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Solid State Sciences 13 (2011) 1869e1873

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Solid State Sciences

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Study of dielectric properties of single phase Bi1�xCaxFeO3 (x¼ 0.1, 0.3, 0.5)

Jaiparkash a,*, Yogesh Kumar b, R.S. Chauhan a, Ravi Kumar b,c

aDepartment of Physics, RBS College, Agra 282 002, IndiabMaterials Science Division, Inter University Accelerator Center, New Delhi 110 067, IndiacCentre for Materials Science and Engineering, National Institute of Technology, Hamirpur 177 005, India

a r t i c l e i n f o

Article history:Received 9 February 2011Received in revised form20 June 2011Accepted 21 July 2011Available online 30 July 2011

Keywords:MultiferroicsCanted antiferromagnetismDielectric modulusActivation energy

* Corresponding author. Tel.: þ91 9711315173.E-mail address: jaiprakash.manu@gmail.com ( Jaip

1293-2558/$ e see front matter � 2011 Elsevier Masdoi:10.1016/j.solidstatesciences.2011.07.021

a b s t r a c t

A series of Bi1�xCaxFeO3 (x¼ 0.1, 0.3, 0.5) multiferroic samples have been prepared in order to study theeffect of different concentrations of Ca on the crystal structure, and dielectric properties. Structuralanalysis has been performed using X-ray diffraction (XRD) measurements. Rietveld refined XRD dataconfirm that all the samples are of single phase, having hexagonal structure with R3c space group.Cell parameters decrease systematically with increase in Ca concentration. Dielectric measurementshave been performed in the temperature range of 200e450 K at selected frequencies in the range 100e1100 kHz. A near room temperature ferroelectric anomaly has been observed in all the samples whichshift toward lower temperature with increase in doping. Conduction activation energies are calculatedand found to decrease with increasing doping.

� 2011 Elsevier Masson SAS. All rights reserved.

1. Introduction

In recent years huge attention is paid to the search for multi-ferroic materials, which offers the coexistence of various ferroicorders [1]. Concurrence of different ferroic properties allows one touse these materials for novel device concepts that would not beattainable by either ferroelectric or magnetic materials only. Thereare very few materials which offer multiferroic (MF) nature. Theperovskite type transition metal oxides having general formulaABO3 are promising candidates for MF applications because theypossess a wide range of electric and magnetic properties. However,most of these materials have low magnetic ordering temperatures[2e7], which limit their application. But, high magnetic orderingtemperature (TNw 643 K) and ferroelectric transition temperature(Tcw 1103 K) of BiFeO3 (BFO)-based multiferroics encourage one tostudy these materials. BFO shows a rhombohedrally distortedperovskite crystal structure with space group R3c and G-typeantiferromagnetism at room temperature [8]. In this system,presence of 6s2 lone pair of electrons in Bi cation may hybridizewith empty p orbital to form a localized lobe causing a structuraldistortion i.e. off-center distortion and hence ferroelectricity [1],while Fe ion is responsible for magnetic properties. But antiferro-magnetic nature of BFO is a big obstacle in its practical applications[9e11]. Never the less partial substitution at A-site (Bi3þ) with La3þ,

arkash).

son SAS. All rights reserved.

Nd3þ, Sm3þ, Ba2þ etc. and/or at B-site with Mn4þ is reported toinduce ferromagnetism in this system [12e16]. Khomchenko et al.[15,16] have recently attempted the heterovalent doping bya particular composition (say 20% and 30%) of Ca2þ, Sr2þ, Pb2þ andBa2þ at A-site of BFO. Magnetization is found to increase with theincrease in ionic radius of doped ion. However, no increase inmagnetization has been observed for Ca and Sr doped samples upto30% doping [15,16]. The effect of more than 30% Ca doping has notbeen reported to the best of our knowledge. But, the appropriateamount of doping may enhance the dielectric properties and maycause the strain in the lattice resulting in the suppression of thespatially modulated spin structure of BiFeO3 [17], hence sponta-neous magnetization. Keeping in mind this fact, we have synthe-sized Ca doped BiFeO3 samples upto 50% doping at Bi-site andstudied structural and dielectric properties of single-phasecompositions Bi1�xCaxFeO3.

2. Experimental details

Bulk samples of Bi1�xCaxFeO3 (0.1� x� 0.5) were synthesizedusing rapid two stage solid state reaction method. It was done inorder to avoid the undesirable effects produced due to volatiliza-tion of Bi whichmay result in secondary phases in thematerial. Thestoichiometric amounts of high purity Bi2O3, Fe2O3 and CaCO3

powders were ground in agate mortar for several hours to makefine powder. Then, fine powder of all the compositions was pressedto the pellet (15 mm diameter) form. All the compacted mixtureswere heated at 850 �C for half an hour in first stage. These mixtures

Table 1Lattice and refinement parameters for Bi1�xCaxFeO3 (x¼ 0.1, 0.3, 0.5).

Composition(x)

Lattice parameter(Å)

Volume of theunit cell (Å3)

c2

0.1 a¼ 5.569702; c¼ 13.83191 371.59 2.270.3 a¼ 5.535005; c¼ 13.559915 359.76 2.480.5 a¼ 5.51586; c¼ 13.511917 356.00 1.476

Jaiparkash et al. / Solid State Sciences 13 (2011) 1869e18731870

were again ground and heated after pressing to the pellet form. Thefinal heat treatment was performed at temperatures 890, 950, and950 �C respectively for x¼ 0.1, 0.3, and 0.5 for 10 min. This heatingwas done using a programmable furnace. In both the stages, thesamples were quenched by removing from the furnace immedi-ately after sintering.

For the determination of crystal structure of all the samples,X-ray diffraction (XRD) measurements were performed usingBruker D8 X-ray diffractometer having Cu Ka radiation. The energydispersive X-ray analysis was carried out using Zeiss EVO40 model.Dielectric measurements were performed using Agilent 4285A(75 kHz to 30MHz) precision LCR meter in the temperature rangeof 200e450 K at selected frequencies in the range 100e1100 kHz.Measurements were performed after the proper cleaning ofsamples. In order to carry out dielectric measurements of samples,the pellets were cut with diamond saw wheel into small pelletshaving dimensions 6.40� 3.60�1.66 mm, 6.28� 3.29�1.81 mm,and 6.28� 3.29�1.81 mm for x¼ 0.1, 0.3, and 0.5, respectively.After this, electrodes were deposited on them using silver paste.Capacitance vs. voltage measurements were performed at 425 K(x¼ 0.1 and 0.3) and 350 K (x¼ 0.5) at a frequency of 1100 kHzusing the same LCR meter. Magnetization measurements of allsamples were performed using vibrating sample magnetometer(VSM) option of physical property measurement setup (PPMS).Magnetization was measured as a function of temperature in therange 20e395 K in both, zero field cooled (ZFC) and field cooled(FC) modes with a field of 1 kOe. For this, first the sample wascooled in zero field upto 20 K and the magnetization as a functionof temperaturewas measured in thewarming process to 395 K. TheFC curve was obtained by measuring the magnetization in thecooling process in the same field.

3. Results and discussions

3.1. X-ray diffraction studies

X-ray diffraction (XRD) patterns of Bi1�xCaxFeO3 (x¼ 0.1, 0.3, 0.5)samples were collected at room temperature. To find out whether

Fig. 1. X-ray diffraction pattern for the samples Bi1�xCaxFeO3 (x¼ 0.1, 0.3, and 0.5)measured at 300 K.

materials formed are of single phase in nature or not, XRD data wasfurther analyzed with Rietveld refinement using FULLPROF code.Fig. 1 shows the XRD pattern along with the fitted curve anddifference line. A good agreement between observed and calculateddata suggests that Bi1�xCaxFeO3 samples are in single phase even upto 50% Ca doping while in previous studies 50% Ca doping has notbeen reported. These were indexed to hexagonal unit cell with R3cspace group. In Bi1�xCaxFeO3, Bi/Ca is associated with Wyckoffposition 6a (0, 0, z), Fe is at position 6a (0, 0, 0) and O is at 18b (x, y,z). All the parameters related to unit cell and refinements are listedin Table 1. It is clearly evident from the tabulated parameters thatwith increased doping, there is a reduction in lattice parametersand hence volume of the unit cell. It could be either due to smallerionic radius of Ca2þ as compared to Bi3þ or due to creation ofoxygen vacancies to accommodate charge imbalance [18,19,22]. Wealso observe from the XRD pattern that Bragg reflections of all thesamples are quite sharp, except for sample x¼ 0.1, which isa signature of rich crystallization. But, with increase in the value of xfrom 0.2 to 0.5 (results are not shown here for x¼ 0.2 and 0.4),a reduction in the intensities of peaks can be observed suggestinga lattice distortion in the samples due to Ca doping [19]. Peaks arenot very sharp for x¼ 0.1 and also intensities of peaks are less ascompared to other samples. Volatile nature of Bi2O3 might beresponsible for the poor crystallization of this sample [20]. Thestoichiometry of the above compositions was calculated by meansof EDX attached to SEM and is shown in Table 2. It is important tomention here that stoichiometry of all samples deviates slightlyfrom the synthesis stoichiometry. Unlike a number of publicationson heterovalent doped BiFeO3 [21,22], no structural phase transi-tion has been observed.

3.2. Dielectric properties

Figs. 2 and 3 display the temperature dependent variation ofdielectric constant and loss tangent at selected frequencies,respectively. From these figures, it can be observed that dielectricconstant ( 30) as well as loss tangent (tan d) remains almost constantfor all the samples at lower temperatures. But, at higher tempera-tures the value of both of these parameters rises significantly withtemperature. Due to Ca doping there may be the creation of oxygenvacancies resulting in a distorted system. Thus the normal conceptof band conduction may be replaced by localized sites, which aresurrounded by very high potential wells which can not be sur-passed by electrons. Dielectric polarization is the result of creationof dipoles at these localized hopping sites. Thus the phenomenon oflocalized charge hopping between spatially fluctuating potentials isknown to give rise to conduction as well as dipolar effects [16]. Theincrease in temperature will increase the hopping of the chargegiving rise to more conduction and dipolar effects which will result

Table 2The compositional analysis of Bi1�xCaxFeO3, extracted from EDX.

Atomic ratio (Ca/Bi) x¼ 10% x¼ 30% x¼ 50%

From stoichiometry calculations 0.111 0.4285 1From EDX data 0.0948 0.4386 1.05

Fig. 2. Temperature dependence of dielectric constant ( 30) for Bi1�xCaxFeO3 (x¼ 0.1,0.3, and 0.5) at selected frequencies.

Fig. 4. Frequency dependence of real and imaginary parts of dielectric constant forBi1�xCaxFeO3 (x¼ 0.1, 0.3, and 0.5) at 400 K.

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in increase in both dielectric constant and loss tangent. Hence, theincrease in both of these parameters with temperature suggeststhat carrier hopping is playing the dominant role at highertemperatures for all the samples. Moreover, real ( 30) and imaginary( 300) parts of dielectric constant becomemore or less parallel to eachother at increased frequencies (Fig. 4) which is also a signature ofexistence of hopping mechanism in these samples [23]. But, atlower temperatures, the carrier hopping contributes a little topolarization due to freezing of dipoles, resulting in negligibly smallvariation in 30 and tan d. This temperature dependent variation ofdielectric constant and loss tangent is followed at all the frequen-cies in the measured frequency range (100e1100 kHz). Theseparameters show a decrease in their values with rise in frequency,

Fig. 3. Temperature dependence of loss tangent (tan d) for Bi1�xCaxFeO3 (x¼ 0.1, 0.3,and 0.5) at selected frequencies.

indicating that at low frequencies dipoles produced due to chargedefects (e.g. due to oxygen vacancies) follow the field reversalsresulting in increased values of 30 and tan d. However, as thefrequency is increased, the defects related dipoles lag behind thefield and hence values of 30 and tan d decrease.

As far as the effect of doping is concerned, the value of bothdielectric constant and loss tangent increases with rise in doping(Figs. 2 and 3). This could be due to the enhanced oxygen vacanciescreated as a result of Ca doping giving rise to hopping current aswell as dipolar effects [16]. A dielectric anomaly is observed for allthe samples in the temperature range 400e439 K in 30 vs. T curves,suggesting some kind of ferroelectric transition (see Fig. 2) possiblydue to some structural transformation and ordering of dipoles [24].No such anomaly has been reported in the earlier studied Ca dopedBiFeO3 samples due to their measurement limit up to 400 K [16].These anomalies are not very sharp, possibly because of large lossfactors and charge hopping at these temperatures masking theseferroelectric transitions. Hence, to make a clear view of these peaks,background has been subtracted for 30 vs. T curves at 100 kHz andshown in the inset of Fig. 2. It is to be noted that there is noobservable shift in this anomaly for x¼ 0.1 and 0.3 but shiftingtoward lower temperature for x¼ 0.5. For further investigation ofthe anomaly, variation in capacitancewasmeasuredwith respect tochange in bias voltage below their respective ferroelectric transi-tion temperature, Tc (shown in Fig. 5), instead of ferroelectrichysteresis loop measurements due to their leaky nature. Thebutterfly nature of CeV curves is a clear indication of weak ferro-electric behavior in these ceramics [25]. Hence, we can say that theanomaly observed in the 30 vs. T curves may be due to some type offerroelectric transition in the samples. Due to the limitation of ourmeasurement setup, we could not confirm whether it is theparental anomaly (Tcw 1103 K) which has been shifted to lowertemperature with doping or new one. However, on the basis oflarge difference between parental anomaly in pure BiFeO3 and theexisting anomaly in Ca doped samples it can be stated that theexisting anomaly is a new one. Further studies are required toconfirm the nature of anomaly.

For getting the information about the relaxation mechanism,dielectric modulus M00 has been plotted against temperature forx¼ 0.3 at selected frequencies and shown in Fig. 6. The nature of

Fig. 5. Capacitance(C) vs. voltage (V) curves Bi1�xCaxFeO3 (x¼ 0.1, 0.3, and 0.5)obtained at 1100 kHz.

Fig. 7. The semi-log plot of relaxation frequency as a function of 1000/T for Bi1�xCax-FeO3 (x¼ 0.1, 0.3, and 0.5).

Jaiparkash et al. / Solid State Sciences 13 (2011) 1869e18731872

curves has been found to be same for other compositions also (notshown here). Clear peaks can be observed in the plots, which areshifting toward higher temperature with rise in frequency. Thepeaks provide the temperature at which the conductivity relaxa-tion frequency (fr) becomes equal to the measuring frequency [23].The fr can be expressed by fr¼ f0 exp(�E/kT) where fo is character-istic frequency and E is the activation energy for loss relaxation. Theplot of ln(fr) as a function of reciprocal of temperature is shown inFig. 7. The value of relaxation frequency reduces with rise intemperature and verifies the above equation. The value of fr isfound to increase with increase in composition. The conductionactivation energies estimated from plots shown in Fig. 7 for x¼ 0.1,0.3 and 0.5 are found to be 0.471, 0.467, 0.448 eV, respectively. Themagnitudes of the activation energies are greater than 0.2 eV forthe samples under study. This suggests that the conductionmechanism is due to hopping [26]. The observed decrease in the

Fig. 6. The plot of dielectric modulus M00against temperature at selected frequencies

for Bi0.7Ca0.3FeO3.

activation energy with Ca content suggests that Ca doping isproviding easy path to the charge carriers for hopping. Also, almostthe same values of activation energies have been observed foroxygen vacancies obtained in another system [27]. Hence, thedielectric behavior observed for our samples might be associatedwith the oxygen vacancies.

3.3. Magnetic properties

Further, to study the effect of Ca doping on the magneticproperties of BiFeO3, temperature dependent magnetization (MeT)measurements have been carried out in the presence of 1 kOemagnetic field in the temperature range of 5e400 K.MeT curves, inboth zero field cooled (ZFC) and field cooled (FC) mode, are shownin Fig. 8 for Bi0.7Ca0..3FeO3. We observe a drop in magnetization atw300 K, to a non-zero magnetization value in both the FC and ZFCcycles. We also note the irreversible behavior in ZFC and FC curvesover awide range of temperature. These observations suggest someferrimagnetic type of ordering present in the sample. Further,magnetic hysteresis loop measurements (MeH) have also beencarried out for all the samples (see Fig. 9). We observe a magnetichysteresis loop, with non-zero coercivity and it is practically same

Fig. 8. Zero field cooled (ZFC) and field cooled (FC) magnetization as a function oftemperature for Bi0.7Ca0.3FeO3 at 1 kOe magnetic field.

Fig. 9. Isothermal magnetization hysteresis for Bi1�xCaxFeO3 (x¼ 0.1, 0.3, and 0.5) atroom temperature.

Jaiparkash et al. / Solid State Sciences 13 (2011) 1869e1873 1873

for all the samples. The observation suggests that at roomtemperature some magnetic ordering exists. However, magnetiza-tion does not saturate up to 1 Tesla magnetic field. Drop inmagnetization at w300 K to a non-zero magnetization value, anobservable coercivity and non saturation of magnetization suggestthat after Ca doping, the antiferromagnetic ordering of parentBiFeO3 is modified to some ferrimagnetic ordering or canted anti-ferromagnetic ordering. After Ca doping in BiFeO3 stoichiometry ofvarious ionic species is modified to accommodate charge neutrality.This would lead to creation of oxygen vacancies [18]. Moreover dueto ionic mismatch of Bi3þ and Ca2þ, the structure is slightlystrained, as also revealed by smaller volume of the sample ascompared to the parent compound [28]. In these settings, it is quitepossible that the antiferromagnetic exchange network ofFe3þeOeFe3þ of parent compound is perturbed, leading to cantedantiferromagnetic interaction or ferrimagnetic interaction.

4. Conclusions

Single-phase polycrystalline Bi1�xCaxFeO3 (x¼ 0.1, 0.3, and 0.5)samples have been prepared using rapid two stage solid statereaction technique. Even for the highest doped sample, Ca substi-tution in BiFeO3 at Bi-site does not change its structure. But, boththe lattice parameters ‘a’ and ‘c’ are found to decrease systemati-cally with the incorporation of Ca. A diffused dielectric anomaly hasbeen found in the temperature dependent dielectric data. On the

basis of CeV measurements, it can be concluded that this anomalymay be due to ferroelectric transition. The activation energies havebeen calculated and found to decrease with the increase in Cacontent. Due to incorporation of Ca, magnetic disorder has beenintroduced into the system.

Acknowledgment

One of the authors (Yogesh Kumar) is grateful to Centre forScientific and Industrial Research (CSIR), India for the financialsupport through fellowship.

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