Journal of Sol-Gel Science and Technology (2018) 88:584–592https://doi.org/10.1007/s10971-018-4835-3

ORIGINAL PAPER: SOL-GEL AND HYBRID MATERIALS FOR DIELECTRIC,ELECTRONIC, MAGNETIC AND FERROELECTRIC

Investigations on the phase transition of Mn-doped BaTiO3multifunctional ferroelectric ceramics through Raman, dielectric,and magnetic studies

K. Madhan1● R. Thiyagarajan2 ● C. Jagadeeshwaran1

● A. Paul Blessington Selvadurai3 ● V. Pazhanivelu1●

K. Aravinth4● Wenge Yang2,5

● R. Murugaraj 1

Received: 24 July 2018 / Accepted: 25 September 2018 / Published online: 4 October 2018© Springer Science+Business Media, LLC, part of Springer Nature 2018

AbstractBaTiO3 (BTO) and BaTi1−xMnxO3 (x= 0.25, 0.50, 0.75 mol%) ceramic materials have been prepared by the sol–gelcombustion method. X-ray diffraction (XRD) has been carried out to characterize the phase purity and crystal structure of theprepared compounds, and all XRD patterns suggest tetragonal structure with the phase group of P4mm. The variation in theestimated lattice parameters confirms the incorporation of Mn atoms at Ti site of BTO. Raman spectroscopy studies undervarious temperatures suggest a phase transition from tetragonal to cubic phase at ~433 K, identified by a distinct Ramanmode at 308 cm−1. As Raman modes are getting softened by Mn doping, phase transition temperature of the Mn-dopedcompounds is significantly decreased from 473 K (x= 0%) to 433 K (x= 0.75%). Dielectric properties such as permittivityand dielectric loss as the function of frequency under various temperatures have two distinct dielectric anomalies (i) at 393 Kassociated to tetragonal to cubic phase transition and (ii) at 550 K due to oxygen vacancy defect in the samples. Observationof weak ferromagnetism at 2, 300, and 400 K in the M (H) and ZFC-FC curve, suppose its origin due to an intriguingexchange interaction between Mn and oxygen vacancies.

Graphical Abstract

Highlights● Mn-doped BaTiO3 is prepared by sol–gel combustion method.● The structural phase transition from tetragonal to cubic phase occurs at ~433 K.

* R. Murugarajr.murugaraj@gmail.com

1 Department of Physics, MIT Campus, Anna University,Chennai 600044, India

2 Center for High Pressure Science and Technology AdvancedResearch (HPSTAR), 201203 Shanghai, People’s Republic ofChina

3 Department of Nuclear Physics, University of Madras,Chennai 600025, India

4 SSN Research Center, SSN College of Engineering,Kalavakkam 603110, India

5 Center for the Study of Matter at Extreme Conditions, FloridaInternational University, Miami, FL 33199, USA

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● Phase transition associated with a distinct Raman mode at ~308 cm−1.● M–H loop shows weak ferromagnetism with intriguing exchange interactions.

Keywords Sol–gel combustion ● Phase transition ● Raman and dielectric spectroscopy ● Ferromagnetism

1 Introduction

Ferroelectric materials in nanoceramic phase are becominga recent interest of research due to its complex physicalproperties and their applications in device fabrication suchas in nonlinear optics, multilayer ceramic capacitors,actuators, transducers, ferroelectric random access mem-ories, micro-electro-mechanical systems, positive tempera-ture co-efficient thermostats, microwave devices,spintronics, and infrared sensors etc [1, 2]. In particular,BaTiO3 (BTO) belongs to ABO3 perovskite-type ferro-electric material (A= Ba and B= Ti) with 12- and 6-foldcoordination of the oxygen in its structural framework [3].While increasing the temperature, it exhibits various struc-tures such as rhombohedral, orthorhombic, tetragonal,cubic, and hexagonal phases till 183, 273, 300, 403, and1773 K, respectively [4]. The phase transition temperatureand intricate electrical properties of BTO can be efficientlycontrolled by doping at Ba and/or Ti sites. In addition, wefound explicitly in the literature that the doping with tran-sition metal cations such as Mn, Cr, Fe, Co, Ni, and Cu at Tisite of BTO have shown significant enhancement in elec-trical and magnetic properties [5]. In particular, Mn dopingat Ti site has been extensively studied and reported byvarious groups [4, 6, 7] and suggested that doping of Mn inBTO acts as an acceptor type, and thereby enhances elec-trical resistivity near the Curie temperature (TC). Further, itfound that Mn doping in BTO induces ferromagnetic (FM)nature and escalates the grain size too. Also, Mn doping inBTO enriches its applicability towards the multifunctionalmaterials, and investigations are still going on.

Till now from most of the reports, Mn-doped BTOsamples were prepared with methods like solid-state reac-tion, hydrothermal synthesis, and thin film depositiontechnology, and focused only on the microstructural anddielectric properties [8–16]. On the other hand, there areonly a few reports on chemically prepared nanoceramics ofMn-doped BTO with the detailed study on structural andmagnetic phase transitions [5, 17]. Among various chemicalmethods, sol–gel combustion technique offers a goodcomposition, homogeneity, purity, and crystallite size of thenanoceramics at a microscopic level and moderate by thesintering temperature [18]. In addition, an inducement ofdefects in the sol–gel method is still a quest to study.Thereby, magnetic characteristics and structural phasetransition of Mn-doped BTO samples prepared by sol–gelcombustion method will be interesting. Herein the present

paper, we have investigated the phase transition of Mn-doped BTO through Raman, dielectric, and magnetic stu-dies under various temperatures, and origin of ferro-magnetism and its enhancement due to Mn doping in BTOare analyzed based on the role of defects.

2 Experimental details

BaTi1−xMnxO3 (x= 0.0, 0.25, 0.50, 0.75 mol%) samples aresynthesized by sol–gel combustion technique. The pre-cursors such as titanium isopropoxide, manganese acetate,barium nitrate, acetic acid, ethanol, polyethylene glycol(PEG), and citric acid are used without further purification,and ammonia solution is taken in corresponding stoichio-metric. In the present work, PEG: citrate ratio are main-tained at 1:2 and pH of the admixture solution were kept at~9. The solution is stirred, dried and burnt into the carbo-naceous product in open atmosphere. This obtained productis calcined and heat treated at 900 °C for 3 h. Finally, thecalcined products are pelletized and sintered at 1000˚C for5 h.

Powder X-ray diffraction (XRD) patterns are measuredby Bruker-D2 phaser diffractometer with Cu-Kα (λ=1.5418 Å) radiation to determine the crystal structure of theprepared samples. Raman spectra are recorded using Ramanspectrometer (RenishawInc.) with excitation laser wave-length 532 nm. For dielectric measurement, the pellet withdimensions of 10 mm (dia) and 1 mm (thickness) are sin-tered and coated with silver paste as contact electrodes onboth sides. The frequency and temperature dependentdielectric measurements are carried out on the sinteredpellets by employing PSM1735NumetriQ, UK. Tempera-ture- and field-dependent magnetization measurements areexecuted using physical property measurement system(PPMS, Quantum design, USA).

3 Results and discussions

3.1 Structural analysis

Figure 1 shows the X-ray diffraction patterns of BaTi1−xMnxO3

(x= 0, 0.25, 0.50, 0.75 in mol%), and all the patterns reveal awell defined tetragonal structure with P4mm space group,which is confirmed via fittings using XRD-A software [19]. Byfitting the XRD patterns, we observe that dopings of Mn up to

Journal of Sol-Gel Science and Technology (2018) 88:584–592 585

0.75mol% do not instigate any impurity phases and structuralchanges in the prepared samples. The lattice parameters andunit cell volume of all the prepared samples are obtained byrefinement, and the results are reported in Table 1. The latticeparameters a, c, and c/a ratio are increased with Mn doping inBTO, which is ascribed to the ionic size difference of (Mn2+(rion= 0.83Å))/(Mn3+(rion= 0.645Å)) and Ti4+ (rion=0.605Å) ions [9]. Also, the linear increase of unit cell volumeis observed from Table 1 and a corresponding shift in the XRDpattern of (110) plane as shown in Fig. 1b. Explicates theVegard’s law trend [20], which implying the doping of Mn2+/Mn3+ cations in Ti site. The crystallite size of the preparedsamples are calculated using Scherrer formula

D ¼ kλ

βcosθð1Þ

Where D is the crystallite size, k is shape factor (0.94), λ isthe wavelength of Cu-Kα line (1.5418 Å), β is full width athalf maximum in radians, and θ is peak position in degree.The average crystallites sizes are found to be 34, 43, 46, and49 nm for pristine and Mn-doped BTO samples respec-tively; thereby, the increasing trend confirms incorporationof Mn at Ti site of BTO by doping.

3.2 Raman spectroscopy analysis

Raman spectroscopy is a powerful technique for investi-gating subtle phases and their transitions either shift or splitin the bands and/or by softening of modes [1, 21]. In thepresent work, temperature-dependent Raman spectra wererecorded on the prepared samples to identity the phasetransition behavior. Figure 2a–d demonstrates the Ramanspectra and its phase transition behavior of BaTi1−xMnxO3

(x= 0.0, 0.25, 0.50, and 0.75%) samples, investigatedbetween 298 and 673 K at the steps of 10 K in the wave-number range 100 to 1000 cm−1.

In case of tetragonal BTO system, the Raman modes forP4mm space group is expected to show 10 Raman-activemodes. Whereas the for cubic Pm-3m space group, thephonon modes are inactive [22]. Further, in BTO tetragonal,the splitting of optical phonon modes into transverse andlongitudinal and extra splitting owing to polarizability in theunit cell cause 18 Raman-active phonon modes. At 298 K,Raman spectra of pristine and Mn-doped BTO exhibit peaksat 181 and 248 cm−1 that are assigned as (E(2TO)+ E(1LO)+A1(1TO)+A1(1LO)) and (A1(2TO)) modes,respectively. The peak at 308 cm−1 is assigned to (E(3TO)

Fig. 1 a Room temperature XRD patterns of BaTi1−xMnxO3 compounds, b shift of (110) peak with increases in Mn concentrations at roomtemperature

Table1 Calculated X-raypowder diffraction structuralparameters on tetragonalcomposition of pristine and Mn-doped BaTiO3 compounds

Sample BaTiO3 BaMn0.0025Ti0.9975O3 BaMn0.005Ti0.995O3 BaMn0.0075Ti0.9925O3

Crystallite size(nm) 34 43 46 49

Lattice strain (10−3) 3.52 2.76 2.60 2.48

X-ray density (gm/cm3) 7.804 5.917 5.662 5.604

Lattice parameters

a (Å) 4.0036 3.9982 4.0035 4.0055

c (Å) 4.0199 4.0169 4.0222 4.0247

c/a 1.0040 1.0046 1.0046 1.0047

Cell volume (Å3) 64.435 64.214 64.471 64.576

586 Journal of Sol-Gel Science and Technology (2018) 88:584–592

+ E(2LO)+ B1). A peak at 512 cm−1 assigned as (E(4TO)+A1(3TO)), and at 717 cm−1 as (E(4LO)+A1(3LO)) thatare the predominate Raman modes occur exclusively for thetetragonal phase of BTO [23, 24]. In addition, the 308 and717 cm−1 peaks indicate the tetragonality of BTO, whichare for identifying the phase transition in pristine and Mn-doped BTO compounds.

On increasing the temperature from 298 to 413 K,Raman spectra reveal that the intensities of the sharp modeat 308 cm−1 and the over-damped soft mode at 717 cm−1

are reduced continuously. Further increasing of temperaturefrom 433 to 673 K, those peaks disappear rapidly, byindicating cubic phase. In regard to Mn doping, those peakintensities diminish and get obliterated in a decreasing spanof increasing temperature, which emphasizes the softeningof phonon modes. The observed broad peak nature at 512cm−1 is ascribed to the disorder of Ti cations on Ti/B sitesin the ferroelectric phase, and the disorder in the paraelectricphase is associated with the position of the Ti ions. As Ti

ions in paraelectric phase are expected to occupy bodycenter position for an ideal cubic phase. However, Ti ionsare randomly displaced along the cube diagonals causingdisorder in the lattice structure. Hence, the transitions are ofdislocate type, i.e., an order–disorder phase transition [21,25]. The existence of polar phase at higher temperature is anindicative of a partial presence of tetragonal phase. Thereason for its existence is the presence of polar phase at thegrain boundary and grain growth during heating and aninternal strain in the samples [26–28]. The contrast peakbroadening of 512 cm−1 also evident that the peak under-goes a significant change during the phase transition.

3.3 Electrical analysis

Figure 3a–d shows the frequency-dependent dielectricconstant of BaTi1−xMnxO3 (x= 0.00, 0.25, 0.50, 0.75%)measured at different temperatures from 313 to 673 K overthe frequency range of 1 Hz–1MHz. The real part of

Fig. 2 Raman spectra of a BaTiO3, b BaMn0.0025Ti0.9975O3, c BaMn0.005Ti0.995O3, d BaMn0.0075Ti0.9925O3 compounds at various temperaturespoints

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dielectric constant is calculated by

ε′ ¼ Cpd

ε0Að2Þ

where d is the thickness of pellet sample, Cp is thecapacitance, ε0 is the permittivity of free space, and A is thecross-sectional area of the pellet [29]. All the samplesexhibit usual dielectric dispersion behavior with increasingof frequency. The in-phase and phase lag with a gradualincrease of frequency depicts the relaxation behavior ofelectrical species present in the grain boundaries and as wellas in the grains [30]. In the high-frequency region, the phaselag is attributed to the refused corporation of some softphonon modes of Ti–O6 distortion in pristine and dopedBTO system [31].

A broad dielectric peak in close proximity to ~393K,between 1 Hz and 1 KHz overshadows the dielectric anomaly,which indicates a diffused ferroelectric and paraelectric phasetransition [29, 32]. A lucid phase transition is observable ataround 1MHz as shown in Fig. 4 for BaTi1−xMnxO3 (x=0.00, 0.25, 0.50, 0.75mol%) samples. The doping of Mn

Fig. 3 Frequency dependence of dielectric constant (ε′) (main plots) and loss tangent (δ) (inset plot) of the a BaTiO3, b BaMn0.0025Ti0.9975O3, cBaMn0.005Ti0.995O3, d BaMn0.0075Ti0.9925O3 compounds at various temperatures points

Fig. 4 Temperature dependence of dielectric constant (ε′) and losstangent (tan δ) (inset) at a particular frequency of 1MHz of BaTi1−xMnxO3 the compounds

588 Journal of Sol-Gel Science and Technology (2018) 88:584–592

decreases the ferro-paraelectric transition temperature. Since,the doping or the substitution of dn cations relaxes the pseudoJahn–Teller distortion of TiO6 complex, which in turn reducesthe phase transition temperature [33, 34]. This observation isin compliance with the disappearance of Raman mode at 308cm−1 [E (TO)] of BaTi1−xMnxO3 samples. In addition to theabove, the doping of Mn increases the magnitude of thedielectric constant up to x= 0.75%. This pitch in the dielectricconstant is probably due to the presence of defect dipoles inthe lattice, theoretically supported by the following Kronger–Vink relations [29].

2MnO ! 2Mn′′Tiþ2OOþ2V��O ð3Þ

Mn2O3 ! 2Mn′Tiþ3OOþV��O ð4Þ

MnþTie� ! Mn′′Ti Mn3þ ! Mn2þ

� � ð5Þ

Which are expected due to the induce domain stability inthe ferroelectric ceramics, by the observation in lowering of

tan δ value for x= 0.25% and x= 0.50%. This wasobserved between 312 and 413 K, which persists in a stableferroelectric domain [14]. A trivial increase of tan δ valuefor x= 0.75% than pristine and other concentrations,exposits the leakage behavior impending due to the multi-valence state of defects viz., Mn2+/Mn3+ and Ti3+/Ti4+.Further, the other dielectric anomaly which is observed near529 K is akin to a thermally activated localized charge byoxygen vacancies as a report for BTO [35]. Such a signatureis also relevant in tan δ plot shown in Fig. 4 as an inset.

3.4 Magnetic analysis

Figure 5a–c shows the field dependence of magnetization[M(H)] of all samples measured up to 2 T at 2, 300, and400 K, respectively. From Fig. 5, one can see an interestingferromagnetic behavior, with an unsaturated hysteresiscurve for the applied magnetic field of 2 T. It is observedthat for BaTi1−xMnxO3 samples, an intriguing magnetizationpresent which ascends from the vacancy-induced ferro-magnetism, vacancy mediated and clustering of Mn in the

Fig. 5 Magnetic-field dependence of magnetization of BaTi1−xMnxO3 compounds at a 2 K, b 300 K, c 400 K

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doped BTO system, which is probed through electronparamagnetic resonances (EPR) analysis. However, theobserved magnetization in the pristine sample is due to theTi3+ existence as a result of oxygen vacancy. Further the M(H) curve is lucid in exhibiting wasp waist hysteresis looptype, which may be the result of different magnetic aniso-tropy in the domains.

With the increasing of Mn2+ ions in BTO system, weobserved that the magnetization varies non-monotonicallyas shown in Fig. 5. In particular, for x= 0.25% doping ofMn in BTO, the magnetization increases remarkably incomparison with other concentrations. The enhancement inthe magnetization for x= 0.25% confer on with theexchange interactions between the defect-induced carriersand Mn2+/Mn3+ magnetic ions. Further, the decrease in theintensity for x= 0.25% with respect to pristine, and thebroad resonances in the spectra instigate the existence ofMn2+/Mn3+ ions in the BTO lattice. On the other hand, forx= 0.50% doping, the M–H curve exhibits a non-magneticphase implying a diamagnetic-type behavior at high field,along with the ferromagnetic phases. Similar behavior isalso reported even for pristine compounds like ZnO, SnO2,TiO2, and BTO3 [36–39]. Such a behavior observed in adoping case, incurs a change in the defect-induced ordefect-mediated ferromagnetism, ascribed to the restraint ofdefect-induced carriers, which is implicitly through EPRstudies.

For x= 0.75%, the M–H curve is a further constraint inits magnetization nature, as a result of MnOx segregation,which is likely possible to occur on higher concentrationdoping. Hence, the M (H) curve resembles to appear as anantiferromagnetic behavior [40]. In all the case of Mndoping, a significant enhancement in the magnetization asnoted at 300 K, which ensuring a decline in the thermalenergy. However, at 2 K, the observed M (H) loops signifya quasi-paramagnetic ordering, encompassing both para andweak ferromagnetic behavior in the samples.

A keen look at the M (H) loops of BaTi1−xMnxO3 revealsan asymmetric hysteresis loop (a small shift in the loop),corresponds to an exchange-bias (EB) signature. Theexchange-bias field (HEB) and the coercive field (HC)were calculated using |HEB|= | (H+ +H−)/2| and |HC|= |(H

+−H−)/2|, where H+ and H− are the positive and negativecoercive field on the magnetic axis [41, 42]. The calculatedvalues are reported in Table 2. The presence of exchangebias implies the existence of surface imperfections andsurface strain anisotropies, which induces distortion on thelarger particles and site in-equivalence on the crystal-lographic structure [43].

Figure 6 shows the magnetization as a function of tem-perature [M (T)] measured with an applied field of 100 Oe,and we observed a notable splitting between Zero FieldCooled (ZFC) and Field Cooled (FC) at below 200 K, Ta

ble2Calculatedparameter

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temperature

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hysteresisloop

ontetragon

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pristin

eandMn-do

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unds

Mn(%

)2K

300K

400K

Ms(emu/g)

Mr(emu/g)

Hc(O

e)EB

(Oe)

Ms(emu/g)

Mr(emu/g)

Hc(O

e)EB

(Oe)

Ms(emu/g)

Mr(emu/g)

Hc(O

e)EB(O

e)

X=0.00

6.74

47×10

−4

1.99

41×10

−5

269.26

7.67

4.29

68×10

−5

1.18

02×10

−5

825.41

76.15

3.17

82×10

−5

1.02

63×10

−5

789.59

17.38

X=0.25

6.03

99×10

−3

6.09

09×10

−5

118.81

4.02

2.34

49×10

−4

4.66

88×10

−5

1621

.90

28.33

1.85

24×10

−4

3.65

52×10

−5

1713

.09

29.31

X=0.50

2.91

01×10

−3

1.14

25×10

−5

55.78

5.25

1.24

11×10

−5

7.47

34×10

−6

1455

.72

42.43

1.23

75×10

−5

5.85

94×10

−6

2206

.71

133.74

X=0.75

6.83

00×10

−3

7.43

83×10

−6

12.95

2.53

1.98

66×10

−4

6.69

54×10

−6

562.98

41.13

1.65

67×10

−4

5.80

34×10

−6

535.44

24.59

590 Journal of Sol-Gel Science and Technology (2018) 88:584–592

which indicates an irreversibility behavior for the pristineand 0.50% Mn-doped samples. This can be ascribed due tothe competing interactions of FM and AFM phases presentin the sample. An overlapping of FC-ZFC at and above 200K reveals that the thermal energy is likely to surpass theirinteractions. But, the ferromagnetic existence in M (H) at300 K for pristine and 0.5% indicate that their interactionsare not completely worn-out. For x= 0.25% doping inBTO, a small curvature is noted both in the ZFC-FC curvein the temperature region between 2 and 400 K, revealing aprominent ferromagnetic signature exhibit by the samples.However, for x= 0.75% Mn doping in BTO, a linear andoverlapped M (T) emphasizes a dominated AFM signaturein the sample. Finally, in all these samples, a quasi-paramagnetic behavior is observed below at 15 K, which issupposed to arise extrinsically from oxygen or due to thesurface defect of the samples [44].

4 Conclusions

BaTi1−xMnxO3 (with x= 0.0, 0.25, 0.50, 0.75%) (BTMO)ceramics were prepared by the sol–gel combustion method.The phase purity and the formation of a tetragonal structurefor all BTMO samples at room temperature are confirmedby XRD analysis. Doping of Mn concentration in BTOincreases the crystallite size, in turn, reduces the internalstrain in the prepared samples. Raman spectral analysis alsoconfirms the tetragonal phase for all BTMO samples. In situtemperature dependent Raman spectra analysis and dielec-tric spectroscopic studies on BTMO indicates a usualstructural transition from tetragonal to cubic phase. Thedoping of Mn reveals a significant decrement in the phasetransition temperature and softening of phonon modes fromthe Raman spectra. Dielectric spectra have shown adielectric anomaly at temperatures 393 and 553 K, which is

attributed to phase transition and defects due to the oxygenvacancies. The temperature dependent dielectric constant (ε′) and loss tangent (tan δ) with varying Mn content wereinvestigated. The enhanced dielectric property is observedfor x= 0.50% and its tan δ found to be significantly lessthan the other Mn concentration, which makes this com-position is a very good candidate for tunable capacitorsapplications. Magnetic studies reveals an enhancement inmagnetic signature for x= 0.25% mol of Mn in BTO, wherethis material essentially imparts a new functional perspec-tives for the development of the spintronic applications.

Compliance with ethical standards

Conflict of Interest The authors declare that they have no conflict ofinterest.

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