Sequence of structural transitions and electrocaloric properties in (Ba1-xCax)(Zr0.1Ti0.9)O3 ceramics

Accepted Manuscript Sequence of structural transitions and electrocaloric properties in (Ba1-xCax) (Zr0.1Ti0.9)O3 ceramics H. Kaddoussi, A. Lahmar, Y. Gagou, B. Manoun, J.N. Chotard, J.-L. Dellis, Z. Kutnjak, H. Khemakhem, B. Elouadi, M. El Marssi PII:

S0925-8388(17)31347-6

DOI:

10.1016/j.jallcom.2017.04.148

Reference:

JALCOM 41552

To appear in:

Journal of Alloys and Compounds

Received Date: 22 November 2016 Revised Date:

7 April 2017

Accepted Date: 13 April 2017

Please cite this article as: H. Kaddoussi, A. Lahmar, Y. Gagou, B. Manoun, J.N. Chotard, J.-L. Dellis, Z. Kutnjak, H. Khemakhem, B. Elouadi, M. El Marssi, Sequence of structural transitions and electrocaloric properties in (Ba1-xCax)(Zr0.1Ti0.9)O3 ceramics, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.148. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Sequence of structural transitions and electrocaloric properties in (Ba1-xCax)(Zr0.1Ti0.9)O3 ceramics H. Kaddoussi1,2, A. Lahmar1,a), Y. Gagou1, B. Manoun3,4, J.N. Chotard5, J-L. Dellis1, Z. Kutnjak6, H. Khemakhem2, B. Elouadi7and M. El Marssi1 1

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Laboratoire de Physique de la Matière Condensée (LPMC), Université de Picardie, Jules Verne, Pôle Scientifique, 33 rue Saint-Leu, 80039 Amiens Cedex 1, France. 2 Laboratoire des Matériaux Ferroélectriques (LMF),LR- Physique-Mathématiques et Applications, Université de Sfax, Faculté des Sciences de Sfax (FSS), Route de Soukra km 3,5 B.P. 1171, 3000 Sfax, Tunisie. 3 Univ Hassan 1er, Laboratoire des Sciences des Matériaux, des Milieux et de la Modélisation (LS3M), 25000, Khouribga, Morocco. 4 Materials Science and nano–engineering (MSN), Mohammed VI Polytechnic University, Lot 660 Hay Moulay Rachid, 43150 Ben Guerir, Morocco. 5 Laboratoire de Réactivité et Chimie des Solides (LRCS), UMR 7314 CNRS, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France 6 Institue de Jozef Stefan, Jamovacesta 39, 1000 Ljubljana, Slovénie. 7 Laboratoire des Sciences de l’Ingénieur pour l’Environnement, Pôle Sciences et Technologie, Avenue Michel Crépeau, 17042 La Rochelle Cedex 1 – France.

Abstract

The influence of the incorporation of Calcium in lead-free ferroelectric Ba(Zr0.1Ti0.9)O3 perovskite on its structural phase transition, dielectric, pyroelectric, ferroelectric, and electrocaloric effect was investigated. Room temperature X-ray diffraction

20

mol%

with

a

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study allowed the identification of a continuous solid solution in the composition range 0 ≤ x≤ rhombohedral

to

orthorhombic

structural

transition

beyond

15mol%.Structural investigation of X-ray diffraction as a function of temperature reveals four different structural regions for the compositions with low content of Ca and three regions for

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the compositions with high Ca-content. Using heat flow measurements and dielectric investigations, two sequences of structural phase transitions were elucidated. The tetragonal-

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to-orthorhombic and orthorhombic-to-rhombohedral phase transitions induced by temperature variation were found to decrease with the increase of the Ca2+ content, while the cubic-totetragonal phase transition temperature remained almost unchanged and exhibited a dielectric permittivity maximum observed around the composition of x = 15mol%.The electrocaloric temperature change ∆T was calculated from two methods: the thermal variation of P–E hysteresis loops and pyroelectric current. The highest electrocaloric ∆T/∆E = 0.24 K.mm/kV was found for the composition x = 0.05.The largest electrocaloric responsivity around 0.30 K.mm/kV was observed for this composition at Curie temperature using direct method. Keywords: BZCT, structural phase transition, dielectric, ferroelectric, electrocaloric effect, pyroelectric coefficient. 1

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Corresponding author: [email protected]

Contents

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1. Introduction .......................................................................................................................... 3 2. Experimental......................................................................................................................... 5 3. Results and discussion .......................................................................................................... 5 3.1. SEM micrographs .......................................................................................................... 5 3.2. Structural investigation ................................................................................................. 6

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3.2.1. Room temperature structural investigation ......................................................... 6 3.2.2. Structural investigation as a function of temperature......................................... 9

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a) Case of 5BCZT composition ....................................................................................... 9 b) Case of 20BCZT composition .................................................................................. 13 3.3. Dielectric properties and specific heat capacity........................................................ 15 3.5. Determination of electrocaloric properties ............................................................... 19 3.5.1. By indirict method using P-E hysteresis loops .................................................. 19 3.5.2. By pyroelectric current indirect method ............................................................ 24

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3.5.3 By direct method .................................................................................................... 27 3.6 Piezoelectric properties ................................................................................................ 29 4. Conclusion ........................................................................................................................... 30

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Acknowledgment .................................................................................................................... 29

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References ............................................................................................................................... 32

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1. Introduction Lead based ferroelectriccomplex perovskites suchas Pb(ZrxTi1-x)O3(PZT) and Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT)have been of great interest for industry and technological applications because of their excellent physical properties,especially piezoelectric and

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ferroelectric ones [1]. Nevertheless, such compounds are dealing with global restriction in industrial application because theyare deemed as toxic substances due to the Pb toxicity. Because of the increasing demand for sustainable and environmentally-friendly materials [2], there is aprimordialinterestfor developing lead-free multifunctional materials as an alternative

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of the Pb-based material in many technological applications. Barium Zirconate-Titanate system Ba(ZrxTi1-x)O3(abbreviate as BZT), is one of interesting and attractive perovskite candidates[3-6] regarding of their specific properties such as the change of ferroelectric

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behavior as function of their composition as well as a shiftable Tc with Zr4+ content. Numerous substitutions have been tried to improve the physical properties of BZT systems. Distinctively, the incorporation of Calcium was found to enhance the piezoelectric properties at the morphotropic phase boundary (MPB) of BZT phase diagram, i.e., close to the triple point, where more than one ferroelectric phase coexist [3,7].

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In the literature, Liu et al.[8] were the first ones to have study the phase diagram of the system x(Ba0.7Ca0.3TiO3)-(1-x)(BaZr0.2Ti0.8O3) (abbreviate as BZT-BCT). The authors reported the existence of two phase transitions for the composition 0.5BZT-0.5BCT with the presence of atriple point composition where rombohedral, tetragonal and cubic phases coexist

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(see Fig.1(a)).In a later work, Keeble et al.[9] revised the structural phase diagram for the same system and evidenced an intermediate Orthorhombic phase beyond the composition of

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x= 50mol%that can be seen in the diagram represented in Fig.1(b). However, for the system Ba1-xCax(Zr0.05Ti0.95)O3, Singh et al.[10] established the phase diagram from the positions of transition temperatures during the dielectric measurements. The authors found a small increase of Cubic to tetragonal transition temperature (TC-T) with the increase of Ca content. Likewise, transition temperatures of tetragonal to orthorhombic (TT-O) and orthorhombic to rhombohedral (TO-C) decreased monotonically with increasing Ca2+ amount. In the case of (Ba1-xCax)(Zr0.05Ti0.95)O3 (BCZT), Being the object of the present study, Fu et al.[11] tried to correlate the piezoelectric response with structural changes depicted on phase diagram determined basing only on the dielectric measurements.

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Fig.1: Phase diagrams of x(Ba0.7Ca0.3TiO3)- (1- x)(BaZr0.2Ti0.8O3); BZT-BCT) : (a) reported by Liu et al.[8];(b) reported by Keeble et al.[9].

Note that a number of issues were out lined; the incorporation of both Ca2+ and Zr4+ in the BaTiO3 matrix may improve the EC responsivity (∆T/∆E). For instance, Singh et al.12reported rather large ∆T/∆E of 0.38Kmm/kV in (Ba0.835Ca0.165)(Zr0.09Ti0.91)O3,near its tetragonal-to-cubic phase transition. In a closely compositions, Asbani et al.[12] reported a value of 0.38

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K.mm/kV for (Ba0.8Ca0.2)(Zr0.04Ti0.96)O3 and only 0.14 K.mm/kV for (Ba0.895Ca0.105)(Zr0.13Ti0.87)O3 reported by Bai et al.[13].The role of Ca2+ and Zr4+ on ECE is not clear and still to be addressed

in detail for a proper understanding of ECE behavior in BCZT systems. In the present work, emphasis is placed on (Ba1-xCax)(Zr0.1Ti0.9)O3 (BCZT) solid

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solution in the composition range 0 ≤ x ≤ 20 mol %. We studied both the structure and the electrical properties as a function of the Ca composition and temperature. The ECE of the

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prepared ceramics was carried out by both calorimetric and ferroelectric hysteresis measurements, i.e., via so called direct and indirect method [14-17]. No structural study as a function of temperature has been reported before in the titled system. It was appeared for us more convenient to strengthen our study with structural refinement to get reliable structural data allowing to evaluate the effect of Ca incorporation on structural changes and related physical properties especially electrocaloric effect. The existence of such a complex phase transitions is beneficial for the electrocaloric effect (ECE) which result from large entropy and polarization variation in the system [18,19]. In fact, both the electrocaloric temperature variation (∆T) and the isothermal entropy (∆S) are important for the electrocaloric applications. 4

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2. Experimental The ceramics of Ba1-xCax(Zr0.1Ti0.9)O3 (xBCZT, with x = 5%, 10%, 15% and 20%) were prepared by a conventional solid state reaction technique. Raw materials of BaCO3, CaCO3, ZrO2, and TiO2 were weighed in the equimolar proportions and calcined at 1420 K

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(12h). After grinding and granulating, the pressed pellets (8 mm in diameter and 1 mm in thickness) were sintered at 1700 K (2h). The detailed experiment set up was reported in Ref.[14-17]. The phase analysis of the ceramic was characterized by X-ray diffraction (Bruker D8 Advanced, Cu-radiation, λKα1=1.54060 Å and λKα2= 1.54439 Å, equipped with a LynxEye

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detector) performed at room temperature and at different temperatures.The microstructures of the sintered samples were examined by a scanning Electron Microscope Philips XL30.Dielectric temperature dependence measurements were carried out by using a Solartron

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Impedance analyzer SI-1200. Heat-capacity measurements were carried out as a function of temperature by a differential scanning calorimeter, NETZSCH DSC 204 F1 apparatus. The ferroelectric polarization was investigated at 1 Hz by using a ferroelectric test system (aixACCT, TF Analyzer 1000) as a function of temperature. The pyroelectric current was measured using a Keithley 2635B Source Meter. Finally, the Direct (DM) measurement was

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performed via a home-made high resolution calorimeter [1,20,21].

3. Results and discussion

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3.1. SEM micrographs

An example of SEM images of 5BCZT, 10BCZT, 15BCZT, and 20 BCZT ceramics

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are shown in Fig. 2. As it can be seen, low Ca-content ceramics exhibit very dense microstructure. However, a clear detachment of the grains is observed by examining the microstructure of the composition 15BCZT, which indicates probably the beginning of a microstructural change. On the electronic micrograph of the composition rich in Calcium, 20BCZT, three dimensional grains were observed with different geometric shapes, interposed on each other, revealing an intergranular porosity. The microstructure is totally changed.

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Fig .2: SEM micrographs of 5BCZT, 10BCZT, 15BCZT and 20 BCZT ceramics,

3.2. Structural investigation

3.2.1. Room temperature structural investigation

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Powder X-ray diffraction patterns of the polycrystalline xBCZT ceramic samples, recorded at room temperature, are plotted in Fig.3 (a). These patterns show pure perovskite phases without any secondary crystalline phases or impurities within the detection limits of

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our instrument. The reflections obtained were indexed according to the mother phase BZT. A small shift of all reflection peakstowards the higher 2θ angles by increasing Ca-content has

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been observed whichindicates a decrease of the unit cell lattice volume. Furthermore, thesereflectionsbroadened.This behavior could be attributed to strain on the cell parameters coming from thecationic disorder. Indeed, partially substituting large Ba2+ (ionic radius=1.61 Å [22]) by small Ca2+ (ionic radius = 1.34 Å [22]) will very likely modify locally the cell parameters and thus induced strain. Neverthless, the presence of the Kα2may contribute to such broadning.These results are in good agreement with the reported works in Refs.[10,11]. The X-ray diffraction patterns were refined by using the FullProf program coupled with WinPLOTR software [23]. A satisfactory agreement factors have been obtained at room temperature for the structure refinement in R3m space group for BZT, 5BCZT, 10BCZT,and 15BCZT.However, for 20BCZT composition, the good agreement was found in Pmm2 space group(Fig. 3 (b)-(d)). 6

ACCEPTED MANUSCRIPT Fig. 3 (e) shows the decrease of the lattice parameters as function of Ca content at room temperature. These results are expected because the substitution of a cation with large radius by a cation with smaller radius induces the decrease of lattice parameters. The Raman spectra recorded at room temperature for different compositions are plotted in Fig. 3 (f), the plot shows some remarkable changes depending on the substitution

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rate relative to the mother phase [24]. At low-frequency, all bands widen and shift to lower frequencies with the introduction of Ca2+ in the BZT matrix reflecting the disorder created by this substitution (table 1). The shift of these bands towards lower frequencies can be explained by the displacement of Ba (Ca) and/or the distortion of the oxygen octahedra.

The most significant change in this study is the gradual reduction of the A1band

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intensity at 260 cm- 1 to completely disappear from the Raman spectrum corresponding to x= 20% of Ca. The disappearance of this band confirms undoubtedly the change of symmetry.

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Recall that from X-ray diffraction, we found that 5BCZT, 10BCZT and 15BCZT compositions crystallize in a rhombohedral symmetry while 20BCZT crystallizes in a orthorhombic symmetry. Hence a good agreement between the different characterization techniques is found.

Table 1: Positions of the different modes of BCZT system. Wavenumbers (cm-1)

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Compound

BZT

A1(TO2)

E(TO2)

A1(TO3)

A1(LO2)/ E(LO)

160*

200*

260*

303*

520*

720*

178

211

263

315

533

730

171

209

261

309

528

730

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5BCZT

A1(TO1)

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BZT [24]

E(TO1)

10BCZT

168

203

259

308

528

729

15BCZT

166

203

255

307

527

729

15BCZT [25]

150*

210*

315*

520*

720*

20BCZT

165

202

305

527

728

*: modes shown in references12 (for BZT) and 13 (for15BCZT)

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Fig.3: (color online) (a) Room temperature X-ray diffraction patternsof (Ba1xCax)(Zr0.1Ti0.9)O3 ceramics. Measured and calculated room temperature X-ray diffraction patterns: (b) for BZT; (c) for 5BCZT in the non centrosymmetric R3m space group. (d) for 20BCZT in the non centrosymmetric Pmm2 space group. The vertical lines show calculated positions of Bragg reflexions and the lower curve is the residual diagram (e) parameters as a function of composition and (f) Raman as a function of composition at room temperature.

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ACCEPTED MANUSCRIPT 3.2.2. Structural investigation of xBCZTas a function of temperature a) Case of 5BCZT composition Figs. 4 (a) shows the X-ray powder diffraction patterns of Ba0.95Ca0.05Zr0.1Ti0.9O3 polycrystalline phase as a function of temperature. X-ray diffraction were indexed patterns by

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means of the computer program Dicvol [26]. Note that the first ten peak positions were used as input data taking in to the account a maximal absolute error of 0.01° (2θ). The peak profiles were then adjusted in pattern matching mode using Fullprof software. In table 2, we gathered the refinement conditions and results obtained in cubic, tetragonal, and orthorhombic

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symmetries.

Table 2: Details of Rietveld refinement conditions of the cubic, tetragonal and orthorhombic

Composition Wavelength (Å)

T=373K λkα1 = 1.5406

Step scan increment (°2θ) 2θ range (°) Program Zero point (°2θ)

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No. of reflections No. of refined parameter Space group a (Å) b(Å) c(Å) V (Å3) RF RB Rp Rwp cRp cRwp

T=348K λkα1 = 1.5406

T=333K λkα1 = 1.5406

0.010526 20-90 FULLPROF

0.010526 20-90 FULLPROF

0.010526 20-90 FULLPROF

-0.0284(11)

0.0123 (16)

-0.0291(10)

ɳ=1.022(9) U= 0.049(4) V= -0.008(4) W= 0.0049(7) 28 20 Pm-3m 4.0304(1)

ɳ= 0.865(9) U= 0.108 (7) V= -0.027(5) W= 0.0083(9) 60 24 P4mm 4.027(10)

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Pseudo-Voigt function PV = ɳ L + (1 − ɳ) G Caglioti parameters

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structures Ba0.95Ca0.05Zr0.1Ti0.9O3 at 373, 348 and 333K.

65.383(2) 3.72 5.41 6.9 9.46 10.9 12.7

4.030(1)

ɳ= 0.952(8) U= 0.062 (5) V= -0.008(3) W= 0.0045(4) 100 26 Pmm2 4.0324(1) 4.0222(1) 4.0268(1)

65.352(3) 2.25 3.51 6.8 9.43 11.1 13

65.352(3) 4.25 6.44 6.47 8.75 10.02 11.8

In order to clarify the variation of structural stability, the cubic, tetragonal and orthorhombic structures of Ba0.95Ca0.05Zr0.1Ti0.9O3 at different temperatures were thereafter verified by the Rietveld refinement of the observed powder XRD profiles. Linear interpolation between a set of background points with refinable heights was used to fit the 9

ACCEPTED MANUSCRIPT background. The profiles were described using a Pseudo-Voigt function. For all the compounds studied here, the refinements of the occupancies of all the atoms show no significant deviation from their stoichiometric values. Significantly good residuals of the refinements are obtained. The structures that occur in Ba0.95Ca0.05Zr0.1Ti0.9O3 were, therefore, modelled in the

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space groups Pm m, P4mm and Pmm2, respectively. All Rietveld refinements using the X-ray powder diffraction data yielded to satisfactory results.

The X-ray powder patterns were fitted to the calculated ones using a full-profile

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analysis program to minimize the profile discrepancy factor Rp. For Ba0.95Ca0.05Zr0.1Ti0.9O3, the refinement of the powder X-ray diffraction pattern was carried out with cubic (Pm-3m) lattice. In this model Ba2+/Ca2+ andTi4+/Zr4+are placed at (½, ½, ½) and (0,0,0) sites,

of

the

reflections

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respectively; the oxygen atoms occupy (½, ½, 0) positions. No splitting or asymmetry of any observed

in

the

cubic

temperature

range

for

Ba0.95Ca0.05Zr0.1Ti0.9O3(358
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data for5BCZT at 373, 348 and 333K. As an example we show, in Fig. 4 (c), the typical refinement patterns by Rietveld method along with the difference plot for 5BCZTat 373K. The refinement for tetragonal structure of Ba0.95Ca0.05Zr0.1Ti0.9O3for 343- 353K temperature range of the powder X-ray diffraction pattern was carried out with tetragonal

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symmetry (P4mm) lattice. In this model Ba2+/Ca2+ and Ti4+/Zr4+ are placed at (0,0,z) and (½,½,z) sites, respectively. There are two distinct crystallographic oxygen atoms positions (O1

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(½, 0, z) and O2 (½,½, z)), present in the unit cell. The refinement of Ba0.95Ca0.05Zr0.1Ti0.9O3 for 313- 338 K temperature range, was carried out with an orthorhombic symmetry with Pmm2 space group. In this model Ba2+/Ca2+ and Ti4+/Zr4+ are placed at (0,0,z) and (½,½,z) respectively. The final plots of the observed and calculated profiles for tetragonal 5BCZT at 348 K and orthorhombic 5BCZTat 333 K are shown respectively in Fig. 4 (d) and (e). The variation of the lattice parameters in the studied temperature range in 5BCZT is shown in Fig. 4(f). As it can be seen there is a gradual decrease in the cell parameters while the temperature decreases. A clear change in the slope with a break around 313 K is observed 10

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reported in x(Ba0,7Ca0,3TiO3)-(1-x)BaZr0,2Ti0,8O3system [8.9].

Fig.4: (a) X-ray powder diffraction patterns for Ba0.95Ca0.05Zr0.1Ti0.9O3 as a function of temperature. (b) Reflexion peak (220) at different temperatures. Final Rietveld plots for the Ba0.95Ca0.05Zr0.1Ti0.9O3 in the(c) cubic at 373 K,(d) tetragonal at 348K and (e) orthorhombic at 333K. The upper symbols illustrate the observed data (circles) and the calculated pattern (solid line). The vertical markers show calculated positions of Bragg reflexions. The lower curve is the difference diagram and (f) Variation of the lattice parameters as a function of temperature for the composition Ba0.95Ca0.05Zr0.1Ti0.9O3. Table 3: Refined structural parameters for Ba0.95Ca0.05Zr0.1Ti0.9O3 at 373, 348 and 333K. 11

ACCEPTED MANUSCRIPT B (A²) 0.65(2) 0.77(3) 1.34(4) 0.63(1) 0.82(2) 0.72(4) 0.72(4) 0.51(1) 0.67(2) 0.49(4) 0.49(4) 0.49(4)

Occupancy 0.95/0.05 0.9/0.1 3 0.95/0.05 0.9/0.1 1 2 0.95/0.05 0.9/0.1 1 1 1

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Atomic coordinate (0,0,0) (0.5, 0.5, 0.5) (0.5, 0.5, 0) (0, 0, 0.1290 ) (0.5, 0.5, 0.6374) (0.5, 0, 0.7374) (0.5, 0.5, 0.0385) (0, 0, -0.0020 ) (0.5, 0.5, 0.4790) (0.5, 0, 0.5310) (0.5, 0.5, 0.0149) (0, 0.5, 0.4700)

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Temperature Atom Ba2+/Ca2+ Ti4+/Zr4+ T=373K O Ba2+/Ca2+ Ti4+/Zr4+ O T=348K O Ba2+/Ca2+ Ti4+/Zr4+ O T=333K O O

Note that in the prototype phase BaTiO3, the transition sequence is governed by the number of Ti4+ moved along the pseudo cubic direction (111)pc [27]. In the rhombohedral

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unit cell, at low temperature, a long-range correlation in the three Cartesian directions between the Ti atoms exists. As soon as the temperature increases, in each phase transition, order is lost in one direction. Thus in orthorhombic phase, Ti atoms occupied one of the two possible positions in the (111)pc direction with the existence of a long-range correlation only in two directions. For the tetragonal phase, however, correlation in two directions is lost and

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four (111)pc positions are occupied by Ti. Finally, no correlation could exist in the cubic phase, and the 8 equivalents Ti sites are occupied randomly. In our present work, the incorporation of 10% Zr and low 5% Ca concentrations did not appear to affect the nature of Ti4+ in terms of occupation sites and moving along the direction (111)pc as the sequence of

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structural transition is still the same, only the transition temperatures are affected. The analysis of the refined crystallographic parameters in Ba0.95Ca0.05Zr0.1Ti0.9O3 at

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373, 348 and 333K indicates that the (Ti/Zr)4+ are octahedrally coordinated with the oxygen atoms. The (Ti/Zr)O6 octahedra are alternatively connected and extended in three dimensions. The O atoms connect the (Ti/Zr)O6 octahedra along the three directions. The analysis of various inter-atomic distances (Table 4) shows that Ba/Ca atoms form Ba/CaO12 polyhedra.

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ACCEPTED MANUSCRIPT Table 4: Selected inter-atomic distances (Å) Ba0.95Ca0.05Zr0.1Ti0.9O3 at 373, 348 and 333K. Ti/Zr-O1 Ti/Zr-O2 Ti/Zr-O3

Ba/Ca-O1

Cubic

6x2.015

12x2.8499

Tetragonal

4x2.001

2x2.093

Orthorhombic 2x2.0221

2.1579 1.8688

2x2.0166

4x2.601

4x2.903

4x3.002

2x2.708 2x2.998

4x2.851

2x2.817 2x2.874

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b) Case of 20BCZT composition

Ba/Ca-O2 Ba/Ca-O3

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Symmetry

X-ray diffraction of 20BCZT ceramic was performed at the same experimental

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conditions as 5BCZTin order to follow the structural changes in this phase rich in Calcium.

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Fig.5 shows the X-ray diffraction patterns as a function of temperature for 20BCZT.

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Fig. 5: X-ray powder diffraction patterns for Ba0.80Ca0.20Zr0.1Ti0.9O3 as a function of temperature. With a similar approach as for 5BCZT phase, we performed the structural refinement

by Rietveld and we extracted the lattice parameters at each temperature. Fig.6 shows the evolution of lattice parameters as a function of temperature. As shown in the plots, three structural regions are distinguished: orthorhombic, tetragonal and cubic. Note that this composition does not show the rhombohedral symmetry at low temperatures. The Long-range correlation between the Ti atoms in the three directions (111) is impossible. The absence of this symmetry seems to depend on the amount of calcium in the BZT matrix.

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Fig. 6:(a) Variation of the lattice parameters as a function of temperature for the composition Ba0.80Ca0.20Zr0.1Ti0.9O3; (b) variation of volume versus temperature. To verify this assumption, we will refer to the previous results reported in the BCZT systems. In the case of x(Ba0,7Ca0,3TiO3)-(1-x)BaZr0,2Ti0,8O3system, for the composition x= 0.5 Keebel et al.[9] reported the existence of three phase transitions. The titled phase coincides well with our composition 15BCZT, that corroborates our results. In contrast, the composition Ba0,94Ca0,06Zr0,16Ti0,84O3(x =0.2), with low concentration of calcium, shows only 14

ACCEPTED MANUSCRIPT one phase transition. Thus, we surmised that the presence or absence of transitions in our BCZT system cannot be attributed only to the concentration of Calcium. A competition between the polar instability in A-and B-sites is likely to influence the number of reducing Ti4+ moved along the direction (111)pc according to previous works by Ravel et al.[27].

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3.3. Dielectric properties and specific heat capacity The temperature and frequency variations of the dielectric constant and dielectric loss of xBCZT ceramics are shown in Fig. 7. To highlight the phase transitions, we plotted the curve of the differential dielectric constant (∂Ɛ’r/∂T) vs temperature in the insets of Fig.7. In

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the case of 5BCZT, 10BCZT and 15BCZT, three dielectric anomalies were detected, similar to that observed in BaTiO3 [28]. In contrast, only two phase transitions were observed for

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20BCZT. Using the phase diagram reported by Keeble et al. [9] (see Fig1.(b)) and starting from the high temperature side, the sequence of phase transitions can be identified as the following:

cubic-to-tetragonal-to-orthorhombic-to-rhombohedral

phase

transitions

in

compounds up to x=15mol% Ca-content, while another sequence cubic-to-tetragonal-toorthorhombic is established in compounds with higher Ca-content (here for x=20mol%). This result is similar to those reported in Ref.10 and Ref.11.

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The amplitude of the dielectric anomalies associated with the tetragonal-toorthorhombic and with the orthorhombic-to-rhombohedral (TT-O and TO-R) phase transitions are weaker (a lower permittivity value at the temperature of the anomalies) than that observed

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at cubic-to-tetragonal phase transition (TC-T).

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Fig.7: (color online) Temperature dependance of the dielectric constant and the dielectric loss for xBCZTceramics: (a) differents compositions measured at 10 Hz (b) BZT, (c) 5BCZT (d) 10BCZT, (e)15BCZT,and (f) 20BCZT. The insets show the derivative of the dielectric permittivity versus temperature for each composition.

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Moreover, the temperature of the cubic-to-tetragonal phase transition shows a weak

variation as a function of Ca-content with a maximum observed around the composition x= 5 mol%. Fu et al.[11] reported a large piezoelectric response around this composition attributed to the existence of a polymorphic phase transition. However, the temperature of the two other transitions (TT-O and TO-R), decrease with Ca-content (Fig.7), probably due to Ca-off centering effects. Furthermore, the value of the dielectric constant at the cubic-to-tetragonal phase transition decreases from about 104 in 5BCZT to nearly 4500 in 20BCZT. Noting that for all investigated compositions, the temperatures of the dielectric anomalies were frequency independent, and no clear relaxor behavior could be observed from these measurements.

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It is worth to note that difference between first or second order transition from dielectric curves is not generally easy, because most ferroelectric ceramics do not provide a clear evidence of the nature of their transition as in the case of BaTiO3 single crystal which undergoes first order phase transition or LiTaO3 with second order phase transition [29] .

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The Cp data obtained in several BCZT materials (see Fig. 9 bellow) indicate the weak first order nature of the phase transitions. Namely the temperature positions of Cp anomalies do not match those of the dielectric ones, but are rather positioned at steep dielectric slopes just below the dielectric peaks, which is typical of the first order transitions. However, all Cp

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anomalies are rather small, suggesting small latent heat of less than 0.03 Jg-1 is involved for all transitions, thus indicating a weakly first order nature for all transitions involved. Such

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weakly first order transitions behave nearly as second order transitions and only small disagreement between the indirect and direct method is expected.

Noting that the dielectric results are showing relatively sharp transitions, with very small frequency dispersion as shown in Fig. 7, which peaks are not shifting very much with temperature, but are more or less concentrated near the same temperature. Such dispersion is typically observed at higher compositions of PMN-xPT system (like PMN-30PT), where

PMN [30].

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transitions are very sharp and are not really diffused as in canonical relaxors systems like

In Figure 8, we show the simplified phase diagram realized from our dielectric results. It is worth mentioning that the obtained phase is somewhat similar to that one reported in the

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literature [9,10] where an intermediate orthorhombic structure was observed.

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Fig. 8: Phase diagram of BCZT based on the present dielectric results.

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Figure 9 presents the xBCZT heat-capacity curve within the temperature range 315– 400 K. Three distinct peaks were observed at the same anomalies temperatures for 5BCZT, 10BCZT, and 15BCZT, corroborating the three phase transitions already evidenced by the dielectric measurements. Similarly, in 20BCZT, only two anomalies were observed in

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accordance with the dielectric results. However, these two anomalies were considerably weaker and broader indicating more disordered structure. The obtained values of the specific

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heat were thereafter, used to evaluate the electro-caloric temperature change in our BCZT system.

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Fig.9: Temperature dependence of the specific heat of xBCZT ceramics.

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3.4. Determination of electrocaloric properties 3.4.1. Indirict method using P-E hysteresis loops

The ferroelectric properties of xBCZT ceramics were confirmed by polarization

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switching measurements recorded at 1Hz testing frequency and plotted in Figs. 10 ((a) to (d)). In order to evaluate the electrocaloric effect for the prepared ceramics, the hysteresis (P-E),

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loops were measured at different temperatures during a cooling process. The hysteresis loops, for all samples, showed typical characteristics of paraelectric to ferroelectric phase transitions with linear-to- nonlinear behavior. The saturation polarization (Ps) obtained for xBCZT ceramics are approximately 9.08 µC/cm2, 7.26µC/cm2, 6.53µC/cm2, 4.60µC/cm2 at 273 K, and the coercive fields (Ec) values are 2.17 kV/cm, 2.18 kV/cm, 2.18 kV/cm and 2.24 kV/cm for 5BCZT, 10BCZT, 15BCZT, and 20BCZT, respectively. It is very interesting to note that these results do not conform to the usual effect expected by the substitution of a big cation with a smaller one. This difference in the size caused often the rotation of octahedra (tilting octahedra) in the perovskites [31]. Maintaining the polarization with the progressive substitution of Ba2+ by Ca2+, suggests the existence of 19

ACCEPTED MANUSCRIPT an additional polarization of compensation. In the system (Ba1-xCax)TiO3, Fu et al.[11] emitted an hypothesis where they attributed the existence of compensation's polarization to a small displacement of Ca2+ ions in Barium sites (Ca off-centering effect). Such displacement from their initial positions generates an additional effective electric dipole moment to that one created by the displacement of Ti4+ as shown in Fig.10. Theoretical studies have also been

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devoted to this effect, they have shown that the displacement of Ca2 + from its initial position stabilizes the total energy of the unit cell in the (Ba1-xCax)TiO3 samples [32-34].

From the temperature and electric-field dependence of the polarization P(T,E), the electrocaloric temperature change ∆T, due can be calculated via the Maxwell relation

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, i.e., from the equation [6]:

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1    ∆ = −        

where ρ is the density of the ceramics which was found between 5.60 and 5.80 g/cm3. E1 and E2 are the initial and final applied fields, respectively, the value of E1 can be set as low as

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zero, and cp is the specific heat capacity of the material.

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Fig.10. (color online) P–E hysteresis loops of (Ba1-xCax)(Zr0.1Ti0.9)O3 ceramics measured at different temperatures((a) to (d)) and the corresponding electrocaloric temperature change (∆T) as a functionof temperature at different applied electric fields (((e) to (h)). The polarization at different applied electric fields was calculated by using the upper branches of the hysteresis loops [19]. The values of ∂P/∂T are obtained from forth-order 21

ACCEPTED MANUSCRIPT polynomial fits of the P–T data.Fig.10((e) to (h)) shows the electrocalorictemperature change as a function of temperature for three selected applied electric fields; 4, 6 and 8 kV/cm. The absolute maxima observed in ∆T curves, were observed at the cubic-to-tetragonal phase transition temperatures. The maximum value of ∆T (TMEC temperature at which maximum of ECE) were between 0.2 for 5BCZT and 0.08 for 20BCZT.The variation of the electrocaloric

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responsivity (ξ = ∆T/∆E) and of the spontaneous polarization PS as a function of composition are plotted in Fig.11. It should be noted that the variation versus composition of the electrocaloric coefficient and that of the polarization are similar.

The maximum value of the electrocaloric responsivity obtained in the present work is

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significantly higher than that found in pure BZT [14] and those observed in various lead-free ferroelectric materials reported in the literature (see table 5).The incorporation of a small amount of Ca ions in the BZT matrix was found beneficial for improving the ECE. Indeed, ∆T

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value increases from 0.23 K.mm/kV for pure BZT to 0.256 K.mm/kV for 5BCZT. While increasing Ca content, the value of ξ, drops monotonically to 0.11K.mm/kVfor20BCZT. The main factor that could influenced the ECE effect in our case is the change of orbital around Ti(Zr)-O distance associated with Goldchmidt tolerance factor which leads to the decrease of the polarisation (see inset of Fig. 11).

The tolerance factor (t) for an ideal perovskite ABO3structure is defined as :

where

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=

 

√  

is the oxygen ionic radius (1.40 Å),

and

are the ionic radii of the A-site and the

B-site atoms. For tetragonal BaTiO3,t = 1.061 and for Ba(Zr0.1Ti0.9)O3 (BZT) is equal to 1.055,

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because the radius of Zr4+ ion (0.72 Å) is bigger than that of Ti4+ (0.605 Å). The incorporation of Ca in the BZT lattice induces a linear decrease of the tolerance factor from t = 1.050 for

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5BCZT (Ba0.95Ca0.05(Zr0.1Ti0.9)O3)to about t = 1.036 for 20BCZT (see inset of Fig.11).

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Fig.11: Variation of the electrocaloric responsivity coefficient and polarization versus Ca content at cubic-to-tetragonal phase transition temperature. The inset shows a linear dependence of the tolerance factor versus composition. As summarized in table 4, the highest value of ξ observed in our work is around 0.256 K.mm/kV near cubic-to-tetragonal phase transition temperature, similar to values reported for (Ba0.92Ca0.08)(Zr0.05Ti0.95)O3[35] and (Ba0.8Ca0.2)(Zr0.06Ti0.94)O3 [9] but with rather lower TMEC

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values.

Table 5: Comparison of ECE obtained in this work with those reported in literature for various BCZT compositions. TMEC (K)

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Material

∆Tmax (K)

E (kV/cm)

ξ=∆T/∆E (K.mm/kV)

References

404

0.46

12

0.38

[36]

(Ba0.8Ca0.2)(Zr0.04Ti0.96)O3

386

0.27

7.95

0.34

[12]

(Ba0.8Ca0.2)(Zr0.08Ti0.92)O3

377

0.22

7.95

0.27

[12]

(Ba0.95Ca0.05)(Zr0.1Ti0.9)O3

368

0.205

8

0.256

This work

(Ba0.8Ca0.2)(Zr0.06Ti0.94)O3

383

0.21

7.95

0.26

[35]

(Ba0.92Ca0.08)(Zr0.05Ti0.95)O3

400

0.38

15

0.25

[35]

(Ba0.8Ca0.2)(Zr0.02Ti0.98)O3

393

0.19

7.95

0.24

[12]

Ba(Zr0.1Ti0.9)O3

363

0.2

8.7

0.23

[14]

(Ba0.95Ca0.05)(Zr0.05Ti0.95)O3

400

0.31

15

0.206

[35]

(Ba0.90Ca0.1)(Zr0.1Ti0.9)O3

373

0.161

8

0.201

This work

(Ba0.85Ca0.15)(Zr0.1Ti0.9)O3

373

0.152

8

0.19

This work

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(Ba0.835Ca0.165)(Zr0.09Ti0.91)O3

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0.33

20

0.17

[13]

(Ba0.9Ca0.1)(Zr0.05Ti0.95)O3

400

0.23

15

0.153

[34]

(Ba0.91Ca0.09)(Zr0.14Ti0.86)O3

328

0.3

20

0.15

[13]

(Ba0.8Ca0.2)TiO3

398

0.12

8

0.15

[12]

(Ba0.895Ca0.105)(Zr0.13Ti0.87)O3

348

0.28

20

0.14

[13]

(Ba0.925Ca0.075)(Zr0.15Ti0.85)O3

325

0.25

20

0.125

[13]

(Ba0.97Ca0.03)(Zr0.18Ti0.82)O3

323

0.22

20

(Ba0.80Ca0.2)(Zr0.1Ti0.9)O3

373

0.088

8

0.11

[13]

0.11

This work

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3.4.2. Pyroelectric current indirect method

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(Ba0.91Ca0.09)(Zr0.14Ti0.86)O3

This method has the supremacy to determine the ECE without using ferroelectric

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measurements, only based on pyroelectric current measurements, which is then used to calculate the electrocaloric temperature variation ∆T.BCZT samples were cooled under an applied electric field of E = 4 kV/cm above C-T temperature with a heating rate of r = 20 K/min. After the field is switched off at room temperature, the pyroelectric current was then measured on heating runs.Note here that during these measurements, a short circuit was

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realized before heating the sample. That allowed to neutralize the interfacial space charges, especially those localized near the electrodes.



The relationship between the pyroelectric current i and the ! "E=0 is given by:

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#=$

&'

    = $ = $ %    

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where &(" is the pyroelectric coefficient measured at zero applied electric field after field cooling,

is the heating rate and A represents the area of electrodes. Noting that in the 

work of Asbani et al.[37], the pyroelectric coefficient! "does not change drastically with the applied field E. In order to avoid all type of conductivity we adopt the approximation 



! "E≠0= ! "E=0. Note that sample were poled at E electric field value but pyroelectric current was measured at zero field. The polarization was determined by integrating the pyroelectric current:

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1 !)  # %$ !*

Figure 12 (a to d) shows the temperature dependence of the pyroelectric coefficient and the calculated polarization for different compositions in order to calculate the adiabatic temperature at a heating rate r = 20 K/min. Note that the pyroelectric coefficient decreases

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with Ca-content followed by a decrease in the electrocaloric coefficient. The values found by this method are consistent with those calculated by the method based on the hysteresis cycles. It should be noted that the saturated polarization could not be reached, because of the limited applied voltage of the experimental device. However, the calculated polarization was

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comparable to remnant polarization taken from P-E hysteresis loops at the same applied electric field E = 4 kV/cm. Hence, the good agreement between the two methods was

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obtained.

The adiabatic temperature change ∆T was determined from experimental pyroelectric current measurements using the expression [29]: ∆ = −  +

!

, -

#∆

Where E1= 0 and E2 is the field used to polarize the sample.

The electrocaloric responsivity (∆T/∆E) is usually used as the appropriate coefficient to

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characterize the material’s ECE performance, it can be estimated from: ∆!

∆

! .!

= −+

, -

The electrocaloric temperature change ∆T for 5BCZT, 10BCZT, 15BCZT and 20BCZT are

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depicted in Figs. 12(e to h), respectively for the temperature rate r = 20 K/min. For all samples ∆T shows a maximum at the C-T temperature phase transitions. The maximum

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values of ∆T/∆E were 0.23 K.mm/kV for 5BCZT and 0.12 K.mm/kV for 20BCZT. Slightly lower value observed in 20BCZT is in accordance with the lower polarization found in this composition.

The maximum ∆T/∆E values calculated from the two performed methods on BCZT system are summarized in Table 6.

A good agreement in EC responsivity at the cubic -to- tetragonal phase transition is obtained between these methods. In contrast to the classic indirect method based P-E hysteresis loops, the data analysis of the new indirect method based on pyroelectric current measurements is much simpler while providing more reliable results.

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Fig.12. (color online) (a) to (d) Pyroelectric coefficient and the corresponding calculated polarization as function of temperature for 5BCZT, 10BCZT, 15BCZT and 20BCZT ceramics measured at E= 4kV/cm; (e) and (h) represent the corresponding electrocaloric temperature change ∆T versus temperature calculated from pyroelectric current measurements. 26

ACCEPTED MANUSCRIPT Table 6. Comparison of electrocaloric parameters obtained from P-E hysteresis loops and pyroelectric methods for BCZT system. E = 4 kV/cm Composition P-E hysteresis loops

∆T (K) ∆T/∆E ∆T/∆E (K.mm/kV) (K.mm/kV) 0.24 0.095 0.23

0.098

10BCZT

0.096

0.24

0.095

15BCZT

0.081

0.20

0.077

20BCZT

0.046

0.11

0.048

0.23

0.19

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5BCZT

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∆T (K)

Pyroelectric current

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0.12

3.4.3. Direct method

Figure 13 illustrates the obtained results of the electrocaloric effect by direct measurements. These were produced using a high resolution calorimetry. Our main objective

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is to compare results obtained by indirect and direct methods and verify the validation of our indirect determination. From where, we are limited to the 5BCZT phase which is a critical composition with optimal EC response found by the direct method. Fig.13(a) shows the variation of the adiabatic electrocaloric temperature(∆T) for two applied electric fields: E = 4 composition. Three anomalies are detected in this figure and

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and 8kV/cm for 5BCZT

correspond to R-O,O-T, and T-C phase transitions. These anomalies are also detected by the

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determination of ∆T from P-E hysteresis cycles, dielectric, calorimetric, pyroelectric measurements and structural resolution. Attesting the good agreement between the various techniques of characterization used in the present work. The maximum of ∆T at the transition temperature (Tc), is found in the order of 0.24 K and 0.09 K for an applied electric field E = 8kV/cm and E = 4kV/cm, respectively. Figure 13(b) shows that the maximum electrocaloric responsivity ∆T/∆E is about 0.30K.mm/kV at E= 8kV/cm. This value is higher and comparable to other values reported in previous studies [13, 35, 38-47]. We compare in Figure 13(c) the results obtained by indirect and direct methods. Comparable values of electrocaloric temperature change (∆T) for 5BCZT were obtained using the various methods at Curie temperature with an uncertainty for the method based on the 27

ACCEPTED MANUSCRIPT hysteresis loops. Never the less, a good agreement between the indirect method using the pyroelectric current and the direct method was obtained. The maximum ∆T/∆E values calculated from different methods performed on 5BCZT are summarized in Table 7. A good agreement in EC responsivity at the cubic -to- tetragonal phase transition is obtained between these methods. It is worth mentioning that the new

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indirect method based on the pyroelectric current measurements is much easier and provides more reliable and comparable results with the direct method of calculating the ECE instead of

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the classical indirect method based to (P-E) hysteresis loops.

Fig.13. (a) Electrocaloric temperature change ∆T as a function of temperature at 4 and 8 kV/cm for 5BCZT obtained by direct method; (b) electrocaloric responsivity as function temperature for 5BCZT at E = 4 kV/cm; (c) and (d) comparison between ∆T obtained by direct and indirect measurements as a function of temperature for 5BCZT.

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ACCEPTED MANUSCRIPT Table 7. Comparison of electrocaloric parameters obtained from P-E hysteresis loops, pyroelectric and direct methods for 5BCZT. 5BCZT Method

Curie. Temp

∆E = 4 kV/cm

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∆T(K) ∆T/∆E (K.mm/kV) TC-T

0.098

0.24

Direct Method

TC-T

0.090

0.22

Pyroelectric method

TC-T

0.095

0.24

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P-E hysteresis loops

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3.5. Piezoelectric properties

Figure 14 shows the piezoelectric coefficient d33 of BCZT ceramics as a function of Ca content at room temperature. It can be observed that the d33 curves possess a peak with increasing Ca content. The d33 of the mother BZT ceramics is about 37pC/N. With increasing Ca content (x= 0.05), the d33 of 5BCZT ceramics reach their maximum values of 160pC/N at

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applied electric field E = 8 kV/cm, then decreases within crease in the concentration of Ca.The highest d33 (160pC/N) of 5BCZT ceramics correlated well with the evolution of the value of remanent polarisation with maximum of Pr = 5.72µC/cm2 and relative low coercive field Ec of 218 V/mm observed for the same composition.

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As we mentioned above, in the Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 system, the piezoelectric response is reported to correlate to the electromechanical coupling effects due to

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the existence of a morphotropic phase [11, 48]. Because of the instability of the polarization, the polarization direction can be rotated by an external electric field, which causes a strong piezoelectricity response. Pisitpipathsin et al.[49] reported a piezoelectric coefficient (d33) equal to 392pC/N in the (Ba0.97Ca0.03)(Zr0.04Ti0.96)O3composition under an electric field of 60kV/cm. Furthermore, Wei et al.[50] reported a value of d33 = 375pC/N under E= 40kV/cm for the composition (Ba0.99Ca0.01)(Ti0.98Zr0.02)O3.

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Fig. 14: (a) Piezoelectric coefficient d33 as a function of the composition at room temperature and at different applied electric fields; (b) comparison of Pr, EC and d33 as a function of Ca content at 8 kV/cm. 4. Conclusion

In summary, lead-free Ba1-xCax(Zr0.1Ti0.9)O3 ceramics (xBCZT, with x = 5, 10, 15 and 20 mol%) were prepared by a conventional solid state reaction technique. The phase purity was checked by X ray diffraction technique. Electronic microscopy images showed

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progressive change of the microstructure with a complete transformation for the composition 20BCZT. The room-temperature x-ray structural analysis and Raman spectroscopy measurements have allowed following the structural changes versus Ca2+ content in the studied system. A rhombohedral structure for the composition with low Ca-content instead of

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orthorhombic structure for the Ca- rich composition. Besides, we were also able to follow the structural change as function of temperature of two compositions 5BCZT and 20BCZT. The

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structural refinement reveals two types of phase transition sequences: rhombohedricorthorhombic-quadratic-cubic for 5BCZT while for 20BCZT the founded sequence is of orthorhombic-quadratic-cubic. Dielectric and specific heat capacity permitted to highlight these sequences of structural phase transitions and confirmed the structural study. From the observed dielectric anomalies, we were able to draw the phase diagram of the studied system. The ferroelectric character of xBCZT ceramics was confirmed by the measurements of P-E hysteresis loops. It is worth to notice that until 20% Ca substitution, the ferroelectricity is still observable; we attributed such behavior to the Ca off-centering effect.

30

ACCEPTED MANUSCRIPT The piezoelectric measurements undertaken at room temperature showed good correlation with the ferroelectric properties with an optimum observed for 5BCZT ceramic of(d33 =160 pC/N; Pr = 5,72 µC / cm2). We were interested more particularly in this work to the determination of the effect of electrocaloric responsivity by the main of indirect and direct methods. In the indirect method,

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we followed two different approaches to calculate the EEC coefficient using Maxwell's relation. Firstly from the thermal variation of P-E hysteresis loops and then, by direct measurements of the pyroelectric current.

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The high resolution calorimetric method was used as direct way to confirm and to estimate the indirect approaches. We concluded in this investigation that the indirect method

good agreement with the direct method.

Acknowledgment

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based on the calculation of the pyroelectric current is easy and allowed to obtain results with

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The authors thank the “Program Hubert Curie-Maghreb” (PHC Maghreb) and the the Slovenian ResearchAgency program P1-0125for the financial support.

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[3] R. Farhi, M. El Marssi, A. Simon, and J. Ravez, A Raman and dielectric study of ferroelectric ceramics, Eur. Phys. J. B. 9 (1999) 599–604.

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high-energy x-ray diffraction and Raman spectroscopy, J. Phys. Condens. Matter. 26 (2014) 065901.

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[5] M.L.V. Mahesh, V.V. Bhanu Prasad, A.R. James, Enhanced dielectric and ferroelectric properties of lead-free Ba(Zr0.15Ti0.85)O3 ceramics compacted by cold isostatic pressing, J. Alloy. Comp. 611 (2014) 43–49.

[6] F. Moura, A.Z. Simões, B.D. Stojanovic, M.A. Zaghete, E. Longo, J.A. Varela, Dielectric and ferroelectric characteristics of barium zirconate titanate ceramics prepared from mixed oxide method, J. Alloy. Comp. 462 (2008) 129–134.

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[7] P. S. Dobal, A. Dixit, R. S. Katiyar, Z. Yu, R. Guo and A. S. Bhalla, Micro-Raman scattering and dielectric investigations of phase transition behavior in the BaTiO3–BaZrO3 system, J. Appl. Phy. 89 (2001) 8085.

[8] Wenfeng Liu and Xiaobing Ren, Large Piezoelectric Effect in Pb-Free Ceramics, Phys.

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Rev. Lett. 103 (2009) 257602.

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Revised structural phase diagram of (Ba0.7Ca0.3TiO3)-(BaZr0.2Ti0.8O3), Appl. Phys. Lett. 102 (2013) 092903.

[10] Gurvinderjit Singh, V. S. Tiwari and P. K. Gupta, Evaluating the polymorphic phase transition in calcium-doped Ba(Zr0.05Ti0.95)O3: a lead-free piezoelectric ceramic, J. Appl. Cryst. 46 (2013) 324–331.

[11] Desheng Fu, YutoKamai, Naonori Sakamoto, Naoki Wakiya, Hisao Suzuki and Mitsuru Itoh. Phase diagram and piezoelectric response of (Ba1−xCax)(Zr0.1Ti0.9)O3 solid solution, J. Phys.: Condens. Matter. 25 (2013) 425901.

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ACCEPTED MANUSCRIPT [12] B. Asbani, J.-L. Dellis, A. Lahmar, M. Courty, M. Amjoud, Y. Gagou, K. Djellab, D. Mezzane, Z. Kutnjak and M. El Marssi, Lead-free Ba0.8Ca0.2(ZrxTi1−x)O3 ceramics with large electrocaloric effect, Appl. Phys. Lett. 106 (2015) 042902. [13] Yang Bai, Xi Han and Lijie Qiao, Optimized electrocaloric refrigeration capacity in leadfree (1−x)BaZr0.2Ti0.8O3-xBa0.7Ca0.3TiO3 ceramics, Appl. Phys. Lett.102 (2013) 252904.

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[14] H. Kaddoussi, Y. Gagou, A. Lahmar, J. Belhadi, B. Allouche, J-L. Dellis, M.Courty, H. Khemakhem and M. El Marssi, Room temperature electro-caloric effect in lead-free Ba(Zr0.1Ti0.9)1−xSnxO3 (x=0, x=0.075) ceramics, Solid state communications. 201 (2015) 6467.

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Figure captions

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Fig.1: Phase diagrams of x(Ba0.7Ca0.3TiO3)- (1- x)(BaZr0.2Ti0.8O3); BZT-BCT) : (a) reported by Liu et al.[8];(b) reported by Keeble et al.[9]. Fig.2: SEM micrographs of 5BCZT, 10BCZT, 15BCZT and 20 BCZT ceramics,

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Fig.3: (color online) (a) Room temperature X-ray diffraction patternsof (Ba1-xCax)(Zr0.1Ti0.9)O3 ceramics. Measured and calculated room temperature X-ray diffraction patterns: (b) for BZT; (c) for 5BCZT in the non centrosymmetric R3m space group. (d) for 20BCZT in the non centrosymmetric Pmm2 space group. The vertical lines show calculated positions of Bragg reflexions and the lower curve is the residual diagram (e) parameters as a function of composition and (f) Raman as a function of composition at room temperature Fig.4: (a) X-ray powder diffraction patterns for Ba0.95Ca0.05Zr0.1Ti0.9O3 as a function of temperature. (b) Reflexion peak (220) at different temperatures. Final Rietveld plots for the Ba0.95Ca0.05Zr0.1Ti0.9O3 in the(c) cubic at 373 K,(d) tetragonal at 348K and (e) orthorhombic at 333K. The upper symbols illustrate the observed data (circles) and the calculated pattern (solid line). The vertical markers show calculated positions of Bragg reflexions. The lower curve is the difference diagram and (f) Variation of the lattice parameters as a function of temperature for the composition Ba0.95Ca0.05Zr0.1Ti0.9O3.

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Fig.5: X-ray powder diffraction patterns for Ba0.80Ca0.20Zr0.1Ti0.9O3 as a function of temperature. Fig.6:(a) Variation of the lattice parameters as a function of temperature for the composition Ba0.80Ca0.20Zr0.1Ti0.9O3; (b) variation of volume versus temperature. Fig.7: (color online) Temperature dependance of the dielectric constant and the dielectric loss

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for xBCZTceramics: (a) differents compositions measured at 10 Hz (b) BZT, (c) 5BCZT (d) 10BCZT, (e) 15BCZT,and (f) 20BCZT. The insets show the derivative of the dielectric permittivity versus temperature for each composition.

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Fig.8: Phase diagram of BCZT based on the present dielectric results. Fig.9: Temperature dependence of the specific heat of xBCZT ceramics. Fig.10. (color online) P–E hysteresis loops of (Ba1-xCax)(Zr0.1Ti0.9)O3 ceramics measured at different temperatures((a) to (d)) and the corresponding electrocaloric temperature change (∆T) as a functionof temperature at different applied electric fields (((e) to (h)). Fig.11: Variation of the electrocaloric responsivity coefficient and polarization versus Ca content at cubic-to-tetragonal phase transition temperature. The inset shows a linear dependence of the tolerance factor versus composition. Fig.12. (color online) (a) to (d) Pyroelectric coefficient and the corresponding calculated polarization as function of temperature for 5BCZT, 10BCZT, 15BCZT and 20BCZT ceramics measured at E= 4kV/cm; (e) and (h) represent the corresponding electrocaloric temperature change ∆T versus temperature calculated from pyroelectric current measurements. 37

ACCEPTED MANUSCRIPT Fig.13. (a) Electrocaloric temperature change ∆T as a function of temperature at 4 and 8 kV/cm for 5BCZT obtained by direct method; (b) electrocaloric responsivity as function temperature for 5BCZT at E = 4 kV/cm; (c) and (d) comparison between ∆T obtained by direct and indirect measurements as a function of temperature for 5BCZT.

Table captions Table 1: Positions of the different modes of BCZT system

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Fig.14: (a) Piezoelectric coefficient d33 as a function of the composition at room temperature and at different applied electric fields;(b) comparison of Pr, EC and d33 as a function of Ca content at 8 kV/cm.

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Table 2: Details of Rietveld refinement conditions of the cubic, tetragonal and orthorhombic structures Ba0.95Ca0.05Zr0.1Ti0.9O3 at 373, 348 and 333K Table 3: Refined structural parameters for Ba0.95Ca0.05Zr0.1Ti0.9O3 at 373, 348 and 333K.

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Table 4: Selected inter-atomic distances (Å) Ba0.95Ca0.05Zr0.1Ti0.9O3 at 373, 348 and 333K. Table 5: Comparison of ECE obtained in this work with those reported in literature for various BCZT compositions. Table 6. Comparison of electrocaloric parameters obtained from P-E hysteresis loops and pyroelectric methods for BCZT system

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Table 7. Comparison of electrocaloric parameters obtained from P-E hysteresis loops, pyroelectric and direct methods for 5BCZT.

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