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Applied Materials Today 1 (2015) 37–44

Contents lists available at ScienceDirect

Applied Materials Today

j ourna l h o mepage: www.elsev ier .com/ locate /apmt

nhanced electrocaloric effect in Fe-doped (Ba0.85Ca0.15Zr0.1Ti0.9)O3 ferroelectriceramics

atyanarayan Patel1, Aditya Chauhan1, Rahul Vaish ∗

chool of Engineering, Indian Institute of Technology, Mandi 175 001, Himachal Pradesh, India

r t i c l e i n f o

rticle history:eceived 15 July 2015eceived in revised form 10 August 2015ccepted 10 August 2015

eywords:

a b s t r a c t

Solid-state refrigeration systems employing bulk ferroelectric materials suffer from two main drawbacksof low temperature change and high depolarization temperature which prevent rapid commercializa-tion. In this regard, the beneficial effects of Fe-doping on the electrocaloric performance of bulk ceramicswith nominal composition Ba0.85Ca0.15Zr0.1Ti0.9−xFexO3 (BCZTO-Fe) have been investigated. Indirect pre-dictions were made using Maxwell’s relations in conjunction with data from experimental observations.

lectrocaloricerroelectricolid-state refrigeration

It was revealed that 1% Fe doping can greatly improve the performance of BCZTO based compositions. Apeak adiabatic temperature change of 0.86 K (345 K) was predicted for 0–37 kV/cm electric field, signif-icantly higher (>100%) than pristine BCZTO ceramic. While a simultaneous increase in specific entropychange was also observed. The values indicate a huge improvement of the caloric capacity at reducedtemperature indicating the benefits of such doping.

© 2015 Elsevier Ltd. All rights reserved.

. Introduction

Research in the field of electrocaloric effect (ECE) in ferroelec-ric materials has undergone rapid development over the last 10ears [1–3]. The interest was first sparked upon the discovery ofiant ECE in PbZr0.95Ti0.05O3 thin films in 2006 [4]. This discoveryed to the speculation that ferroelectric materials could be useds potential solid refrigerants for field-driven cooling applications.ince then, ferroic transitions in ferroelectric materials have under-one extensive scrutiny, with the objective of conceiving a compactnd efficient solid-state refrigeration system [5–7]. This will ushern an era of new cooling technology that might help replace theonventional vapour compression system. Such devices would beighly efficient and environmental friendly. Other benefits includeilent operation, faster response time and low energy requirementshich make them best suited for on-board installation in modern

lectronics. Thus ferroelectric materials are projected as capable ofvercoming the limitations of current vapour compression technol-gy and usher in a new era of advanced refrigeration devices [5–7].

hese would be miniaturized and mobile, capable of scavengingaste energy to provide sustainable cooling solutions. However,

efore large scale commercialization can be undertaken, a few

∗ Corresponding author.E-mail address: rahul@iitmandi.ac.in (R. Vaish).

1 These authors contributed equally to this work.

ttp://dx.doi.org/10.1016/j.apmt.2015.08.002352-9407/© 2015 Elsevier Ltd. All rights reserved.

major limitations need to be overcome. The first and foremost is thechoice of material size or morphology. Ferroelectric thin films havebeen credited with the largest caloric effects and require moderateamount of electric potential to generate the desired polarizationchange. Thus, for viable heat extraction a comparatively larger vol-ume of thin film would be required which is practically difficultto realize. Conversely, bulk ferroelectrics benefit from a compar-atively higher thermal inertia, most suited for practical coolingapplications [1,8,9]. However, macroscopic samples suffer fromlower adiabatic temperature change (�T) and have restrictions onthe upper limit of their caloric capacity. This is due to the strictlimitations on their dielectric breakdown strength. Consequently,application of a lower electric field results in lower caloric effect.Lastly, the peak adiabatic temperature change is only realized nearthe ferro-para transition temperature which is much higher thanroom temperature, for most materials [10,11]. This further servesto limit the applicability of such systems.

Many solutions have been proposed to overcome or mitigatethese drawbacks. These include chemical modifications, use ofrelaxor ferroelectrics, employing biasing fields and domain engi-neering [8,9,11–14]. We recently demonstrated stress-mediatedtuning of ECE in bulk ceramics wherein an improvement of 200%was obtained with respect to �T [15]. Other benefits included

the ability to tune the caloric response and create a broad tran-sition response for enhanced temperature change capacity. Eventhough the results displayed promising potential, it is often diffi-cult to induce directional confinement or compressive pre-stresses

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8 S. Patel et al. / Applied M

n a practical application. Therefore, a viable solution is requiredhich does not hinder practical realization of such systems. Chem-

cal modification in the form of doping has been proven efficiento ameliorate many ferroelectric properties. Furthermore, effectf doping to enhance electrocaloric response is a relatively unex-lored field. Thus, this study aims to investigate the ECE in bulka0.85Ca0.15Zr0.1Ti0.9−xFexO3 (BCZTO-Fe) ceramics. To the best ofur knowledge no similar attempt has been made till date.

. Materials and methods

.1. Material

Lead-free (Ba1−xCax)(Zr1−yTiy)O3 based ferroelectric ceramicsave recently garnered global attention owing to their excellentlectromechanical properties [16–20]. Within a finite range ofperating temperature, the ceramics based on this composition areapable of displaying large piezoelectric strain coefficient and evenarger strain response, at relatively lower electric field strength21,22]. The values clearly surpass most lead-free compositionsnd even a few lead-based materials. Hence, they have been exten-ively explored as low field actuator materials and more recentlys potential candidates for ferroelectric based cooling systems.hese exceptional material attributes have been credited to thexistence of an orthorhombic phase. This phase which is interme-iate to dominating rhombohedral and tetragonal phases allowsor polarization rotation [23,24]. Together with extrinsic contribu-ion peaking around morphotropic phase boundary (MPB) allows

broad minimum in switching energy barrier, resulting in aug-ented ferroelastic switching [23,24]. This allows the material to

xhibit exceptionally large strain response around room tempera-ure.

Incorporation of Fe into the BCZTO system destabilizes the for-ation of orthorhombic phase. This in turn inhibits larger strain

esponse reducing its piezoelectric coefficients. With respect tolectrocaloric application it could actually be beneficial in order tovoid cracking and failure [25]. Furthermore, it has been reportedhat acceptor-doped (Fe3+) BCZTO exhibits ferroelectric aging,estricted domain wall motion and smaller grain formation [25].owever, despite the initial lack of appeal, it was observed that

ncorporation of Fe within the BCZTO system could help to furtherts EC response by a large magnitude, as will be explained in theubsequent sections.

. Methods

Gibb’s free energy due to polarization in ferroelectric materialss described as [8,26,27]:

= 12

˛D2 + 12

�D4 + 12

�D6 (1)

Here, = ˇ(T1 − T), T being the material’s initial temperature and being a temperature dependent material constant. However, �nd � are temperature independent constants. This phenomeno-ogical theory can be used to evaluate the entropy change in

ferroelectric material due to change produced in its polariza-ion. The term (T1 − T) is dependent on T1 in accordance to theurie–Weiss relation and changes its sign at T. Furthermore, and

are positive constants, while � assumes a negative value or firstrder transformation and positive for second order transformation.or a constant value of polarization, the rate of change of free energyith respect to temperature can be used to evaluate the entropy

hange of the system as follows [8,26,27]:

∂G

∂T

)D

= −�S = 12

ˇD2 (2)

ls Today 1 (2015) 37–44

Thus, adiabatic temperature change �T can be determined usingthe relation [8,26,27]:

�T = 12cE

ˇTD2 (3)

In Eq. (3) cE has been used to represent the specific heat capac-ity of the ferroelectric material at constant electric field. From thenature of Eq. (3) it can be established that electric displacement isthe prime factor influencing the magnitude of temperature change.Thus, for a given volume of material, higher polarization changewould lead to a more pronounced caloric effect. Incorporation ofFe into the pristine BCZTO composition not only raises its satura-tion polarization but also increases its polarization gradient withrespect to temperature. Hence, it is expected that optimally dopedcompositions are capable of exhibiting pronounced caloric effect.The magnitude of temperature change thus achieved can be indi-rectly estimated using the following Maxwell’s relation for entropychange [8,28]:

�TEC = − T

CE

∫ E2

E1

(∂P

∂T

)E

dE (4)

Here the symbols denote the usual quantities of temperature(T), polarization (P), electric field (E) and heat capacity at constantelectric field (CE). Eq. (4) has been used to predict �TEC for manyferroelectric compositions. When electric field is applied to thematerial, ferroelectric domain rotation commences, aligning indi-vidual dipoles along the direction of electric field, giving rise to ECE.Here lower value of electric field in Maxwell’s relation correspondsto ‘zero’ while the higher intensity is set to be at saturation, whichis also the value at which polarization data has been gathered. Itis evident from the nature of Eq. (4) that a material possessinghigher polarization gradient (∂P/∂T) will exhibit a higher �TEC.Here P is the dependent variable while E is the driving field. How-ever, a material possessing a higher saturation electric field E isalso expected to possess a larger temperature change due to thehigher energy required for polarization back-switching. Therefore,there exist two methods to enhance ECE in ferroelectric ceram-ics, either by increasing the polarization gradient or by increasingelectric field intensity. It is for this reason that thin films havebeen credited with exceptionally large �TEC, as they can withstandextremely high electric field. Bulk ferroelectric ceramics suffer fromlow dielectric breakdown strength, hence have lower ECE. Ceramicsbelonging to BCZTO family are low field actuator materials, imply-ing they saturate at lower E. This is the primary reason for the lowtemperature change capacity. However, Fe doping can help to mit-igate such drawbacks by enhancing the polarization gradient andsaturation polarization.

3.1. Experimental and characterization details

Ba0.85Ca0.15Zr0.1Ti0.9−xFexO3 (BCZTO-Fe) ceramics were pre-pared through solid oxide reaction route. Reagent grade powders(>98% pure) of BaCO3, CaCO3, ZrO2, TiO2 and Fe were mixed in stoi-chiometric proportions containing (0%, 0.5% 1.0% and 1.5%) Fe andsubjected to rigorous physical mixing using mortar and pestle. Theground powder was calcined twice at 1325 ◦C and 1350 ◦C for 6 h.After intermediate re-grinding, the calcined product was pressedinto cylindrical samples of 12 mm * 1 mm (dia * height) and sinteredat 1400 ◦C for 4 h. X-ray diffraction analysis was performed on bothcalcined powder and sintered pellets to determine the phase forma-tion and purity of the perovskite. The surface profile of the sintered

samples was confirmed using scanning electron microscopy (SEM).The density of the pellets was determined using Archimedes prin-ciple and found to be over ∼93% of the theoretical density. The flatfaces of the pellets were painted with silver electrodes to create

S. Patel et al. / Applied Materials Today 1 (2015) 37–44 39

FF

ewcmrc

4

iosa

ig. 1. X-ray diffraction data for sintered samples containing 0%, 0.5%, 1.0% and 1.5%e doping.

lectrical contacts for hysteresis measurements. Sintered samplesere subjected to bipolar electric field cycling to obtain electri-

al (P–E) hysteresis characteristics at different temperatures. Theeasurements were conducted under varying conditions of mate-

ial temperature for all compositions under study. The alternatingurrent frequency was kept fixed at 50 Hz for all measurements.

. Results and discussions

Fig. 1 shows the X-ray diffraction data of the sintered samples

ndicating phase purity of the fabricated perovskite, due to absencef unidentified peaks. Fig. 2 displays the SEM images of the sinteredamples and it can be observed that the samples are free fromny major porosity or anomaly. Fig. 3 displays the P–E data for

Fig. 2. Scanning electron micrographs for sintered samples containing (a) 0

Fig. 3. Polarization versus electric field hysteresis loops for all compositions under-study as observed for room temperature (300 K).

all compositions understudy as obtained at room temperature.Similarly, Fig. 4 contains the P–E data for all compositions as afunction of operating temperature. The data thus obtained hasbeen analysed in conjunction with Eq. (4) to predict the variouscaloric response values.

Fig. 5(a)–(d) describes the EC behaviour of all compositionsunderstudy as a function of material temperature and electricfield intensity. It can be clearly observed for the figure that themaximum adiabatic temperature change increases with increasingintensity of electric field. This behaviour is in accordance with thenature of Eq. (4). However, another important observation that canbe made is the fact that with increasing Fe content, the EC capacity

of the material first increases then decreases. The undoped sampledisplays a maximum ECE of 0.4 K at 370 K under an applied electricfield of 0–21.5 kV/cm. However, upon incorporation of 0.5% Fe,

%, (b) 0.5%, (c) 1.0% and (d) 1.5% Fe doping at different magnifications.

40 S. Patel et al. / Applied Materials Today 1 (2015) 37–44

F ning (

tfioppici

TC

ig. 4. Polarization versus electric field hysteresis loops for sintered samples contai

his value is increased to 0.63 K along with an enhanced saturationeld value of 33.0 kV/cm. This marks an improvement of 57%ver the undoped sample while simultaneously lowering theeak EC temperature to ∼345 K, a substantial 25 K lower than the

arent material. Furthermore, this value is again improved by an

mpressive 125% to 0.85 K, thereby more than doubling the originalaloric capacity in the 1% Fe doped sample. This peak responses also obtained at 345 K albeit at an improved saturation electric

able 1omparison of electrocaloric properties of the BCZTO family ceramics.

Material T [K] |�T| [K] |�0.45(BaZr0.2 Ti0.8O3)–0.55(Ba0.7Ca0.3TiO3)a 404 0.46 12(Ba0.92Ca0.08)(Zr0.05Ti0.95)O3 398 0.37 150.65Ba(Zr0.2Ti0.8)O3–0.35(Ba0.7Ca0.3)TiO3 353 0.27 200.3Ba(Zr0.2Ti0.8)O3–0.7(Ba0.7Ca0.3)TiO3 378 0.34 200.93Ba(Zr0.2Ti0.8)O3–0.07(Ba0.7Ca0.3)TiO3 358 0.55 40Ba0.98 Ca0.02 (Zr0.085Ti0.915)O3 358 0.6 40Ba0.8Ca0.2Zr0.04Ti0.96O3 386 0.27 70.65Ba(Zr0.2Ti0.8)O3–0.35(Ba0.7Ca0.3)TiO3 338 0.33 200.65Ba(Zr0.2Ti0.8)O3–0.35(Ba0.7Ca0.3)TiO3 345 0.23 20Ba0.865Ca0.135Zr0.1078-Ti0.8722Fe0.01Nb0.01O3 307 0.5 30Ba0.865Ca0.135Zr0.1089-Ti0.8811Fe0.01O3 347 0.45 30BaSn0.15Ti0.85O3 301 0.42 20BaSn0.12Ti0.88O3 335 0.45 20BaTi1−xSnxO3 301 0.61 20BaTiO3

a 402 0.9 12BaTiO3 391 0.4 7Ba0.85Ca0.15Zr0.1Ti0.9O3 370 0.40 21Ba0.85Ca0.15Zr0.1Ti0.895Fe0.005O3 345 0.60 33Ba0.85Ca0.15Zr0.1Ti0.89Fe0.01O3 345 0.86 37Ba0.85Ca0.15Zr0.1Ti0.885Fe0.015O3 345 0.54 33

a Single crystal.

a) 0%, (b) 0.5%, (c) 1.0% and (d) 1.5% Fe doping plotted as a function of temperature.

field of 37.0 kV/cm. However, upon subsequently increasing thedoping concentration to 1.5%, the EC is severely affected andonly a minor temperature change of 0.55 K can be obtained. Ourpredicted value corresponds well to the data reported in literature

for other variations of the BCZTO family (Table 1). Furthermore,it is also substantially higher than other caloric effects recentlyreported for BCZTO based compositions (Table 1). Moreover, wehave recently published electrocaloric potential in Fe-Nb co-doped

E| kV/cm �T/�E (mK cm/kV) Method Reference

38.3 Indirect [31] 24.6 Indirect [32] 13.5 Indirect [2] 17 Indirect [2] 13.7 Direct [33] 15 Direct [32].95 33.9 Indirect [34]

16.5 Direct [35] 11.5 Indirect [35] 16.7 Indirect [29] 15 Indirect [30] 21 Indirect [36] 22.5 Indirect [36] 30.5 Indirect [37] 75 Indirect [3].5 53 Indirect [3].5 18.6 Indirect Present work

18.2 Indirect Present work 23.2 Indirect Present work 16.37 Indirect Present work

S. Patel et al. / Applied Materials Today 1 (2015) 37–44 41

F ing (aa

aBoraMifielac

marrfmc

obmptoef

ig. 5. The electrocaloric adiabatic temperature change profile for samples containnd applied electric field intensity.

nd Fe-doped Ba0.865Ca0.135Zr0.1078-Ti0.8722Fe0.01Nb0.01O3 anda0.865Ca0.135Zr0.1089-Ti0.8811Fe0.01O3 compositions which werebserved to be 0.5 K and 0.45 K, respectively [29,30]. The valueseported in the present work exceed both these previous attemptss well as a host of related materials as is evident from Table 1.oreover, BaTiO3 single crystal is only known to exhibit a max-

mum caloric effect of 0.9 K whereas this value is reduced to 0.4 Kor the polycrystalline sample, as presented in table. Therefore,t can be said that incorporation of Fe significantly improveslectrocaloric capacity. Additionally, the most intriguing featureays in the fact that peak adiabatic temperature change has beenchieved substantially closer to room temperature than its otherounter parts.

All these improvements combined boast of an overall advance-ent that can help mitigate most of the drawbacks currently

ssociated with ferroelectric solid-state refrigeration. Fe3+ plays theole of a b-site acceptor dopant in BCZTO-Fe. Fe doping has beeneported to reduce remnant polarization, coercive field value anderro-para transition temperature [25]. However, an increase in the

aximum polarization has been achieved as compared to undopederamic.

From the presented results, it is evident that 1% Fe doping is theptimum amount to obtain ameliorated EC performance in BCZTOased ferroelectric materials. This not only more than doubles theaximum adiabatic temperature change but also lowers the peak

erformance temperature closer to room temperature. However,

emperature change alone is insufficient data to determine theverall caloric performance of a ferroelectric material. Mass specificntropy change is a measure of the overall heat extraction capacityor a material, per unit mass. Thus, it is an important parameter

) 0%, (b) 0.5%, (c) 1.0% and (d) 1.5% Fe doping a function of operating temperature

which will help to evaluate the relative heat extraction capacityof the material used. Fig. 6(a)–(d) represents the specific entropychange of all compositions understudy, as a function of appliedelectric field and temperature. It is evident from the displayedresults that a trend corresponding to the ECE can be observedfor entropy change as well. The pristine composition has a max-imum entropy change capacity of 0.45 J/kg K which is improvedto 0.68 J/kg K and 0.97 J/kg K for 0.5% and 1% Fe doped samples,respectively. Thus, once again it is revealed that 1% Fe doping is thebest composition from among the selected compositions under-study, for heat extraction and ferroelectric solid-state refrigerationpurposes.

Even though the presented values are a good indicator in itself,these numbers only represent a fraction of the full caloric capacityof the compositions understudy. The electric field used to deter-mine the adiabatic temperature change has been limited to thatrequired to attain saturation polarization. Further, due to limita-tions of the experimental setup the maximum value of electric fieldthat could be applied is 40 kV/cm. From the nature of Eq. (4), itcan be determined that the ultimate value of ECE will scale withincreasing value of electric field. It is reported in the literature thatthe dielectric breakdown strength of oxide-derived bulk ceram-ics is >125 kV/cm. Thus, in order to better evaluate their coolingpotential the EC temperature change data has been projected forall compositions up to a safe electric field limit of 100 kV/cm. Lin-ear regression analysis was utilized to predict the temperature and

entropy change at higher electric field value and the same has beenrepresented in the form of Fig. 7(a) and (b), respectively. It can beclearly observed from Fig. 7(a) that composition containing 1% Feis capable of exhibiting a maximum temperature change of 2.3 K at

42 S. Patel et al. / Applied Materials Today 1 (2015) 37–44

F ) 0.5%e

1FrccbcFB

ioipZtapaFpmimpTttdd

ig. 6. The mass specific entropy change profile for samples containing (a) 0%, (blectric field intensity.

00 kV/cm. While the compositions containing 0%, 0.5% and 1.5%e are expected to deliver a maximum ECE of 1.9 K, 1.8 K and 1.5 K,espectively. A corresponding trend is observed for specific entropyhange in all compositions, Fig. 7(b). Again, a maximum entropyhange of 2.6 J/kg K is expected from 1% Fe composition followedy 2.3 J/kg K, 2.1 J/kg K and 1.6 J/kg K for 0% 0.5% and 1.5% dopedompositions. All these observations lead to the consensus that 1%e doping is the most beneficial for improving EC performance ofCZTO based compositions.

It has already been mentioned that the broad minimumn switching energy barrier originates from the presence ofrthorhombic phase in BCZTO based materials. This phase whichs intermediate to the dominating rhombohedral and tetragonalhases is metastable in nature. Further, it is very sensitive to ther/Ti ratio and is only observed in compositions fabricated closeo their morphotropic phase boundary. Incorporation of Fe3+ ions a b-site dopant disrupts the formation of this orthorhombichase. Consequently, the strain response in the material is severelyffected and defect induced electrical hardening is observed upone doping. However, Fe doping simultaneously lowers remnantolarization while improving saturation polarization. The maxi-um polarization is achieved between 298 and 328 K after which

t decreases monotonically with temperature. Furthermore, theaximum polarization gradient is observed closer to room tem-

erature as corroborated by the dielectric response of the material.he increased polarization and reduced depolarization tempera-

ure append to produce an enhanced ECE at a lower operatingemperature. The beneficial effects are observed only up to 1% Feoping. Upon increasing the doping concentration to 1.5%, a sud-en decrease in performance is observed. This could be credited to

, (c) 1.0% and (d) 1.5% Fe doping a function of operating temperature and applied

the complete disappearance of orthorhombic phase and the onsetof rhombohedral phase. However, at this point a complete expla-nation for the same is not available. Notwithstanding, the optimumFe doping concentration is revealed to be 1%. Further, the per-formance could be greatly enhanced if the dielectric breakdownstrength of the bulk ceramics could be increased by either of thefollowing approaches: glass-ceramic composites, advanced fabri-cation processes (spark plasma sintering) and domain engineering.Thus, there still lies ample scope of improvement and therefore,will form a part of the subsequent investigations. As a concludingremark, the authors would like to state that the scope of currentwork is limited to three Fe doped compositions only. However,in order to establish a clear solid structure–property relationshipdoping over a wider range of Fe would be needed; which would besubsequently undertaken and presented in the form of a separatestudy.

5. Conclusions

In this study, the authors have presented the beneficial effects ofFe doping on the electrocaloric performance of bulk BCZTO ceram-ics. Four compositions were selected for the investigation with(atomic) 0%, 0.5%, 1.0% and 1.5% Fe doping concentrations. The P–Edata was gathered as a function of electric field and temperatureand used in conjunction with Maxwell’s relation for entropy andtemperature change in ferroelectric materials. It was observed that

a peak adiabatic temperature change of 0.86 K is observed for 1%composition which is a 125% larger than the pristine sample (0.4 K).Additionally, this value is obtained at a much lower temperatureof 345 K (25 K below) the peak response for parent ceramic 370 K.

S. Patel et al. / Applied Materia

Fig. 7. The figure summarizes the actual and predicted (a) electrocaloric adiabatictsl

FomT1ac1Fricfmtfwatlwu

A

I

emperature change and (b) specific entropy change data for all compositions under-tudy. The data has been extrapolated for electric field intensity of 100 kV/cm usinginear regression analysis.

urthermore, the volume specific entropy change was alsobserved to be the largest for 1% composition, exhibiting a maxi-um of 0.97 J/kg K as opposed to 0.45 J/kg K for undoped sample.

he values were extrapolated to an electric field intensity of00 kV/cm to assess the full potential, using linear regressionnalysis. It was observed that composition containing 1% Fe isapable of exhibiting a maximum temperature change of ∼2.3 K at00 kV/cm. While the compositions containing 0%, 0.5% and 1.5%e are expected to deliver a maximum ECE of 1.9 K, 1.8 K and 1.5 K,espectively. The results are indicative of the fact that 1% Fe dopings very beneficial for augmenting the EC performance of BCZTOeramics and the huge untapped potential of ferroelectric materialsor cooling applications. This study is expected to greatly aug-

ent the progress of ferroelectric based solid-state refrigerationechnology while simultaneously advocating the efficacy of bulkerroelectric materials as suitable candidates for caloric devicesith a huge margin for improvement. As a concluding remark, the

uthors would like to state that the scope of current work is limitedo three Fe doped compositions only. However, in order to estab-ish a clear solid structure–property relationship doping over a

ider range of Fe would be needed; which would be subsequentlyndertaken and presented in the form of a separate study.

cknowledgments

One of the authors (Rahul Vaish) acknowledges support from thendian National Science Academy (INSA), New Delhi, India, through

ls Today 1 (2015) 37–44 43

a grant by the Department of Science and Technology (DST), NewDelhi, under INSPIRE faculty award-2011 (ENG-01) and INSA YoungScientists Medal-2013.

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