders were placed into an oven at 80 oC for 24h, and then the powders were calcined at 800 oC for 4h. The mixtures were milled again under the same conditions and dried. Before cold isostatic pressing (LDJ100/320-300, Sichuan, China), the calcined mixtures were uniaxially pressed at 7 Mpa into discs of 10 mm in diameter and 1.5 mm in thickness. Obtained discs were being cold isostatic pressed at 200 Mpa for 300 s. These samples were sintered at 1030 oC for 2 h. Silver paste was covered on opposite sides of the ceramics for the properties measurement. The crystal structure and morphology of the ceramics were studied by X-ray diffraction (XRD D/max-2200PC, RIGAKU, Tokyo, Japan) patterns with Cu kα radiation and field emission scanning electron microscope (SEM, JSM-6700, JEOL Ltd., Tokyo, Japan), respectively. The structure of ceramics were confirmed by Raman spectroscopy (Renishaw-invia, Renishaw, UK) in the range of 100-1000 cm-1 at room temperature. The permittivity, dielectric loss and complex impedance of samples were measured using Agilent (LCR, E 4980A, USA). The polarization hysteresis (P-E) loops and magnetic hysteresis (M-H) loops were characterized using a ferroelectric test system (Premier II, Radiant, USA) and vibrating sample magnetometer (VSM) 113 (Lake Shore 7410, Westerville, OH), respectively. To further investigate the valence and chemical state of the samples, X-ray Photoelectron spectroscopy (XPS)

measurement were performed (XPS, AXIS SUPRA, UK.). 3. Result and discussion The crystalline phases were identified by XRD. The XRD patterns of SBNF ceramics powder prepared by conventional solid state method are shown in Fig. 2(a). A single phase layered perovskite structure is confirmed with the XRD data, which is consistent with Srinivas’s report[9]. None of the observable diffraction peaks can be attributed to BiFeO3 and SrBi2Nb2O9. Three-layered SBNF synthesized by inserting method that [FeO6] octahedral layer randomly inserts into [NbO6] octahedral layer of SrBi2Nb2O9. The Raman spectrum of SBNF specimen in the range of 100-1000 cm-1 at room temperature is shown in Fig.2 (b). In order to illustrate microstructure of SBNF, we have fitted curves into individual Lorentzian components. Among them, the mode at 574 cm-1 is assigned to the equivalent and opposite displacement of negative and positive ions in the (a,b)-plane and in [Bi2O2]2+. The modes at 203 cm-1 and 140 cm-1 correspond to O-A-O bending located in the perovskite layers and O-B-O bending, respectively[10]. The mode at 290 cm-1 corresponds to the characteristic of fluorite and layered perovskite of SBNF ceramics, which was also observed in Nelis’s report[11]. The Nb-O stretching in the [NbO6] octahedral results in vibration mode at 839 cm-1 along c-axis in the SBNF[12]. Compared with SrBi2Nb2O9-based materials[13-15], the number of perovskite layers in SBNF is transformed into three, therefore the vibration mode at 832~834 cm-1 (for SrBi2Nb2O9) moves to 839 cm-1 (for SBNF). Meanwhile, a new vibration mode emerges at 796 cm-1 (for SBNF). These vibration mode changes

can be attributed to a destroyed local symmetry induced by the introduced [FeO6] octahedral[16]. The Fig. 2(c) shows SEM image of as-prepared SBNF ceramics after polishing and thermal-etching. It is obvious that the ceramics are pore-free and plate-like grains orientated randomly. The plate-like morphology of the grains is a typical feature of layer-structured Aurivillius compounds which results from a high grain growth rate of the (001) plane in the direction perpendicular to the c-axis due to the lower surface energies induced predominantly in sintering[17]. In order to observe grain morphology more clearly, SEM micrograph of polished SBNF ceramics which were etched with acid are shown in Fig. 2(d). The grain size of SBNF ceramics is 400~600 nm in thickness and 2~4 μm in length. Fig. 3(a) shows the permittivity and dielectric loss as a function of frequency ranging from 20 to 2M Hz for SBNF ceramics at room temperature. The relative high permittivity at lower frequency region is due to the presence of electronic, ionic, orientational and space charge polarization. With frequency further increasing, the smaller value of dielectric permittivity contributed from electronic and ionic polarizationa due to part of polarization mechanism could not respond to electric field. Their permittivity and dielectric loss at 1 MHz are 135 and 0.018, respectively. Temperature dependence of permittivity and dielectric loss of SBNF ceramics at different frequency is shown in Fig 3(b). The two permittivity peaks (marked as peak 1 and peak 2) can be detected. The first peak (peak 1), which possesses obvious frequency-dependent, is related to large differences of ion valence and radii in

B-site[18]. And the second peak (peak 2) accompanied with a dramatic increase in dielectric loss, which is corresponding to the phase transition between ferroelectric and paraelectric. The P-E loop of SBNF ceramics at room-temperature and 100 Hz is shown in Fig. 4(a). Such relative low-quality hysteresis seems quite usual for bismuth layered-structure multiferroic materials because of low melting point of Bi2O3 (825 o

C). During the sintering process, substantial oxygen vacancies arose in the samples,

which deteriorated materials performance by enhancing interface polarization and leakage current. As a result, P-E loops become so leaky and low-quality. In addition, [FeO6] octahedral is not sensitive when applied electric field. The insensitive [FeO6] octahedral restrains the polarization by blocking the strain of [NbO6] octahedral. The magnetic field dependences of magnetization (M-H) curve was measured at room temperature for SBNF ceramics, as shown in Fig. 4(b). For the SBNF ceramics sample, the M-H curve shows a linear behavior indicating the antiferromagnetism of SBNF. According to previously reported literature, SrBi2Nb2O9 shows the diamagnetic characteristics due to empty d-orbital of Nb5+ ions. The partially filled d-orbital Fe3+ ions were inserted into perovskite-like layer. These Fe3+ ions give rise to short-range magnetic ordering. To analyze the magnetic properties, XPS spectrums of Fe p3/2 and O 1s of SBNF ceramic powder are shown in Fig. 4(a) and 4(b). The bonding energy of Fe 2p2/3 located at 709.5 eV and 713.3 eV, which is divided by the Lorenzian and Gaussian fitting. The fitting curves indicate that the Fe ion has two valence state (Fe2+ and

Fe3+)[19]. In figure 4(b), the peak OӀ at 527.4 eV is ascribed to O2- of SBNF. The peak OӀӀ at 529.5 eV attributed to the surface hydroxyl such as oxygen vacancy/defects formation on the surface. The peak OӀӀӀ usually attributed to chemisorbed oxygen on the surface[20]. In oxides, magnetic ions are easy to combine with oxygen vacancies forming bound magnetic polarons, and the ferromagnetic interaction could arise with increasing concentration of bound magnetic polarons[21]. The weak antiferromagnetic behavior results from adjacent antiferromagnetic super exchange interaction between equivalent Fe (such as Fe3+-O-Fe3+) at room temperature. Conclusions In conclusion, we have successfully prepared the three-layered SBNF ceramics by solid state reaction method. The structure, dielectric and multiferroic properties of ceramics have been studied. XRD result indicates the SBNF ceramics are pure-phase, and Raman spectroscopy suggests that [FeO6] octahedral inserted into perovskite-like layered. Besides, SEM images display plate-like grain with 400~600 nm in thickness and 2~4 μm in length, respectively. Their permittivity and dielectric loss at 1 MHz are 135 and 0.018, respectively. Two permittivity peaks were observed in the temperature dependent permittivity curves. Most importantly, multiferroics property was obtained simultaneously at room temperature in the three-layered Aurivillius SBNF ceramics. SBNF ceramics that maintain the antiferromagntism form BiFeO3 and ferroelectricity from SrBi2Nb2O9 can be used as a kind of multiferroics in further research. Acknowledgements This work was financed by the National Natural Science Foundation of China

(58172175),

the

International

Cooperation

Projects

of

Shaanxi

Province

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Fig. 1. Schematic diagram of SBNF prepared by inserting method. Fig. 2. (a) XRD patterns of SBNF ceramics powder; (b) Raman spectra of SBNF ceramics at room temperature; (c) SEM image of SBNF ceramics with thermal-etched; (d) SEM image of SBNF ceramics with acid-etched. Fig. 3 (a) Frequency dependence of permittivity and dielectric loss of SBNF ceramics; (b) Temperature dependence of permittivity and dielectric loss of SBNF ceramics at different frequency. Fig. 4 (a) Room temperature ferroelectric hysteresis loop of SBNF ceramics under an applied electric field of 90 kV/cm at 100 Hz; (b) Magnetic field dependence of magnetization in SBNF at room temperature; XPS spectrums of (c) Fe p3/2 and (d) O 1s of SBNF ceramic powder.

Fig. 1

Fig. 2

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Fig. 4