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Materials Chemistry and Physics

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Improved microstructure and ferroelectric properties in B-site Ti4+-substituted (Bi0.86Sm0.14)FeO3 polycrystalline ceramicsHaidee Mana-aya,∗,1, Jesuraj Anthoniappena, Chi-Shun Tub, Raymund Sarmiento Jr.a,Cheng-Sao Chenc, Pin-Yi Chend, Flora Mae Ruizaa Department of Physics, University of San Carlos, Talamban Campus, Cebu City, 6000, PhilippinesbDepartment of Physics, Fu Jen Catholic University, New Taipei City, 24205, Taiwanc Department of Mechanical Engineering, Hwa Hsia University of Technology, New Taipei City, 23567, TaiwandDepartment of Mechanical Engineering, Ming Chi University of Technology, New Taipei City, 24301, Taiwan

H I G H L I G H T S

• Reduction of grain size upon Ti4+ doping.

• Ti4+ doping stabilized phase coexistence in (Bi0.86Sm0.14)FeO3 ceramics.

• Enhanced ferroelectric order due to Ti4+ doping.

• Ti4+ doping enhanced ferroelectric and piezoelectric properties.

• Ti4+ doping resulted in dielectric homogeneity and reduced dielectric loss.

A R T I C L E I N F O

Keywords:BiFeO3

MultiferroicsMorphotropic phase boundaryDielectric homogeneityFerroelectric order

A B S T R A C T

Multiferroic (Bi Sm )(Fe Ti )O0.86 0.14 1 x x 3 ceramics (BFO14Sm100xTi) for =x 0.000, 0.005, and 0.010 were synthe-sized using solid-state reaction method to investigate the influence of Ti4+ doping on their microstructure, di-electric, and ferroelectric behaviors. Scanning electron micrographs reveal that the Ti4+ doping facilitates slowoxygen ion diffusion leading to reduced grain size (2.13, 1.75, and 1.50 µm for BFO14Sm, BFO14Sm0.5Ti, andBFO14Sm1Ti, respectively). High-resolution synchrotron X-ray diffraction (HR-XRD) analysis and Rietveld re-finements confirm the phase coexistence of polar R3c rhombohedral and nonpolar PbZrO3-like Pbam orthorhombicphases. The temperature dependent HR-XRD shows that the Ti4+ ions stabilize phase coexistence up to °250 C. Theenhanced dielectric homogeneity and decreased conductivity are caused by Ti4+ substitution, which are attributedto reduction of oxygen vacancies. The improved piezoelectric coefficients (d33) and symmetric P–E hysteresis loopsof BFO14Sm0.5Ti, and BFO14Sm1Ti indicate enhanced ferroelectric order due to Ti4+ ion doping.

1. Introduction

Multiferroics are a group of smart materials which exhibit couplingbetween two or more ferroic orders, such as ferroelectricity and ferro-magnetism, making them potential candidates for technological applica-tions in multistate information-storage devices, modern electronics, energyharvesting, spintronics, and sensors [1–3]. Among these multiferroics,bismuth ferrite (BiFeO3, BFO) has gained enormous research interest dueto the presence of magnetoelectric coupling at room temperature [2,4].Thus, BFO is a promising alternative for a “green” multifunctional materialas compared with lead-based compounds, such as Pb(Zr Ti )O (PZT),x 1 x 3

Pb(Mg Nb )O PbTiO ,1/3 2/3 3 3 and Pb(Zn Nb )O1/3 2/3 3 PbTiO3 [5]. It has ahigh ferroelectric Curie temperature °T of 830 CC and anti-ferromagnetic Néel temperature °T of 370 CN [6], enabling it to beutilized reliably at and above room temperature.

It has been well established that many perovskites exhibit maximalelectromechanical responses in the vicinity of the morphotropic phaseboundary (MPB) [7,8]. The A-site samarium substitution (Bi1-xSmxFeO3) at room temperature exhibits a ferroelectric rhombohedralphase for =x 0–12%, a coexistence of ferroelectric triclinic and orthor-hombic phases for =x 12.5–20%, and a non-polar orthorhombic phasefor =x 25% [9]. It has been also proved that the smaller ionic radius of

https://doi.org/10.1016/j.matchemphys.2018.12.092Received 2 July 2018; Received in revised form 3 December 2018; Accepted 28 December 2018

∗ Corresponding author. Physics Department, SC 341, 3rd Floor, Science Complex, Silliman University, 1 Hibbard Avenue, Dumaguete City, 6200, Philippines.E-mail address: haideeimana-ay@su.edu.ph (H. Mana-ay).

1 Department of Physics, Silliman University, Dumaguete City, 6200, Philippines.

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T

Sm3+ (1.079 ) compared to Bi3+ (∼1.4 ) [10] induces structuraldistortions which may significantly change the ferroelectric phase be-havior [11,12]. The existence of MPB in 14% samarium-doped BFO thinfilms was reported by many researchers [7,13–17] highlighting struc-tural stability up to °400 C and nanoscale twinned triclinic domains[14]. A recent report by Chen et al. [18] confirmed the coexistence ofR3c and Pbam structures in 14% Sm3+-doped BFO ceramics. The ef-fective piezoelectric value displays a rapid increase around=x 13 15% peaking at =x 14% with d 110 pm/V33 ) [14], which arecomparable to the values for epitaxial thin films of Pb-based com-pounds such as PZT [19]. A square-shaped FE hysteresis loop with goodsaturation and robust switchable polarization ( 70 µC/cm2) was re-ported for compositions from =x 0 up to x 14% [14]. Although Sm3+-doped BFO ceramics exhibited improved polarizations, the P–E hys-teresis loops did not show saturation due to the leaky nature [20]. Theelectrical conduction in Sm3+-doped BFO was suggested to be due tooxygen vacancies and irrespective of Sm3+ compositions [21].

Interestingly, B-site substitution with transition metals (Mn, Zr, Sn,and Ti) in BFO has showed enhanced multiferroic properties [22–25].Among these transition metals, a great interest is given to Ti4+ sub-stituent as it readily reacts with Bi2O3 [26] and is a highly polarizableion which might couple with Bi displacements [25]. The Ti4+ ions inthe d state allow 3d–O 2p hybridization which is essential for stabilizingferroelectric distortion [27]. Hence, enhanced ferroelectricity is ex-pected in the Ti4+-doped BFO. Secondly, the high leakage current andresistivity in BFO-based ceramics are due to the reduction of Fe3+ toFe2+ ions, resulting in formation of oxygen vacancies [7,28–30]. It ispostulated that the donor dopants could reduce concentration ofoxygen vacancies by compensating for Fe2+, hence reducing number ofcharge carriers [22–24]. Shi et al. reported that the Ti4+ has loweredconcentration of oxygen vacancies in BFO thin films by charge com-pensation which remarkably reduced conductivity [24]. In a study byKalantari et al., the dielectric and impedance spectroscopy data showedthat the Ti4+ doping reduced conductivity in the (Bi0.85Nd0.15)(Fe1-yTiy)O3 system [25]. Qi et al. also reported decreased leakage current inBFO by more than three orders of magnitude with Ti4+ ion doping [23].

In this work, 14% Sm3+-doped BFO is chosen to study effects of theB-site Ti4+ substitution on the microstructure, dielectric, piezoelectric,and ferroelectric properties. Small Ti4+ doping concentrations of 0.5%and 1% were chosen to stabilize the phase coexistence in 14%Sm3+-doped BFO and ensure a homogeneous distribution of Ti4+ ions intoBFO matrix. This study reveals that the Ti4+ ions facilitate a sloweroxygen ion diffusion resulting in reduced oxygen vacancies. It alsoenhanced dielectric homogeneity by effectively reducing the ionicconductivity caused by space charges. The higher d33 values and sym-metric P-E loops of the Ti4+-doped samples indicate enhanced ferro-electric order.

2. Materials and methods

Bi Sm Fe Ti O( )( )x x0.86 0.14 1 3 ceramics with =x 0.0, 0.005, and 0.01(labelled as BFO14Sm, BFO14Sm0.5Ti, and BFO14Sm1Ti, respectively)were fabricated using solid-state reaction method. High-purity (> 99%)powders of Bi O2 3, Sm O2 3, Fe O2 3, and TiO2 were used as starting re-agents. Stoichiometric amounts of powders were mixed thoroughlywith zirconia balls in an agate mortar for 48 h with ethanol as medium.The mixture was calcined at °830 C (3 h) for BFO14Sm andBFO14Sm0.5Ti and °810 C (3 h) for BFO14Sm1Ti with a heating rate of

° min5 C/ . A high-energy planetary ball-mill (Retsch PM100) was usedto reduce particle sizes. The powders were uniaxially pressed (2MPa)and pelletized into ∼1 cm disks and sintered at °870 C (4 h) forBFO14Sm and °880 C (3 h) for BFO14Sm0.5Ti and BFO14Sm1Ti. Theas-sintered samples were polished and then were annealed at °650 C for30min in order to remove the residual stress. In situ XRD analysis wasconducted using Rigaku Multiplex X-Ray Diffractometer with radiationsof K (1.5444 )1 and K 2 (1.5406 ). The Rietveld refinements were

performed using HighScore Plus software to determine structural spacegroups. Temperature-dependent in situ HR-XRD was performed at theNational Synchrotron Radiation Research Center in Taiwan (BL17B1).Morphologies of as-sintered ceramics were obtained using a scanningelectron microscope (HITACHI S-3400N FE-SEM) equipped with energydispersive X-ray (EDS) spectroscopy system. Raman spectroscopy wasconducted using a micro-Raman instrument (Nanobase, XperRam 200)equipped with a green laser of = 532 nm. Wayne-Kerr AnalyzerPMA3260A was used to obtain dielectric permittivity at zero-fieldheating and cooling. Direct piezoelectric coefficients d33 were measuredusing ZJ-6B piezometer at various dc poling E fields. Curves of polar-ization versus E field (P–E hysteresis loop) were studied using Sawyer-Tower circuit at f= 46 Hz.

3. Results and discussion

Fig. 1(a–c) shows the cross-sectional SEM micrographs of all sam-ples. The micrographs reveal dense but inhomogeneous distribution ofgrains. The average grain size (D̄) of BFO14Sm ( 2.13 µm) is sig-nificantly higher than BFO14Sm0.5Ti ( 1.75 µm) and BFO14Sm1Ti( 1.50 µm). It has been argued that Ti4+ plays the role of a donordopant due to its higher valence compared to Fe3+; thus, Ti4+ ionsuppresses the formation of oxygen vacancies leading to a sloweroxygen ion diffusion and consequently slower grain growth rate[31–33]. The EDS spectra of the samples found in Fig. 1(d–e) show thepresence of the individual elements of the basic chemical compositionof each sample. Furthermore, Fig. 2 shows the Raman spectra of TiO2

powder and BFO14Sm100xTi (x = 0, 0.005, 0.01) ceramics taken atroom temperature. The peaks marked by asterisk (*) are attributed tothe characteristic peaks of TiO2. The EDS and Raman spectra reveal thattitanium was successfully doped in BFO14SM0.5Ti and BFO14SM1Ti.

Fig. 3 shows the in situ XRD spectra at room temperature forBFO14Sm1Ti, BFO14Sm0.5Ti, and BFO14Sm. The spectra confirm the

Fig. 1. (a–c) Cross-sectional SEM micrographs of BFO14Sm, BFO14Sm0.5Ti,and BFO14Sm1Ti ceramics at room temperature with the corresponding (d–f)energy dispersive X-ray analysis spectra showing the constituent elementspresent.

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perovskite structure with minor secondary phases (Bi25FeO40 andBi2Fe4O9) as indicated by the pound sign (#) [18]. These secondaryphases are likely due to phase decompositions at high-temperaturesintering process [34]. There is no obvious 2 shift position of reflectionpeaks due to similar atomic radii of Ti4+ (0.605 ) and Fe3+ (0.64 ) atthe B-site [10]. All three samples exhibit similar peak splitting, in-dicating R3c rhombohedral and Pbam orthorhombic phase coexistenceas evidenced by visible splitting of the peaks at (110), (111), and (200)positions [7,34–37]. The Pbam orthorhombic phase is shown by anasterisk (*). The phase coexistence is clearly visible from the short anglescans of (111) and (200) peaks obtained from synchrotron HR-XRD asshown in the insets of Fig. 3(a–c). These phases are indicated by the letters “R” (rhombohedral R3c) and “O” (orthorhombic Pbam). This is

further supported by the Rietveld refinements shown in Fig. 4 con-firming that Ti4+-doping stabilized the phase coexistence in 14%Sm3+-doped BFO. Table 1 shows the calculated space groups and their phaseratios for each composition. Rietveld refinements suggest that therhombohedral R3c phase is most dominant at 1% Ti4+ dopant con-centration, indicating that Ti4+ doping induces a more polar order.Although a phase coexistence of R3c and Pnma has been predictedtheoretically at a critical dopant concentration (14% for samarium) [7],the present results are consistent with recent reports by Chen et al. andKhomchenko et al. which confirmed the coexistence of polar R3c andantipolar PbZrO3-like orthorhombic Pbam structures in BFO14Sm byTEM diffraction and Rietveld refinements [19,38].

Fig. 5 shows temperature-dependent synchrotron XRD spectra of(111) and (200) peaks for the samples. The spectra reveal that theroom-temperature phase coexistence is stable until °250 C in all three

Fig. 2. Raman spectra of TiO2 powder and BFO14Sm100xTi (x = 0, 0.005,0.01) ceramics taken at room temperature. Asterisk (*) denote characteristicpeaks of TiO2.

Fig. 3. In situ XRD diffraction patterns of BFO14Sm, BFO14Sm0.5Ti, andBFO14Sm1Ti ceramics at room temperature. Insets are the short angle (111)and (200) diffractions patterns obtained from high-resolution synchrotron XRDat room temperature.

Fig. 4. Experimental XRD patterns (Iobs), Rietveld refinements (Icalc), and de-viations (Iobs-Icalc) are represented by black lines, red “×” symbols, and greenlines, respectively. Rwp, Rexp, and 2 denote weighted R profile, statisticallyexpected R value, and goodness-of-fit, respectively. The peak ticks of Rietveldrefinements for various space groups are given under each sample.

Table 1Space groups and their phase ratios in BFO14Sm, BFO14Sm0.5Ti, andBFO14Sm1Ti ceramics determined by the Rietveld refinements.

Space groups Phase Ratio (%)

BFO14Sm BFO14Sm0.5Ti BFO14Sm1Ti

Rhombohedral R3c 11.8 14.4 74.7PbZrO3-like orthorhombic Pbam 85.6 82.2 23.7Orthorhombic Pbam (Bi2Fe4O9) 0 0.8 0Cubic I23 (Bi25FeO40) 2.4 2.5 1.6SmFeO3-like orthorhombic Pnma 0.2 0.1 0

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compositions. Above °250 C, three peaks (R and O phases) at the (111)diffraction merged into 2 peaks (R phase) and four peaks (R and Ophases) at the (200) diffraction converged into a single peak (R phase).This indicates a possible phase transition from two-phase coexistence toa single R3m rhombohedral structure around °300 C as evidenced byJeong et al. [39]. This suggests that Ti4+ doping does not affect thephase stability in BFO14Sm.

Fig. 6(a–c) shows the frequency dependence of the dielectric per-mittivity ( ) at room temperature measured in the frequency range of500 Hz–1MHz. At lower frequencies, BFO14Sm shows a large fre-quency-dependent dispersion of the dielectric permittivity compared tothe Ti4+-doped samples and attains a nearly constant value at higherfrequencies, in agreement with the report of Gu et al. [40]. This be-havior is typically attributed to space charge contributions [41]. It wassuggested that in polycrystalline ceramics, the displacement of chargecarriers takes place when the field is applied and they align at grainboundaries if resistance of grain boundaries is large. At grain bound-aries, space charge polarization governed by free charges is built up,resulting to the large value of dielectric permittivity [42]. However, thismechanism only takes place at low frequencies due to the large inertial

mass of space charges. At higher fields, the space charges no longerrespond to the field and have no contribution to the polarization.Therefore, the frequency dispersion of dielectric permittivity inBFO14Sm is caused by space charges lagging behind the applied field athigher frequencies, hence contributing less to the polarization [43]. Incontrast, Ti4+-doped samples only show a minimal frequency disper-sion behavior, indicating that the reduction of oxygen vacancies evi-denced by the decreased grain sizes in the Ti4+-doped samples resultsin the suppression of space charge polarization [36]. The minimal de-crease in dielectric permittivity with increasing frequency was alsoobserved by Shi et al. for (Sm/Ti) co-doped BFO ceramics [44].

The right insets in Fig. 6(a–c) show the temperature dependence ofdielectric permittivity ( ) and dielectric loss (tan ) of Ti4+-doped andundoped samples. The room-temperature dielectric permittivities at1MHz for BFO14Sm, BFO14Sm0.5Ti, and BFO14Sm1Ti are 143, 108,and 131, respectively. The enhanced room-temperature dielectric per-mittivity in BFO14Sm1Ti as compared with BFO14Sm0.5Ti is attributedto the more dominant polar rhombohedral structure in BFO14Sm1Ti asrevealed by the phase ratios in Table 1 determined through the Rietveldrefinements. As the temperature increases, the dielectric permittivityincreases sharply in BFO14Sm, exhibiting sharp frequency-dependentpeaks around °200 C , which is likely due to thermally inducedhopping conduction and space charge polarization [42,43]. A similarbehavior was also observed for other BFO-based materials reported inliteratures [42,45]. In contrast, the Ti4+-doped samples show fre-quency-independent broad peaks, which suggests that Ti4+ doping canenhance dielectric homogeneity [46].

The room-temperature dielectric loss at 1MHz decreases with Ti4+

Fig. 5. Temperature-dependent high-resolution synchrotron (111) and (200)diffraction patterns of BFO14Sm, BFO14Sm0.5Ti, and BFO14Sm1Ti ceramics.Peak labelled with + is attributed to minor secondary phase.

Fig. 6. (a–c). Frequency-dependent dielectric permittivity at room temperatureof BFO14Sm, BFO14Sm0.5Ti, and BFO14Sm1Ti. Right insets are the tempera-ture-dependent dielectric permittivity and dielectric loss at 500 kHz and 1MHz.Left insets are the thermal hysteresis taken at 1MHz.

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doping from ∼0.5360 in BFO14Sm to ∼0.0140 and ∼0.0170 inBFO14Sm0.5Ti and BFO14Sm1Ti, respectively. This suggests that Ti4+

doping has effectively reduced leakage current and electrical con-ductivity, in agreement with the reports of Wang et al. and Deng et al.[33,35]. At higher temperatures, the thermally activated process asso-ciated with oxygen vacancies results in the increase in conductivity andthus leads to an exponential increase of dielectric loss [47]. The strongthermal dielectric hystereses at 1MHz shown in the left insets ofFig. 6(a–c) indicate a first-order transition in these compositions.

Fig. 7 shows piezoelectric coefficient (d )33 for Ti4+-doped and un-doped samples measured at room temperature for different polingelectric fields. The maximal piezoelectric coefficients (d33) obtained at100 kV/cm are 43, 51, and 53 pC/N for BFO14Sm, BFO14Sm0.5Ti, andBFO14Sm1Ti, respectively. These values are comparably higher thanthose reported by Chen et al. for Sm-doped BFO ceramics [9] and Shiet al. for (Sm/Ti)-codoped BFO ceramics [44]. The enhanced piezo-electric coefficients can be correlated to reduced grain size in the Ti4+-doped samples and enhanced ferroelectric order as observed from theSEM micrographs and Rietveld refinements, respectively. It has beenreported that the increase in piezoelectric properties with decreasinggrain size is influenced by domain-wall contributions. The domainwidth decreases with decreasing grain size, resulting in increased do-main wall density and activity, and thus contributing to enhancedpiezoelectric properties [48,49].

The effects of Ti4+ doping in ferroelectric polarization have beenstudied from polarization–electric field (P–E) loops at various polingelectric fields as shown in Fig. 8. Due to the higher dielectric loss,BFO14Sm can only withstand the applied field up to 50 kV/cm, whereasTi4+-doped samples can withstand applied field as high as 80 kV/cm.The BFO14Sm exhibits asymmetric and rounded hysteresis loops whichare characteristics of materials with higher leakage current and electricalconductivity [50,51], as reported in Sm-doped BFO [40] and Ba-dopedBFO [33]. The gap between the initial and final values of the polarizationat negative fields can be attributed to an asymmetric space-charge dis-tribution between the top and bottom electrodes. Positively-chargedoxygen vacancies tend to be more concentrated near the bottom surfacewhich generate high leakage current contributing to more polarizationloss during the negative cycle of the applied field. Similar behavior wasalso observed by Lee et al. in Na0.5K0.5NbO3 thin films attributing to anasymmetric oxygen vacancy and Na/K vacancy charge distribution be-tween bottom and top electrodes, respectively [52]. On the other hand,Ti4+-doped samples show the tendency of a typical hysteresis loop withan ellipse-like feature which is consistent with those reported in the lit-erature [31,33,53–55], suggesting enhanced ferroelectric nature incomparison with BFO14Sm. The improvement in the shape of the

ferroelectric hysteresis loops observed with the addition of Ti4+ can beattributed to the decrease of oxygen related defects brought about byTi4+ doping, which reduces the leakage current and thus improves thepolarization [56]. Shi et al. also reported enhanced ferroelectricity inBi0.86Sm0.14FeO3 ceramics with 1% Ti-doping although they argued thatsuch enhancement cannot be attributed to the generally believed me-chanism of suppression of leakage current [24]; further investigation isworth conducting on this issue. It should be noted that all three sampleswere not able to reach a saturated polarization before the leakage cur-rents became significant. Similar unsaturated and lossy loop behaviorwas observed for (Eu/Sr)-doped BFO even under large electric field [57].It has been reported that hysteresis loop in BFO-based polycrystallineceramics does not attain saturation at low poling fields due to its highcoercive field (150 kV/cm) [44,58–60]. The problem on BFO's highcoercive field and low resistance makes it very difficult to achieve asaturated hysteresis loop [29,61–63].

Fig. 7. Piezoelectric coefficient (d33) at different poling electric fields ofBFO14Sm, BFO14Sm0.5Ti, and BFO14Sm1Ti ceramics.

Fig. 8. Room-temperature polarization hysteresis (P–E) loops of BFO14Sm,BFO14Sm0.5Ti, and BFO14Sm1Ti ceramics at various poling electric fields.

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4. Conclusions

SEM micrographs reveal reduced grain sizes in (Sm, Ti) co-dopedbismuth ferrite ceramics due to slower oxygen ion diffusion and loweroxygen vacancies by the B-site Ti4+ substitution. EDS and Ramanspectra confirm the presence of titanium in the doped ceramic samples.In situ XRD spectra and Rietveld refinements confirm a phase coex-istence of rhombohedral R3c and orthorhombic Pbam phases. Rietveldrefinements further reveal that Ti4+-doping stabilizes the phase coex-istence and induces a more polar structure. High-resolution synchrotronXRD reveals that the phase coexistence was stabilized up to °250 C forall three samples. The Ti4+-doped samples have enhanced dielectrichomogeneity and reduced dielectric loss. This suggests that Ti4+ dopinghas effectively reduced electric conductivity caused by space charges.The enhanced d33 values and symmetric P–E hysteresis loops in theTi4+-doped samples indicate enhanced ferroelectric order. This worksuggests that the Ti4+ ions could effectively enhance dielectric andferroelectric properties in multiferroics which are important from theapplication perspective.

Declarations of interest

None.

Acknowledgment

This work is supported by the Research Enrichment Program Grantunder the Accelerated Science and Technology Human ResourceDevelopment Program of the Department of Science and TechnologyPhilippines. The authors express their sincere thanks to the MultiferroicResearch Laboratory at Fu Jen Catholic University (Taiwan), theMaterials Research Laboratory at Ming Chi University of Technology(Taiwan), and the National Synchrotron Radiation Research Center(Taiwan).

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