- Email: [email protected]
Effects of O2 and N2 sintering atmospheres on electric properties of 0.9SrTiO3–0.1NiFe2O4 composite ceramics
Physica B: Condensed Matter 572 (2019) 273–278
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Effects of O2 and N2 sintering atmospheres on electric properties of 0.9SrTiO3–0.1NiFe2O4 composite ceramics
T
Hongjun Zhanga,b,c, Hua Kea,b,∗, Junjie Zhoua,b, Huijiadai Luoa,b, Fangzhe Lia,b, Bin Yangc,∗∗, Yan Fand, Qingqin Ged, Dechang Jiaa,b, Yu Zhoua,b a
Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150080, China Key Laboratory of Advanced Structure-Function Integrated Materials and Green Manufacturing Technology (Ministry of Industry and Information Technology), Harbin Institute of Technology, Harbin, 150001, China c Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin, 150080, China d Thermo Fisher Scientific China, Building 6, No. 27, Xin Jinqiao Road, Shanghai, 201206, China b
ARTICLE INFO
ABSTRACT
Keywords: 0.9SrTiO3−0.1NiFe2O4 XRD refinement XPS Dielectricity Impedance
The 0.9SrTiO3–0.1NiFe2O4 (STO/NFO) composite ceramics sintered at O2/N2 atmospheres were investigated. The N2-sintered STO/NFO (STO/NFON) had a larger microstrain (0.092(47)%) than the O2-sintered STO/NFO (STO/NFOO) revealed by the Rietveld refinement and the Williamson-Hall analysis. The maximum polarization of STO/NFON increased to ~7.1 μC/cm2, but the larger leakage current in STO/NFOO resulted in a 20% larger false remanent polarization value. Moreover, STO/NFON presented a lower activation energy (Ea, 1.22 eV), which implied more oxygen vacancies (VO ). The amount of Ti3+ ions (60%) in STO/NFON increased as well, which formed defect dipoles [Ti4+·e−VO −Ti4+·e] and fixed VO . The composite of STO/NFON also showed decreased VO content (~53%) owing to the fixing effect and presented low leakage current, increased impedances, and the improved ferroelectric hysteresis loop. The anomaly dielectric constants peaks indicated different conductivity mechanisms related to VO , and the STO/NFON ceramic possessed a low rate of dielectric loss.
1. Introduction Magnetoelectric multiferroics, which are widely used on inductors, filters and tuneable microwave devices [1,2] have persistently attracted research attention for decades. In particular, leadless ferroelectric ceramics and ferromagnetic materials are frequently used for preparing magnetoelectric composites and modulating textural structures [3–5] for the enhancement of magnetoelectric coupling coefficient. SrTiO3 (STO) is an interesting quantum paraelectric materials because it presents quantum fluctuations [6] in its atomic positions. These fluctuations suppress ferroelectric transition and results in the so-called incipient ferroelectricity [7,8]. The ferroelectric properties of STO is induced by introducing stress to or reducing the dimensions of the STO phase. These processes delicately disrupt Ti4+ fluctuations and reorder the intrinsic nanopolar regions of STO [9,10]. The phase transitions from cubic (Pm-3m) STO to tetragonal (I4/mcm) or orthorhombic (Pbnm/Bmmb) STO at various strain conditions [11,12] have already been reported. Under these conditions, STO exhibits elevated Curie
∗
temperature (Tc) that is nearly equal to room temperature and shows ferroelectric properties. These features enable stress-induced ferroelectricity and facilitate the magnetic field modulation of electric signals, rendering STO useful in the preparation of composite magnetoelectric materials. In our recent publication [13], SrTiO3/NiFe2O4 (STO/NFO) composite ceramics presented ferroelectric hysteresis loops, and the magnetodielectric change rate reached approximately 1.6%, which indirectly reflected the magnetoelectric coupling effect. As is well known, the introduction of magnetic phases into perovskite ferroelectric matrices usually increases leakage currents due to the existence of defects, such as oxygen vacancies and space-charges [14–16]. A large leakage current would cumber the precision and reliability of measurement methods for ferroelectric properties [17]. Meanwhile, despite being a kind of defects, oxygen vacancies account for the electrical properties of perovskites and can be controlled by doping or annealing at different oxygen partial pressure conditions. The effects of oxygen vacancies on the conductivity has been confirmed by lots of previous studies on
Corresponding author. Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150080, China. Corresponding author. Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin, 150080, China E-mail addresses: [email protected] (H. Ke), [email protected] (B. Yang).
∗∗
https://doi.org/10.1016/j.physb.2019.07.023 Received 14 May 2019; Received in revised form 11 July 2019; Accepted 13 July 2019 Available online 07 August 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al.
Fig. 1. The XRD Rietveld refinement of (a) the STO/NFOO ceramic and (b) the STO/NFON ceramic; and the calculated microstrains borne by STO phase in (c) the STO/NFOO ceramic and (c) the STO/NFON ceramic. Insets of Fig. 1c and d are the SEM images of the STO/NFOO and STO/NFON ceramics.
perovskite materials. Ang et al. discovered three sets of dielectric peaks stemming from oxygen vacancies in Bi:SrTiO3 ceramics, and verified that the conduction carriers were from the first ionization of oxygen vacancies [18]. Schooley et al. even discussed the superconductive transitions in the reduced STO samples with high charge carrier concentration [19]. Additionally, the latest study by Trabelsi et al. [20] revealed that oxygen vacancies concentrations also affected the magnetic and optical properties of STO, and there existed two types of electronic contributions to magnetism. Given the importance of oxygen vacancies and the corresponding conductivity mechanisms, detailed studies on electric properties of STO/NFO composite ceramics under different sintering atmospheres would be necessary. In the present work, we chose the 0.9STO−0.1NFO composite as a specific example to explore the effects of sintering atmospheres (highpure O2/N2) owing to its good ferroelectric property [13]. The structure refinements, strain conditions, and electric/dielectric properties of the 0.9STO−0.1NFO composite ceramics were characterized, and defect concentrations were calculated by X-ray photoelectron spectroscopy (XPS).
pellets at 200 MPa. Each pellet was 10 mm in diameter and 1 mm in thickness. The binder was thoroughly burned away at 550 °C. The green ceramic pellets were randomly divided into two groups and separately sintered in the oxygen atmosphere (STO/NFOO) and the nitrogen atmosphere (STO/NFON) at 1350 °C for 5 h. Both gases were dry with purities of 99.9% and the atmospheric pressures were at 1 atm. The sintered ceramics had relative densities of above 95%. Samples were thoroughly polished to ~200 μm from ~1 mm in thickness. Sputtered gold electrodes of 1.5 mm in diameter were used to investigate ferroelectric properties. As to the dielectric and impedance performances, samples were coated using silver paste as electrodes and the electrical connections to the experimental setup were achieved through silver wires. X-ray diffraction (XRD) data were collected using an automated diffractometer (40 kV/40 mA, CuKα, radiation 0.1540598 nm, Empyrean, PANalytical, Netherlands). X-ray photoelectron spectrometry (XPS) data was obtained with an ESCALAB 250Xi Instrument (ThermoFisher Scientific, USA). The collected XPS spectra were deconvoluted using XPSPeak software by employing a Gaussian-Lorentzian mixed function with the background and the calibration using Carbon 1s spectrum under considerations. Ferroelectric properties and leakage currents were measured on a ferroelectric tester system (precision Premier II, Radiant Technologies, Inc., America). Dielectric/impedance measurements were conducted with an LCR Meter TH2827C (TongHui Inc. China), and the temperature was controlled by a furnace with proportional-integral-derivative (PID) controller.
2. Experiments The 0.9STO−0.1NFO composite powder was synthesized using the sol-gel method. Ni(NO3)2, Fe(NO3)3, (C2H3O2)2Sr, and Ti(C4H9O)4 were the raw materials for the metal ion sources. Acetic acid and ethylene glycol were the solvent and complexing agent, respectively. All chemical reagents used were of analytical grade. Ni(NO3)2, Fe(NO3)3 and (C2H3O2)2Sr were first weighed stoichiometricly according to the molar ratio between STO and NFO (9:1). All these three metal salts were dissolved in boiling acetic acid with mechanical agitation. Then, at room temperature (RT), ethylene glycol was added to the obtained transparent complex acetic acid solution. The resulting solution was dropwise added into the Ti(C4H9O)4 solution with ethylene glycol at RT. Taking different metal ionic activities of the complex sol-gel system into consideration, the respective reaction temperatures of and dosage ratio of the solvent and the complexing agent played key roles in the balancing of hydrolysis and gelation of all the metallic ions. A two-step calcination process [13,21] was conducted to obtain NFO phase and STO phase at 350 °C and 1050 °C, respectively, in the 0.9STO−0.1NFO system. The calcined composite ceramic powders were mixed with 2 wt% PVA aqueous solution and compacted into
3. Results and discussion Fig. 1a and b provide the information on the structures of STO/ NFOO and STO/NFON. The peaks arising from the STO and NFO could be easily to tell apart. Variations in sintering atmosphere had no obvious effects on the phase compositions. The Rietveld refinements displayed that the Rp values of STO/NFOO and STO/NFON were 10.6% and 9.3%, respectively. The relatively low fitting errors indicated the suitability of the refinement strategy. The calculated cell parameters revealed that both lattice constants a of STO in STO/NFOO and STO/ NFON were 3.904 Å and nearly equal to that in the used STO crystal model (COD No. 9002806). While the lattice constant of NFO in STO/ NFON (8.373 Å) was slightly larger than that in STO/NFOO (8.334 Å). The different lattice constants of NFO implied different strain states. The microstrains borne by STO in both composite ceramics were 274
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al.
The Arrhenius plots in Fig. 4 show the function of the conductivity depending on the reciprocal temperature (1/T). The AC conduction curves relying on frequency were displayed as insets. The different slopes of the conduction curves are attributed to various conductive mechanism, such as the relaxation or hopping of charge carriers, charge dissociation from traps or defects and the Schottky emission [26,27]. In the inset of Fig. 4a, two different conductive trends were observed via distinct curvatures for STO/NFOO, and the critical temperature was around 600 °C. While the conductive curves above 600 °C for STO/ NFON showed the same tendency of that at 600 °C. The data collected at 100 Hz from 720 °C to 800 °C were used for fitting progresses according to the following Arrhenius relation [26,28]:
=
calculated on the basis of the XRD data and Williamson–Hall patterns [22,23], as shown in Fig. 1c and d. The equation used for the Williamson–Hall model was as follows:
K× + 4 × Strain × sin( ) Size
exp( Ea/KB T )
(2)
where σ0 is the pre-exponent constant, kB is the Boltzmann's constant, and Ea is the activation energy of conduction. Both samples showed appropriate fitting results when the least square method was used. Moreover, only one single linear section appeared, indicating only one conductive mechanism at this mentioned temperature range. The activation energy was 1.34 eV for STO/NFOO and 1.22 eV for STO/NFON. The activation energy around 1.2 eV is usually identified as the conductivity from Schottky emission in perovskite composite [27]. The lower activation energy in STO/NFON hinted the increased oxygen vacancy content and the enhanced mobility of oxygen ion vacancies [25]. These enhancements would usually result in the larger conductivity but seemed incompatible with the above analyses of which the STO/NFON sample showed the lower leakage current. To further understand the underlying mechanism, XPS was used to uncover the ion valences. Fig. 5a and b displays the O1s XPS spectra for STO/NFOO and STO/ NFON respectively. All the binding energies were measured in reference to the C1s peak (284.8 eV) [29]. Each O1s high resolution XPS spectrum was deconvoluted into two Gaussian peaks. The peaks of STO/NFOO were detected at 529.1 and 531.3 eV, whereas the corresponding peaks of STO/NFON were at 529.2 and 530.9 eV, respectively. On the basis of a great deal of literature studies [30,31], the peak I at about 529 eV was attributed to the Ti–O band, namely associated with the O2− ions in the STO crystal structure. And the peak II at ~531 eV stemmed from the oxygen vacancy [32]. For the meticulous surface ion etching before the XPS data collection, those interference peaks (above 533 eV) assigned to the loosely bound oxygen on the samples' surfaces [33] were excluded here. It can be obviously observed that the shoulder peak II in Fig. 5a was stronger than that in Fig. 5b. According to the ratio of peaks I and II, the proportions of oxygen vacancies at RT were semi-quantitatively calculated as 65% in STO/NFOO and 53% in STO/NFON, respectively. The higher content of the oxygen vacancy in STO/NFOO was aligned with the increased leakage current of the sample sintered in O2 in Fig. 2 but was contrary to the inference of more oxygen vacancies in STO/NFON from Fig. 4b. Fig. 6a and b shows the typical Ti2p XPS spectra. The binding energies of Ti2p3/2 and Ti2p1/2 [34] were at ~458 eV and ~464 eV, respectively. Through optimized fitting processes, peaks associated with the Ti4+ and Ti3+ ions were distinguished carefully. Taking the Ti2p3/2 as an example, the binding energies of Ti4+ and Ti3+ [35] were respectively 458.1 and 457.4 eV in STO/NFOO and 458.5 and 457.7 eV in STO/NFON. According to the area ratios of peaks representing Ti4+ and Ti3+ ions, the fitting results also indicated that STO/NFON had a higher Ti3+ ion content than that of STO/NFOO (~60% and ~39%, respectively). The oxygen vacancy (VO ) is a kind of donor defects which turns an insulator into an n-type semiconductor. The Ti3+ ions (also known as the combination of Ti4+·e) as a type of acceptor defect would form a designed defect dipole [Ti4+·e−VO −Ti4+·e] to fixed VO locally [36,37]. The fixed local VO resulted in the low measurement value of VO content in STO/NFON shown in Fig. 5b. The contributions of local oxygen vacancies to conductivity also decreased because of the limited ionic mobility, which was the reasonable mechanism behind the low
Fig. 2. Ferroelectric hysteresis loops of STO/NFO ceramics. Inset: the corresponding leakage currents.
FW (s ) × cos( ) =
0
(1)
where FW(s) is the full-width at half-maximum (FWHM) of the peak, θ is the Bragg angle, Size is the grain size and Strain is the microstrain. The various FWHM values would reveal diversified microstrains in each sample. Owing to the micro-sized STO grains seen in the insets of Fig. 1c and d, the broadening effect of nanograins on FWHM can be excluded. The calculations showed that the STO phase in the STO/ NFON ceramic possessed a larger average microstrain (0.092(47)%) than that in STO/NFOO (0.068(65)%). As mentioned above, it is plausible to infer that the larger strain usually emphasizes ferroelectric properties. Fig. 2 showed the polarization–electric field (P–E) hysteresis loops of STO/NFOO and STO/NFON. The maximum polarization (Pmax) reached ~7.1 μC/cm2 for STO/NFON, which was definitely larger than that of the sample sintered in oxygen atmosphere (~6.6 μC/cm2). This improved Pmax value was consistent with the high strain of STO/NFON. Meanwhile, the remanent polarization (Pr) of STO/NFOO was 20% larger compared to that of STO/NFON. For this discordance between Pmax and Pr, some clue can be noticed from the leakage currents in the inset of Fig. 2. The leakage current of STO/NFOO increased fivefold relative to that of the STO/NFOO ceramic. The large leakage may have produced additional false signals on the ferroelectric measurement and contributed to the apparently high apparent Pr, as well as the more rounded loop shape [17,24] near the maximum polarization. To further evaluate the effects of O2/N2 atmospheres on the electric conductivities of the 0.9STO−0.1NFO ceramics, the impedance spectra with their derived alternating current conductivities were investigated. In Fig. 3, both impedance spectra decreased in the real part (Z') of the impedance with temperature increasing from 600 °C to 800 °C. The Z' value of the STO/NFON ceramic at each test temperature was from three to four times of that of STO/NFOO, and the high impedance agreed well with the low leakage current in STO/NFON. In addition, the impedance patterns of STO/NFON (inset of Fig. 3b) at 720 °C and 800 °C can conspicuously be divided into two semicircles of which the semicircles at high test frequencies, representing the influence of grain boundaries, possessed much larger proportions compared with STO/ NFOO. In other words, the semicircles arising from grains were dominant in STO/NFOO, which meant the higher concentration of oxygen vacancies and free electrons [25] and resulted in the larger leakage current. 275
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al.
Fig. 3. Impedance spectra acquired for (a) the STO/NFOO ceramic and (b) the STO/NFON ceramic.
Fig. 4. The Arrhenius plots of conductivities vs. 1/T arising from (a) the STO/NFOO ceramic and (b) the STO/NFON ceramic.
Fig. 5. O1s XPS spectra for (a) the STO/NFOO sample and (b) the STO/NFON sample.
Fig. 6. Ti2p XPS spectra for (a) the STO/NFOO sample and (b) the STO/NFON sample. 276
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al.
Fig. 7. The temperature dependence of the permittivities for (a) STO/NFOO and (b) STO/NFON; The temperature dependence of the dielectric loss for (c) STO/NFOO and (d) STO/NFON.
leakage current but large number of oxygen vacancies inferred from the low activation energy (Fig. 4b) in STO/NFON. Moreover, the formation of defect dipole with low leakage current considerably reduced dielectric loss [26,36]. Fig. 7 shows the curves of the dielectric constant and dielectric loss of samples sintered at different atmospheres. By comparing Fig. 7a to b, a series of anomaly peaks of dielectric constants was found from ~620 °C to ~750 °C in STO/NFOO. With increased test frequencies, the maximum peak values decreased, and the peak positions moved to high temperatures, revealing typical relaxation characteristics. These anomaly peaks in STO/NFOO can be attributed to the different conductive behaviors related to oxygen vacancies, which has been widely verified to exist in numerous ABO3 perovskites at a temperature range of 400 °C–800 °C [28,38,39]. The absence of anomalous dielectric relaxation in STO/NFON agreed well with the single conductive behavior observed over a wide temperature range (inset of Fig. 4b). Owing to the effects of defect dipoles, all the dielectric loss values in STO/NFON remained low (< 4 at 10 kHz) but increased rapidly in STO/NFOO (> 35 at 10 kHz) at high temperatures. The low dielectric loss was consistent with the low leakage current and high impedances in the STO/NFON ceramic.
anomaly permittivity peaks around 620 °C − 750 °C substantiated the presence of different conductivity mechanisms related to oxygen vacancies, and the STO/NFON ceramic possessed the low rate of dielectric loss. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 51772065, 51472063 and 51621091). References [1] W. Eerenstein, N.D. Mathur, J.F. Scoot, Multiferroic and magnetoelectric materials, Nature 442 (2006) 759–765. [2] M. Bibes, A. Barthelemy, Multiferroics: towards a magnetoelectric memory, Nat. Mater. 7 (2008) 425–426. [3] C. Israel, N.D. Mathur, J.F. Scott, A one-cent room-temperature magnetoelectric sensor, Nat. Mater. 7 (2008) 93–94. [4] A. Testino, L. Mitoseriu, V. Buscaglia, M.T. Buscaglia, I. Pallecchi, A.S. Albuquerque, V. Calzona, D. Marre, A.S. Siri, P. Nanni, Preparation of multiferroic composites of BaTiO3–Ni0.5Zn0.5Fe2O4ceramics, J. Eur. Ceram. Soc. 26 (2006) 3031–3036. [5] Z.G. Wang, L. Yan, Y.D. Yang, J.F. Li, J. Das, A.L. Geiler, A. Yang, Y. Chen, V.G. Harris, D. Viehland, Magnetoelectric effect in crystallographically textured BaTiO3 films deposited on ferromagnetic metallic glass foils, J. Appl. Phys. 109 (2011) 034102. [6] H.W. Jang, A. Kumar, S. Denev, M.D. Biegalski, P. Maksymovych, C.W. Bark, C.T. Nelson, C.M. Folkman, S.H. Baek, N. Balke, C.M. Brooks, D.A. Tenne, D.G. Schlom, L.Q. Chen, X.Q. Pan, S.V. Kalinin, V. Gopalan, C.B. Eom, Ferroelectricity in strain-free SrTiO3 thin films, Phys. Rev. Lett. 104 (2010) 197601. [7] V.V. Lemanov, Phase transitions in perovskite solid solutions with incipient ferroelectrics, Ferroelectrics 302 (2004) 169–173. [8] E.A. Eliseev, A.N. Morozovska, M.D. Glinchuk, R. Blinc, Anion vacancy-driven magnetism in incipient ferroelectric SrTiO3 and KTaO3 nanoparticles, J. Appl. Phys. 109 (2011) 094105. [9] J.H. Haeni, P. Irvin, W. Chang, R. Uecher, P. Reiche, Y.L. Li, S. Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefe, H. Levy, D.G. Schlom, Room-temperature ferroelectricity in strained SrTiO3, Nature 430 (2004) 758–761. [10] D. Lee, H. Lu, Y. Gu, S.Y. Choi, S.D. Li, S. Ryu, T.R. Paudel, K. Song, E. Mikheev, S. Lee, S. Stemmer, D.A. Tenne, S.H. Oh, E.Y. Tsymbal, X. Wu, L.Q. Chen, A. Gruverman, C.B. Eom, Emergence of room-temperature ferroelectricity at
4. Conclusion The 0.9STO −0.1NFO composite ceramics were sintered at different atmospheres (high-pure O2/N2). The XRD Rietveld refinements accompanied with the Williamson–Hall mode revealed the larger average microstrain of 0.092(47)% existed in STO/NFON. The Pmax in STO/ NFON increased to ~7.1 μC/cm2 owing to its large microstrain, but the leakage current resulted in a false larger value of Pr in STO/NFOO. The low activation energy (Ea = 1.22 eV) implied more oxygen vacancies in STO/NFON and the more Ti3+ ion content (60%). The existence of Ti3+ ions enabled the formation of the defect dipole [Ti4+·e−VO −Ti4+·e] to fixed VO locally in STO/NFON, which resulted in the low measured value of the VO content (~53%), the decreased leakage current, increased impedances and the improved ferroelectric hysteresis loop. The 277
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al. reduced dimensions, Science 349 (2015) 1314–1317. [11] D.G. Schlom, L.Q. Chen, C.B. Eom, K.M. Rabe, S.K. Streiffer, J.M. Triscone, Strain tuning of ferroelectric thin films, Annu. Rev. Mater. Res. 37 (2007) 589–626. [12] S. Qin, A.I. Becerro, F. Seifert, J. Gottsmann, J.Z. Jiang, Phase transitions in Ca1−xSrxTiO3 perovskites: effects of composition and temperature, J. Mater. Chem. 10 (2000) 1609–1615. [13] H. Ke, H.J. Zhang, J.J. Zhou, D.C. Jia, Y. Zhou, Room-temperature multiferroic and magnetodielectric properties of SrTiO3/NiFe2O4 composite ceramics, Ceram. Int. 45 (2019) 8238–8242. [14] B. Xu, K.B. Yin, J. Lin, Y.D. Xia, X.G. Wan, J. Yin, X.J. Bai, J. Du, Z.G. Liu, Roomtemperature ferromagnetism and ferroelectricity in Fe-doped BaTiO3, Phys. Rev. B 79 (2009) 134109. [15] F.Z. Li, H. Ke, H.J. Zhang, L.W. Zhang, H.J.D. Luo, L. Cao, D.C. Jia, Y. Zhou, Structural evolution and electrical properties of Na0.5Bi0.5TiO3-CoFe2O4 ceramics with embedded structures, Ceram. Int. 44 (2018) 22549–22555. [16] X.H. Liu, Z. Xu, X.Y. Wei, Z. H Dai, X. Yao, Ferroelectric, ferromagnetic, and magnetoelectric characteristics of 0.9(0.7BiFeO3–0.3BaTiO3) – 0.1CoFe2O4 ceramic composite, J. Am. Ceram. Soc. 93 (2010) 2975–2977. [17] J.F. Scott, Ferroelectrics go bananas, J. Phys. Condens. Matter 20 (2008) 021001. [18] C. Ang, Z. Yu, L.E. Cross, Oxygen-vacancy-related low-frequency dielectric relaxation and electrical conduction in Bi:SrTiO3, Phys. Rev. B 62 (2000) 228–236. [19] J.E. Schooley, W.R. Hosler, M.L. Cohen, Phys. Rev. Lett. 12 (1964) 474. [20] H. Trabelsi, M. Bejar, E. Dhahri, M.A. Valente, M.P.F. Graca, M. Djermouni, A. Zaoui, Evaluation of the relationship between the magnetism and the optical properties in SrTiO3-δ defective systems: experimental and theoretical studies, J. Magn. Magn. Mater. 478 (2019) 175–186. [21] H.J. Zhang, H. Ke, W. Wang, D.C. Jia, Y. Zhou, Crystallization behavior and multiferroic properties of Bi3.15Nd0.85Ti3O12/CoFe2O4 powders synthesized by sol–gel method, J. Am. Ceram. Soc. 99 (2016) 2334–2340. [22] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, third ed., Prentice–Hall, Englewood Cliffs, NJ, 2002, p. 377. [23] S. Sendhilnathan Hirthna, Enhancement in dielectric and magnetic properties of Mg2+ substituted highly porous super paramagnetic nickel ferrite nanoparticles with Williamson-Hall plots mechanistic view, Ceram. Int. 43 (2017) 15447–15453. [24] G.D. Hu, S.H. Fan, C.H. Yang, W.B. Wu, Low leakage current and enhanced ferroelectric properties of Ti and Zn codoped BiFeO3 thin film, Appl. Phys. Lett. 92 (2008) 192905. [25] H. Trabelsi, M. Bejar, E. Dhahri, M. Sajieddine, K. Khirouni, P.R. Prezas, B.M.G. Melo, M.A. Valente, M.P.F. Graça, Effect of oxygen vacancies on SrTiO3
electrical properties, J. Alloy. Comp. 723 (2017) 894–903. [26] L.J. Liu, M.X. Wu, Y.M. Huang, Z. Yang, L. Fang, C.Z. Hu, Frequency and temperature dependent dielectric and conductivity behavior of 0.95(K0.5Na0.5) NbO3–0.05BaTiO3 ceramic, Mater. Chem. Phys. 126 (2011) 769–772. [27] W. Kim III, C.W. Ahn, J.S. Kim, T.K. Song, J.S. Bae, B.C. Choi, J.H. Jeong, J.S. Lee, Low-frequency dielectric relaxation and ac conduction of SrBi2Ta2O9 thin film grown by pulsed laser deposition, Appl. Phys. Lett. 80 (2002) 4006. [28] C.C. Wang, C.M. Lei, G.J. Wang, X.H. Sun, T. Li, S.G. Huang, H. Wang, Y.D. Li, Oxygen-vacancy-related dielectric relaxations in SrTiO3 at high temperatures, J. Appl. Phys. 113 (2013) 094103. [29] H. Khan, I.K. Swati, Fe3+-doped anatase TiO2 with d−d transition, oxygen vacancies and Ti3+ centers: synthesis, characterization, UV−vis photocatalytic and mechanistic studies, Ind. Eng. Chem. Res. 55 (2016) 6619–6633. [30] S.M. Park, T. Ikegami, K. Ebihara, Effects of substrate temperature on the properties of Ga-Doped ZnO by pulsed laser deposition, Thin Solid Films 513 (2006) 90–94. [31] H.Q. Tan, Z. Zhao, W.B. Zhu, E.N. Coker, B.S. Li, M. Zheng, W.X. Yu, H.Y. Fan, Z.C. Sun, Oxygen vacancy enhanced photocatalytic activity of perovskite SrTiO3, ACS Appl. Mater. Interfaces (6) (2014) 19184–19190. [32] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-Doped ZnO films, Appl. Surf. Sci. 158 (2000) 134–140. [33] Y. Lv, W. Yao, X. Ma, C. Pan, R. Zong, Y. Zhu, The surface oxygen vacancy induced visible activity and enhanced UV activity of a ZnO1−x photocatalyst, Catal. Sci. Technol. 3 (2013) 3136–3146. [34] S. Shibagaki, K. Fukushima, XPS analysis on Nb-SrTiO3 thin films deposited with pulsed laser ablation technique, J. Eur. Ceram. Soc. 19 (1999) 1423–1426. [35] H.L. Cai, X.S. Wu, J. Gao, Effect of oxygen content on structural and transport properties in SrTiO3-x thin films, Chem. Phys. Lett. 467 (2009) 313–317. [36] Z.J. Wang, M.H. Cao, Z.H. Yao, Q. Zhang, Z. Song, W. Hu, Q. Xu, H. Hao, H.X. Liu, Z.Y. Yu, Giant permittivity and low dielectric loss of SrTiO3 ceramics sintered in nitrogen atmosphere, J. Eur. Ceram. Soc. 34 (2014) 1755–1760. [37] N.N. Wang, M.H. Cao, Z.C. He, C.L. Diao, Q. Zhang, Y.P. Zhang, J.Q. Dai, F.Z. Zeng, H. Hao, Z.H. Yao, H.X. Liu, Structural and dielectric behavior of giant permittivity SrNbxTi1-xO3 ceramics sintered in nitrogen atmosphere, Ceram. Int. 42 (2016) 13593–13600. [38] R. Stumpe, D. Wagner, D. Bauerle, Influence of bulk and interface properties on the electric transport in ABO3 perovskites, Phys. Status Solidi 75 (1983) 143–154. [39] S.K. Choi, B.S. Kang, Y.W. Cho, Y.M. Vysochanskii, Diffuse dielectric anomaly in ferroelectric materials, J. Electroceram. 13 (2004) 493–502.
278
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Effects of O2 and N2 sintering atmospheres on electric properties of 0.9SrTiO3–0.1NiFe2O4 composite ceramics
T
Hongjun Zhanga,b,c, Hua Kea,b,∗, Junjie Zhoua,b, Huijiadai Luoa,b, Fangzhe Lia,b, Bin Yangc,∗∗, Yan Fand, Qingqin Ged, Dechang Jiaa,b, Yu Zhoua,b a
Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150080, China Key Laboratory of Advanced Structure-Function Integrated Materials and Green Manufacturing Technology (Ministry of Industry and Information Technology), Harbin Institute of Technology, Harbin, 150001, China c Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin, 150080, China d Thermo Fisher Scientific China, Building 6, No. 27, Xin Jinqiao Road, Shanghai, 201206, China b
ARTICLE INFO
ABSTRACT
Keywords: 0.9SrTiO3−0.1NiFe2O4 XRD refinement XPS Dielectricity Impedance
The 0.9SrTiO3–0.1NiFe2O4 (STO/NFO) composite ceramics sintered at O2/N2 atmospheres were investigated. The N2-sintered STO/NFO (STO/NFON) had a larger microstrain (0.092(47)%) than the O2-sintered STO/NFO (STO/NFOO) revealed by the Rietveld refinement and the Williamson-Hall analysis. The maximum polarization of STO/NFON increased to ~7.1 μC/cm2, but the larger leakage current in STO/NFOO resulted in a 20% larger false remanent polarization value. Moreover, STO/NFON presented a lower activation energy (Ea, 1.22 eV), which implied more oxygen vacancies (VO ). The amount of Ti3+ ions (60%) in STO/NFON increased as well, which formed defect dipoles [Ti4+·e−VO −Ti4+·e] and fixed VO . The composite of STO/NFON also showed decreased VO content (~53%) owing to the fixing effect and presented low leakage current, increased impedances, and the improved ferroelectric hysteresis loop. The anomaly dielectric constants peaks indicated different conductivity mechanisms related to VO , and the STO/NFON ceramic possessed a low rate of dielectric loss.
1. Introduction Magnetoelectric multiferroics, which are widely used on inductors, filters and tuneable microwave devices [1,2] have persistently attracted research attention for decades. In particular, leadless ferroelectric ceramics and ferromagnetic materials are frequently used for preparing magnetoelectric composites and modulating textural structures [3–5] for the enhancement of magnetoelectric coupling coefficient. SrTiO3 (STO) is an interesting quantum paraelectric materials because it presents quantum fluctuations [6] in its atomic positions. These fluctuations suppress ferroelectric transition and results in the so-called incipient ferroelectricity [7,8]. The ferroelectric properties of STO is induced by introducing stress to or reducing the dimensions of the STO phase. These processes delicately disrupt Ti4+ fluctuations and reorder the intrinsic nanopolar regions of STO [9,10]. The phase transitions from cubic (Pm-3m) STO to tetragonal (I4/mcm) or orthorhombic (Pbnm/Bmmb) STO at various strain conditions [11,12] have already been reported. Under these conditions, STO exhibits elevated Curie
∗
temperature (Tc) that is nearly equal to room temperature and shows ferroelectric properties. These features enable stress-induced ferroelectricity and facilitate the magnetic field modulation of electric signals, rendering STO useful in the preparation of composite magnetoelectric materials. In our recent publication [13], SrTiO3/NiFe2O4 (STO/NFO) composite ceramics presented ferroelectric hysteresis loops, and the magnetodielectric change rate reached approximately 1.6%, which indirectly reflected the magnetoelectric coupling effect. As is well known, the introduction of magnetic phases into perovskite ferroelectric matrices usually increases leakage currents due to the existence of defects, such as oxygen vacancies and space-charges [14–16]. A large leakage current would cumber the precision and reliability of measurement methods for ferroelectric properties [17]. Meanwhile, despite being a kind of defects, oxygen vacancies account for the electrical properties of perovskites and can be controlled by doping or annealing at different oxygen partial pressure conditions. The effects of oxygen vacancies on the conductivity has been confirmed by lots of previous studies on
Corresponding author. Institute for Advanced Ceramics, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150080, China. Corresponding author. Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin, 150080, China E-mail addresses: [email protected] (H. Ke), [email protected] (B. Yang).
∗∗
https://doi.org/10.1016/j.physb.2019.07.023 Received 14 May 2019; Received in revised form 11 July 2019; Accepted 13 July 2019 Available online 07 August 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al.
Fig. 1. The XRD Rietveld refinement of (a) the STO/NFOO ceramic and (b) the STO/NFON ceramic; and the calculated microstrains borne by STO phase in (c) the STO/NFOO ceramic and (c) the STO/NFON ceramic. Insets of Fig. 1c and d are the SEM images of the STO/NFOO and STO/NFON ceramics.
perovskite materials. Ang et al. discovered three sets of dielectric peaks stemming from oxygen vacancies in Bi:SrTiO3 ceramics, and verified that the conduction carriers were from the first ionization of oxygen vacancies [18]. Schooley et al. even discussed the superconductive transitions in the reduced STO samples with high charge carrier concentration [19]. Additionally, the latest study by Trabelsi et al. [20] revealed that oxygen vacancies concentrations also affected the magnetic and optical properties of STO, and there existed two types of electronic contributions to magnetism. Given the importance of oxygen vacancies and the corresponding conductivity mechanisms, detailed studies on electric properties of STO/NFO composite ceramics under different sintering atmospheres would be necessary. In the present work, we chose the 0.9STO−0.1NFO composite as a specific example to explore the effects of sintering atmospheres (highpure O2/N2) owing to its good ferroelectric property [13]. The structure refinements, strain conditions, and electric/dielectric properties of the 0.9STO−0.1NFO composite ceramics were characterized, and defect concentrations were calculated by X-ray photoelectron spectroscopy (XPS).
pellets at 200 MPa. Each pellet was 10 mm in diameter and 1 mm in thickness. The binder was thoroughly burned away at 550 °C. The green ceramic pellets were randomly divided into two groups and separately sintered in the oxygen atmosphere (STO/NFOO) and the nitrogen atmosphere (STO/NFON) at 1350 °C for 5 h. Both gases were dry with purities of 99.9% and the atmospheric pressures were at 1 atm. The sintered ceramics had relative densities of above 95%. Samples were thoroughly polished to ~200 μm from ~1 mm in thickness. Sputtered gold electrodes of 1.5 mm in diameter were used to investigate ferroelectric properties. As to the dielectric and impedance performances, samples were coated using silver paste as electrodes and the electrical connections to the experimental setup were achieved through silver wires. X-ray diffraction (XRD) data were collected using an automated diffractometer (40 kV/40 mA, CuKα, radiation 0.1540598 nm, Empyrean, PANalytical, Netherlands). X-ray photoelectron spectrometry (XPS) data was obtained with an ESCALAB 250Xi Instrument (ThermoFisher Scientific, USA). The collected XPS spectra were deconvoluted using XPSPeak software by employing a Gaussian-Lorentzian mixed function with the background and the calibration using Carbon 1s spectrum under considerations. Ferroelectric properties and leakage currents were measured on a ferroelectric tester system (precision Premier II, Radiant Technologies, Inc., America). Dielectric/impedance measurements were conducted with an LCR Meter TH2827C (TongHui Inc. China), and the temperature was controlled by a furnace with proportional-integral-derivative (PID) controller.
2. Experiments The 0.9STO−0.1NFO composite powder was synthesized using the sol-gel method. Ni(NO3)2, Fe(NO3)3, (C2H3O2)2Sr, and Ti(C4H9O)4 were the raw materials for the metal ion sources. Acetic acid and ethylene glycol were the solvent and complexing agent, respectively. All chemical reagents used were of analytical grade. Ni(NO3)2, Fe(NO3)3 and (C2H3O2)2Sr were first weighed stoichiometricly according to the molar ratio between STO and NFO (9:1). All these three metal salts were dissolved in boiling acetic acid with mechanical agitation. Then, at room temperature (RT), ethylene glycol was added to the obtained transparent complex acetic acid solution. The resulting solution was dropwise added into the Ti(C4H9O)4 solution with ethylene glycol at RT. Taking different metal ionic activities of the complex sol-gel system into consideration, the respective reaction temperatures of and dosage ratio of the solvent and the complexing agent played key roles in the balancing of hydrolysis and gelation of all the metallic ions. A two-step calcination process [13,21] was conducted to obtain NFO phase and STO phase at 350 °C and 1050 °C, respectively, in the 0.9STO−0.1NFO system. The calcined composite ceramic powders were mixed with 2 wt% PVA aqueous solution and compacted into
3. Results and discussion Fig. 1a and b provide the information on the structures of STO/ NFOO and STO/NFON. The peaks arising from the STO and NFO could be easily to tell apart. Variations in sintering atmosphere had no obvious effects on the phase compositions. The Rietveld refinements displayed that the Rp values of STO/NFOO and STO/NFON were 10.6% and 9.3%, respectively. The relatively low fitting errors indicated the suitability of the refinement strategy. The calculated cell parameters revealed that both lattice constants a of STO in STO/NFOO and STO/ NFON were 3.904 Å and nearly equal to that in the used STO crystal model (COD No. 9002806). While the lattice constant of NFO in STO/ NFON (8.373 Å) was slightly larger than that in STO/NFOO (8.334 Å). The different lattice constants of NFO implied different strain states. The microstrains borne by STO in both composite ceramics were 274
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al.
The Arrhenius plots in Fig. 4 show the function of the conductivity depending on the reciprocal temperature (1/T). The AC conduction curves relying on frequency were displayed as insets. The different slopes of the conduction curves are attributed to various conductive mechanism, such as the relaxation or hopping of charge carriers, charge dissociation from traps or defects and the Schottky emission [26,27]. In the inset of Fig. 4a, two different conductive trends were observed via distinct curvatures for STO/NFOO, and the critical temperature was around 600 °C. While the conductive curves above 600 °C for STO/ NFON showed the same tendency of that at 600 °C. The data collected at 100 Hz from 720 °C to 800 °C were used for fitting progresses according to the following Arrhenius relation [26,28]:
=
calculated on the basis of the XRD data and Williamson–Hall patterns [22,23], as shown in Fig. 1c and d. The equation used for the Williamson–Hall model was as follows:
K× + 4 × Strain × sin( ) Size
exp( Ea/KB T )
(2)
where σ0 is the pre-exponent constant, kB is the Boltzmann's constant, and Ea is the activation energy of conduction. Both samples showed appropriate fitting results when the least square method was used. Moreover, only one single linear section appeared, indicating only one conductive mechanism at this mentioned temperature range. The activation energy was 1.34 eV for STO/NFOO and 1.22 eV for STO/NFON. The activation energy around 1.2 eV is usually identified as the conductivity from Schottky emission in perovskite composite [27]. The lower activation energy in STO/NFON hinted the increased oxygen vacancy content and the enhanced mobility of oxygen ion vacancies [25]. These enhancements would usually result in the larger conductivity but seemed incompatible with the above analyses of which the STO/NFON sample showed the lower leakage current. To further understand the underlying mechanism, XPS was used to uncover the ion valences. Fig. 5a and b displays the O1s XPS spectra for STO/NFOO and STO/ NFON respectively. All the binding energies were measured in reference to the C1s peak (284.8 eV) [29]. Each O1s high resolution XPS spectrum was deconvoluted into two Gaussian peaks. The peaks of STO/NFOO were detected at 529.1 and 531.3 eV, whereas the corresponding peaks of STO/NFON were at 529.2 and 530.9 eV, respectively. On the basis of a great deal of literature studies [30,31], the peak I at about 529 eV was attributed to the Ti–O band, namely associated with the O2− ions in the STO crystal structure. And the peak II at ~531 eV stemmed from the oxygen vacancy [32]. For the meticulous surface ion etching before the XPS data collection, those interference peaks (above 533 eV) assigned to the loosely bound oxygen on the samples' surfaces [33] were excluded here. It can be obviously observed that the shoulder peak II in Fig. 5a was stronger than that in Fig. 5b. According to the ratio of peaks I and II, the proportions of oxygen vacancies at RT were semi-quantitatively calculated as 65% in STO/NFOO and 53% in STO/NFON, respectively. The higher content of the oxygen vacancy in STO/NFOO was aligned with the increased leakage current of the sample sintered in O2 in Fig. 2 but was contrary to the inference of more oxygen vacancies in STO/NFON from Fig. 4b. Fig. 6a and b shows the typical Ti2p XPS spectra. The binding energies of Ti2p3/2 and Ti2p1/2 [34] were at ~458 eV and ~464 eV, respectively. Through optimized fitting processes, peaks associated with the Ti4+ and Ti3+ ions were distinguished carefully. Taking the Ti2p3/2 as an example, the binding energies of Ti4+ and Ti3+ [35] were respectively 458.1 and 457.4 eV in STO/NFOO and 458.5 and 457.7 eV in STO/NFON. According to the area ratios of peaks representing Ti4+ and Ti3+ ions, the fitting results also indicated that STO/NFON had a higher Ti3+ ion content than that of STO/NFOO (~60% and ~39%, respectively). The oxygen vacancy (VO ) is a kind of donor defects which turns an insulator into an n-type semiconductor. The Ti3+ ions (also known as the combination of Ti4+·e) as a type of acceptor defect would form a designed defect dipole [Ti4+·e−VO −Ti4+·e] to fixed VO locally [36,37]. The fixed local VO resulted in the low measurement value of VO content in STO/NFON shown in Fig. 5b. The contributions of local oxygen vacancies to conductivity also decreased because of the limited ionic mobility, which was the reasonable mechanism behind the low
Fig. 2. Ferroelectric hysteresis loops of STO/NFO ceramics. Inset: the corresponding leakage currents.
FW (s ) × cos( ) =
0
(1)
where FW(s) is the full-width at half-maximum (FWHM) of the peak, θ is the Bragg angle, Size is the grain size and Strain is the microstrain. The various FWHM values would reveal diversified microstrains in each sample. Owing to the micro-sized STO grains seen in the insets of Fig. 1c and d, the broadening effect of nanograins on FWHM can be excluded. The calculations showed that the STO phase in the STO/ NFON ceramic possessed a larger average microstrain (0.092(47)%) than that in STO/NFOO (0.068(65)%). As mentioned above, it is plausible to infer that the larger strain usually emphasizes ferroelectric properties. Fig. 2 showed the polarization–electric field (P–E) hysteresis loops of STO/NFOO and STO/NFON. The maximum polarization (Pmax) reached ~7.1 μC/cm2 for STO/NFON, which was definitely larger than that of the sample sintered in oxygen atmosphere (~6.6 μC/cm2). This improved Pmax value was consistent with the high strain of STO/NFON. Meanwhile, the remanent polarization (Pr) of STO/NFOO was 20% larger compared to that of STO/NFON. For this discordance between Pmax and Pr, some clue can be noticed from the leakage currents in the inset of Fig. 2. The leakage current of STO/NFOO increased fivefold relative to that of the STO/NFOO ceramic. The large leakage may have produced additional false signals on the ferroelectric measurement and contributed to the apparently high apparent Pr, as well as the more rounded loop shape [17,24] near the maximum polarization. To further evaluate the effects of O2/N2 atmospheres on the electric conductivities of the 0.9STO−0.1NFO ceramics, the impedance spectra with their derived alternating current conductivities were investigated. In Fig. 3, both impedance spectra decreased in the real part (Z') of the impedance with temperature increasing from 600 °C to 800 °C. The Z' value of the STO/NFON ceramic at each test temperature was from three to four times of that of STO/NFOO, and the high impedance agreed well with the low leakage current in STO/NFON. In addition, the impedance patterns of STO/NFON (inset of Fig. 3b) at 720 °C and 800 °C can conspicuously be divided into two semicircles of which the semicircles at high test frequencies, representing the influence of grain boundaries, possessed much larger proportions compared with STO/ NFOO. In other words, the semicircles arising from grains were dominant in STO/NFOO, which meant the higher concentration of oxygen vacancies and free electrons [25] and resulted in the larger leakage current. 275
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al.
Fig. 3. Impedance spectra acquired for (a) the STO/NFOO ceramic and (b) the STO/NFON ceramic.
Fig. 4. The Arrhenius plots of conductivities vs. 1/T arising from (a) the STO/NFOO ceramic and (b) the STO/NFON ceramic.
Fig. 5. O1s XPS spectra for (a) the STO/NFOO sample and (b) the STO/NFON sample.
Fig. 6. Ti2p XPS spectra for (a) the STO/NFOO sample and (b) the STO/NFON sample. 276
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al.
Fig. 7. The temperature dependence of the permittivities for (a) STO/NFOO and (b) STO/NFON; The temperature dependence of the dielectric loss for (c) STO/NFOO and (d) STO/NFON.
leakage current but large number of oxygen vacancies inferred from the low activation energy (Fig. 4b) in STO/NFON. Moreover, the formation of defect dipole with low leakage current considerably reduced dielectric loss [26,36]. Fig. 7 shows the curves of the dielectric constant and dielectric loss of samples sintered at different atmospheres. By comparing Fig. 7a to b, a series of anomaly peaks of dielectric constants was found from ~620 °C to ~750 °C in STO/NFOO. With increased test frequencies, the maximum peak values decreased, and the peak positions moved to high temperatures, revealing typical relaxation characteristics. These anomaly peaks in STO/NFOO can be attributed to the different conductive behaviors related to oxygen vacancies, which has been widely verified to exist in numerous ABO3 perovskites at a temperature range of 400 °C–800 °C [28,38,39]. The absence of anomalous dielectric relaxation in STO/NFON agreed well with the single conductive behavior observed over a wide temperature range (inset of Fig. 4b). Owing to the effects of defect dipoles, all the dielectric loss values in STO/NFON remained low (< 4 at 10 kHz) but increased rapidly in STO/NFOO (> 35 at 10 kHz) at high temperatures. The low dielectric loss was consistent with the low leakage current and high impedances in the STO/NFON ceramic.
anomaly permittivity peaks around 620 °C − 750 °C substantiated the presence of different conductivity mechanisms related to oxygen vacancies, and the STO/NFON ceramic possessed the low rate of dielectric loss. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, Grant No. 51772065, 51472063 and 51621091). References [1] W. Eerenstein, N.D. Mathur, J.F. Scoot, Multiferroic and magnetoelectric materials, Nature 442 (2006) 759–765. [2] M. Bibes, A. Barthelemy, Multiferroics: towards a magnetoelectric memory, Nat. Mater. 7 (2008) 425–426. [3] C. Israel, N.D. Mathur, J.F. Scott, A one-cent room-temperature magnetoelectric sensor, Nat. Mater. 7 (2008) 93–94. [4] A. Testino, L. Mitoseriu, V. Buscaglia, M.T. Buscaglia, I. Pallecchi, A.S. Albuquerque, V. Calzona, D. Marre, A.S. Siri, P. Nanni, Preparation of multiferroic composites of BaTiO3–Ni0.5Zn0.5Fe2O4ceramics, J. Eur. Ceram. Soc. 26 (2006) 3031–3036. [5] Z.G. Wang, L. Yan, Y.D. Yang, J.F. Li, J. Das, A.L. Geiler, A. Yang, Y. Chen, V.G. Harris, D. Viehland, Magnetoelectric effect in crystallographically textured BaTiO3 films deposited on ferromagnetic metallic glass foils, J. Appl. Phys. 109 (2011) 034102. [6] H.W. Jang, A. Kumar, S. Denev, M.D. Biegalski, P. Maksymovych, C.W. Bark, C.T. Nelson, C.M. Folkman, S.H. Baek, N. Balke, C.M. Brooks, D.A. Tenne, D.G. Schlom, L.Q. Chen, X.Q. Pan, S.V. Kalinin, V. Gopalan, C.B. Eom, Ferroelectricity in strain-free SrTiO3 thin films, Phys. Rev. Lett. 104 (2010) 197601. [7] V.V. Lemanov, Phase transitions in perovskite solid solutions with incipient ferroelectrics, Ferroelectrics 302 (2004) 169–173. [8] E.A. Eliseev, A.N. Morozovska, M.D. Glinchuk, R. Blinc, Anion vacancy-driven magnetism in incipient ferroelectric SrTiO3 and KTaO3 nanoparticles, J. Appl. Phys. 109 (2011) 094105. [9] J.H. Haeni, P. Irvin, W. Chang, R. Uecher, P. Reiche, Y.L. Li, S. Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefe, H. Levy, D.G. Schlom, Room-temperature ferroelectricity in strained SrTiO3, Nature 430 (2004) 758–761. [10] D. Lee, H. Lu, Y. Gu, S.Y. Choi, S.D. Li, S. Ryu, T.R. Paudel, K. Song, E. Mikheev, S. Lee, S. Stemmer, D.A. Tenne, S.H. Oh, E.Y. Tsymbal, X. Wu, L.Q. Chen, A. Gruverman, C.B. Eom, Emergence of room-temperature ferroelectricity at
4. Conclusion The 0.9STO −0.1NFO composite ceramics were sintered at different atmospheres (high-pure O2/N2). The XRD Rietveld refinements accompanied with the Williamson–Hall mode revealed the larger average microstrain of 0.092(47)% existed in STO/NFON. The Pmax in STO/ NFON increased to ~7.1 μC/cm2 owing to its large microstrain, but the leakage current resulted in a false larger value of Pr in STO/NFOO. The low activation energy (Ea = 1.22 eV) implied more oxygen vacancies in STO/NFON and the more Ti3+ ion content (60%). The existence of Ti3+ ions enabled the formation of the defect dipole [Ti4+·e−VO −Ti4+·e] to fixed VO locally in STO/NFON, which resulted in the low measured value of the VO content (~53%), the decreased leakage current, increased impedances and the improved ferroelectric hysteresis loop. The 277
Physica B: Condensed Matter 572 (2019) 273–278
H. Zhang, et al. reduced dimensions, Science 349 (2015) 1314–1317. [11] D.G. Schlom, L.Q. Chen, C.B. Eom, K.M. Rabe, S.K. Streiffer, J.M. Triscone, Strain tuning of ferroelectric thin films, Annu. Rev. Mater. Res. 37 (2007) 589–626. [12] S. Qin, A.I. Becerro, F. Seifert, J. Gottsmann, J.Z. Jiang, Phase transitions in Ca1−xSrxTiO3 perovskites: effects of composition and temperature, J. Mater. Chem. 10 (2000) 1609–1615. [13] H. Ke, H.J. Zhang, J.J. Zhou, D.C. Jia, Y. Zhou, Room-temperature multiferroic and magnetodielectric properties of SrTiO3/NiFe2O4 composite ceramics, Ceram. Int. 45 (2019) 8238–8242. [14] B. Xu, K.B. Yin, J. Lin, Y.D. Xia, X.G. Wan, J. Yin, X.J. Bai, J. Du, Z.G. Liu, Roomtemperature ferromagnetism and ferroelectricity in Fe-doped BaTiO3, Phys. Rev. B 79 (2009) 134109. [15] F.Z. Li, H. Ke, H.J. Zhang, L.W. Zhang, H.J.D. Luo, L. Cao, D.C. Jia, Y. Zhou, Structural evolution and electrical properties of Na0.5Bi0.5TiO3-CoFe2O4 ceramics with embedded structures, Ceram. Int. 44 (2018) 22549–22555. [16] X.H. Liu, Z. Xu, X.Y. Wei, Z. H Dai, X. Yao, Ferroelectric, ferromagnetic, and magnetoelectric characteristics of 0.9(0.7BiFeO3–0.3BaTiO3) – 0.1CoFe2O4 ceramic composite, J. Am. Ceram. Soc. 93 (2010) 2975–2977. [17] J.F. Scott, Ferroelectrics go bananas, J. Phys. Condens. Matter 20 (2008) 021001. [18] C. Ang, Z. Yu, L.E. Cross, Oxygen-vacancy-related low-frequency dielectric relaxation and electrical conduction in Bi:SrTiO3, Phys. Rev. B 62 (2000) 228–236. [19] J.E. Schooley, W.R. Hosler, M.L. Cohen, Phys. Rev. Lett. 12 (1964) 474. [20] H. Trabelsi, M. Bejar, E. Dhahri, M.A. Valente, M.P.F. Graca, M. Djermouni, A. Zaoui, Evaluation of the relationship between the magnetism and the optical properties in SrTiO3-δ defective systems: experimental and theoretical studies, J. Magn. Magn. Mater. 478 (2019) 175–186. [21] H.J. Zhang, H. Ke, W. Wang, D.C. Jia, Y. Zhou, Crystallization behavior and multiferroic properties of Bi3.15Nd0.85Ti3O12/CoFe2O4 powders synthesized by sol–gel method, J. Am. Ceram. Soc. 99 (2016) 2334–2340. [22] B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, third ed., Prentice–Hall, Englewood Cliffs, NJ, 2002, p. 377. [23] S. Sendhilnathan Hirthna, Enhancement in dielectric and magnetic properties of Mg2+ substituted highly porous super paramagnetic nickel ferrite nanoparticles with Williamson-Hall plots mechanistic view, Ceram. Int. 43 (2017) 15447–15453. [24] G.D. Hu, S.H. Fan, C.H. Yang, W.B. Wu, Low leakage current and enhanced ferroelectric properties of Ti and Zn codoped BiFeO3 thin film, Appl. Phys. Lett. 92 (2008) 192905. [25] H. Trabelsi, M. Bejar, E. Dhahri, M. Sajieddine, K. Khirouni, P.R. Prezas, B.M.G. Melo, M.A. Valente, M.P.F. Graça, Effect of oxygen vacancies on SrTiO3
electrical properties, J. Alloy. Comp. 723 (2017) 894–903. [26] L.J. Liu, M.X. Wu, Y.M. Huang, Z. Yang, L. Fang, C.Z. Hu, Frequency and temperature dependent dielectric and conductivity behavior of 0.95(K0.5Na0.5) NbO3–0.05BaTiO3 ceramic, Mater. Chem. Phys. 126 (2011) 769–772. [27] W. Kim III, C.W. Ahn, J.S. Kim, T.K. Song, J.S. Bae, B.C. Choi, J.H. Jeong, J.S. Lee, Low-frequency dielectric relaxation and ac conduction of SrBi2Ta2O9 thin film grown by pulsed laser deposition, Appl. Phys. Lett. 80 (2002) 4006. [28] C.C. Wang, C.M. Lei, G.J. Wang, X.H. Sun, T. Li, S.G. Huang, H. Wang, Y.D. Li, Oxygen-vacancy-related dielectric relaxations in SrTiO3 at high temperatures, J. Appl. Phys. 113 (2013) 094103. [29] H. Khan, I.K. Swati, Fe3+-doped anatase TiO2 with d−d transition, oxygen vacancies and Ti3+ centers: synthesis, characterization, UV−vis photocatalytic and mechanistic studies, Ind. Eng. Chem. Res. 55 (2016) 6619–6633. [30] S.M. Park, T. Ikegami, K. Ebihara, Effects of substrate temperature on the properties of Ga-Doped ZnO by pulsed laser deposition, Thin Solid Films 513 (2006) 90–94. [31] H.Q. Tan, Z. Zhao, W.B. Zhu, E.N. Coker, B.S. Li, M. Zheng, W.X. Yu, H.Y. Fan, Z.C. Sun, Oxygen vacancy enhanced photocatalytic activity of perovskite SrTiO3, ACS Appl. Mater. Interfaces (6) (2014) 19184–19190. [32] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-Doped ZnO films, Appl. Surf. Sci. 158 (2000) 134–140. [33] Y. Lv, W. Yao, X. Ma, C. Pan, R. Zong, Y. Zhu, The surface oxygen vacancy induced visible activity and enhanced UV activity of a ZnO1−x photocatalyst, Catal. Sci. Technol. 3 (2013) 3136–3146. [34] S. Shibagaki, K. Fukushima, XPS analysis on Nb-SrTiO3 thin films deposited with pulsed laser ablation technique, J. Eur. Ceram. Soc. 19 (1999) 1423–1426. [35] H.L. Cai, X.S. Wu, J. Gao, Effect of oxygen content on structural and transport properties in SrTiO3-x thin films, Chem. Phys. Lett. 467 (2009) 313–317. [36] Z.J. Wang, M.H. Cao, Z.H. Yao, Q. Zhang, Z. Song, W. Hu, Q. Xu, H. Hao, H.X. Liu, Z.Y. Yu, Giant permittivity and low dielectric loss of SrTiO3 ceramics sintered in nitrogen atmosphere, J. Eur. Ceram. Soc. 34 (2014) 1755–1760. [37] N.N. Wang, M.H. Cao, Z.C. He, C.L. Diao, Q. Zhang, Y.P. Zhang, J.Q. Dai, F.Z. Zeng, H. Hao, Z.H. Yao, H.X. Liu, Structural and dielectric behavior of giant permittivity SrNbxTi1-xO3 ceramics sintered in nitrogen atmosphere, Ceram. Int. 42 (2016) 13593–13600. [38] R. Stumpe, D. Wagner, D. Bauerle, Influence of bulk and interface properties on the electric transport in ABO3 perovskites, Phys. Status Solidi 75 (1983) 143–154. [39] S.K. Choi, B.S. Kang, Y.W. Cho, Y.M. Vysochanskii, Diffuse dielectric anomaly in ferroelectric materials, J. Electroceram. 13 (2004) 493–502.
278