Ferroelectric and piezoelectric properties of Ca2+ and Sn4+ substituted BaTiO3 lead-free electroceramics on the emphasis of phase coexistence

Journal Pre-proof 2+ 4+ Ferroelectric and piezoelectric properties of Ca and Sn substituted BaTiO3 leadfree electroceramics on the emphasis of phase coexistence Pravin S. Kadhane, Bharat G. Baraskar, Tulshidas C. Darvade, Ajit R. James, Rahul C. Kambale PII:

S0038-1098(19)30618-0

DOI:

https://doi.org/10.1016/j.ssc.2019.113797

Reference:

SSC 113797

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Solid State Communications

Received Date: 13 July 2019 Revised Date:

12 November 2019

Accepted Date: 15 November 2019

Please cite this article as: P.S. Kadhane, B.G. Baraskar, T.C. Darvade, A.R. James, R.C. Kambale, 2+ 4+ Ferroelectric and piezoelectric properties of Ca and Sn substituted BaTiO3 lead-free electroceramics on the emphasis of phase coexistence, Solid State Communications (2019), doi: https:// doi.org/10.1016/j.ssc.2019.113797. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

Ferroelectric and piezoelectric properties of Ca2+ and Sn4+ substituted BaTiO3 lead-free electroceramics on the emphasis of phase coexistence Pravin S. Kadhane1, Bharat G. Baraskar1, Tulshidas C. Darvade1, Ajit R. James2, and Rahul C. Kambale1* 1

Department of Physics, Savitribai Phule Pune University, Pune 411 007, Maharashtra, India 2

Defence Metallurgical Research Laboratory, Hyderabad 500 058, Telangana, India *

E-mail: [email protected]; [email protected]

Abstract The Ba1-xCaxTi1-ySnyO3 (abbreviated as BCxTSy, x=0.01, y=0.01; x=0.03, y=0.015; x=0.05, y=0.02; and x=0.07, y=0.025 mol.) electroceramics were synthesized by solid-state reaction method and investigated their structural, dielectric, ferroelectric and piezoelectric properties. Rietveld refinement for X-ray diffraction data for the composition x=0.03, y=0.015 reveals the phase co-existence of two non-centrosymmetric orthorhombic (Amm2) (7.71%) + tetragonal (P4mm) (92.29%) lattice symmetries respectively near room temperature which is also evidenced by temperature-dependent Raman and dielectric studies. All compositions reveal the dense microstructure having relative density ~ 92%-96% and average grain size ~ 10.7 - 22.5 µm. -The phase diagram based on dielectric studies suggests that the composition x=0.03, y=0.015 reveals T(R-O) ~ -60 oC , T(O-T) ~ 19 oC and TC ~ 126 oC with higher Pr = 11.80 µC/cm2, Ec = 3.5 kV/cm, Pmax = 21.92 µC/cm2, d33* = 505.5 pm/V, d33 = 287 pC/N and electrostrictive coefficient (Q33) = 0.036 m4/C2 properties which could be useful for developing the piezoelectric Ac device application. Here, we have achieved the phasecoexistence of two non-centrosymmetric lattice symmetries near room temperature to invoke the reliable ferroelectric and piezoelectric properties and discussed the results with the structure-property-composition relation. Keywords: BaTiO3; Curie temperature; Ferroelectrics and Piezoelectrics; Raman Spectra.

1. Introduction PbZrxTi1-xO3 (PZT) materials have the capability to couple mechanical and electrical polarization in response to an applied mechanical stress and/or mechanical strain in response to the applied electric field [1, 2]. PZT ceramics with Ti:Zr = 48:52 composition exhibits the morphotropic phase boundary (MPB) having dielectric permittivity (εr ≥ 700), piezoelectric coefficient (d33~250-600 pC/N) and electromechanical coupling factor (kp ≥ 0.50) [1- 4]. Due to such excellent properties of PZT it has been used to date in many electronic devices, however, PZT comes with critical drawbacks of toxicity since it contains more than 60 % lead (Pb) by weight and it is a toxic element. With increasing globalized use of PZT as piezoelectric material in electronics industries, releases more and more lead contamination mainly in the form of lead oxide into the environment [5]. To avoid the toxic effects of lead and lead-based materials, worldwide regulatory agencies and European Union (EU) restricted the use of lead under restriction of hazardous substances (RoHS) legislation [1,2]. As an alternative to lead-based material, recently lead-free BaTiO3 (BT) is mostly investigated ceramics for their electrical and electromechanical properties, can be useful in sonar, actuator, transducer, sensor devices. However, it is well recognized that the dielectric, ferroelectric and piezoelectric properties of pure phase BaTiO3 are inferior than the lead-based ceramics, also has major issue of lower Curie temperature (TC) ~ 75 ºC -120 ºC [1-6]. Whenever scientific community tried to improve the dielectric, ferroelectric and piezoelectric properties of BT based ceramics the mainly used substituents are Ca2+ and Zr4+/Sn4+ at A and B sites of ABO3 structure respectively. Also, it is well understood that the substitution of Zr4+/ Sn4+ for Ti4+ significantly improves the dielectric and piezoelectric properties of BT based ceramics, but suddenly lowers the TC ~ 75-90 oC which hampers the use of BT based ceramics for real practical applications [1-3]. Therefore, an attempt should make in context to increase the TC or to control the TC with improved ferroelectric and piezoelectric properties for BT based

ceramics [1-4]. Thus, to improve the properties of BT ceramics many attempts such as composition modification, synthesis techniques are used by researchers to achieve the morphotropic phase boundary (MPB) composition or to shift the polymorphic phase transition (PPT) temperatures close to room temperature to get the phase coexistence of two ferroelectric lattices. Polarization is depended on total number of axes of polarization present in unit cells of ceramic material. For tetragonal crystal symmetry there are 6 axes of polarization, orthorhombic crystal symmetry gives 12 axes of polarization while rhombohedral crystal symmetry has 8 axes of polarization. Coexistence of two-phase results addition of polarization axes enhances the polarization and so ferroelectric and piezoelectric properties of ceramics [1-3]. A new approach of cation replacement of host species by a suitable isovalent guest cation at dodecahedral A site and octahedral B site of the perovskite i.e. ABO3 structure in stoichiometric proportion is being under investigation. Although the dopant concentration is very small it modifies the structure of host species, causes the enhancement in the dielectric, ferroelectric and piezoelectric properties of the material [7,8]. In 1952 Berlincourt and Kulcsar reported that 20 mol.% Ca2+ doping for dodecahedral Ba2+ in BaTiO3 gives a negligible change in Curie temperature [9]. Later, in 1987 Zhuang et.al. found that doping of very small quantity (5 mol. %) at the octahedral side of Ti4+ gives the diffused phase transition with drastic decrease in Curie temperature [10]. S. Markovic in 2007 reported that the dielectric permittivity of the BaTiO3 can be improved by the partial replacement of octahedral Ti4+ by Sn4+ (15 mol. %) which can be used as a lead-free relaxor, for ceramics capacitor and sonar application [11]. Tiwari et.al. in 1989 and Varatharajan et.al. in 2000 reported that Ca2+ substitution for Ba2+ in BaTiO3 ceramic causes slightly increase in the TC [11-13]. Therefore Ca2+ can play an important role in stabilization of the Curie temperature. With substitution of Sn4+ and Zr4+ at B site of BaTiO3 the tetragonal to cubic phase transition

temperature (Tc) abruptly decreases, while orthorhombic to tetragonal (TO-T) and rhombohedral to orthorhombic (TR-O) phase transition temperatures increases [14]. Among all the approaches of improving the ferroelectric and piezoelectric properties of BaTiO3 and stabilization of its Curie temperature becomes the challenging task as well as need for today’s scenario. Therefore, concerning to above-mentioned issues of BT based ceramics especially TC, we have tried to investigate the ferroelectric, dielectric and piezoelectric properties of BT ceramics by simultaneous substitution of Ca2+ and Sn4+ at A and B sites of ABO3 structure respectively. Under this investigation we could able to control the TC in between 120 oC to 126 oC along with reliable dielectric and piezoelectric properties of BT based ceramics and tried to explain the observed results by structure-composition-property relation along with the phase diagram. 2. Experimental details The (Ba1-xCax) (Ti1-ySny) O3 (BCxTSy, x=0.01, y=0.01; x=0.03, y=0.015; x=0.05, y=0.02; and x=0.07, y=0.025 mol.) electroceramics were synthesized by solid-state reaction. The raw materials, BaCO3 (99%), CaCO3 (99%), TiO2 (99%), and SnO2 (99.9%) (all are analytical reagent grade from Sigma Aldrich) were weighted in their stoichiometric proportion and ball milled for 24 hrs. using ethanol as a milling medium. The ball-milled mixture was dried at 80 ºC overnight and grounded for 1 hr. This grounded powder was calcined at 1200 ºC for 10 hours with heating and cooling rate of 2 ºC/min. For proper homogeneity, the calcined powder was grounded further and again ball milled for 24 hrs. The ball-milled calcined powder was dried at 80 ºC overnight then pelletized having 10 mm diameter and 0.8 – 1 mm thickness and sintered at 1300 ºC for 10 hours with heating and cooling rate of 2 ºC/min in air atmosphere. Phase formation was examined using the X-ray diffraction (XRD) with a CuKα radiation (λ=1.5406 Å; D8 Advance; Bruker Inc., Karlsruhe, Germany) in the range of

scattering angle 2θ = 20º-80º with 2θ step increment of 0.02º/s recorded at room temperature. The bulk density was calculated by using liquid immersion techniques with Xylene liquid. Raman measurements were performed using Raman spectroscopy (Renishaw In Via microscope Raman) in the range of 100-1000 cm-1 using a He-Ne laser of 532 nm line. To obtain temperature-dependent data, a Linkam heating-cooling stage (THMS 600) ensuring a temperature stability ± 0.1 ºC was used. Surface morphology and microstructure was observed using scanning electron microscopy (JEOL-JSM 6306A, Tokyo, Japan). To ensure better ohmic contacts during electrical property measurement the silver paste was applied on both sides of the well-polished pellets and fired at temperature 550 ºC for 30 minutes. The dielectric constant (εr) was measured as a function of temperature from -180 ºC to 180 ºC at frequency 10 kHz using inductance-capacitance resistance (LCR) meter (HIOKI- 3532-50, Nagano, Japan). The polarization-electric field i.e. P-E hysteresis loop and the strain induced by applied electric field (S-E curves) were measured on unpoled samples at frequency of 0.1 Hz using ferroelectric test system (TF Analyzer 2000 of M/s. aixAcct Systems, GmbH, Germany). The synthesized samples were electrically poled at D.C. bias of 1 kV for 30 minutes and after 24 hrs. of aging the piezoelectric coefficient (d33) was measured using the YE 2730A d33 meter (APC international Ltd.) at applied force of 250mN. 3. Results and discussion 3.1. Structural and microstructural analysis Figure 1 shows the room temperature Rietveld refined X-ray diffraction (XRD) patterns for Ba1-xCaxTi1-ySnyO3 (BCxTSy, x=0.01, y=0.01; x=0.03, y=0.015; x=0.05, y=0.02; and x=0.07, y=0.025; mol.) electroceramic system. The BCxTSy compositions reveal the perovskite structure having some minor trace of undiffused CaTiO3 phase as indicated by * in supporting Figure S1 [15-18]. The Rietveld refinement of XRD data for compositions x=0.01, y =0.01; x=0.05, y=0.02; and x=0.07, y=0.025 was in good agreement with the

tetragonal structure P4mm (ICSD# 168783) symmetry [18-20]. While for composition x = 0.03, y=0.015 refinement made using individual P4mm symmetry as well as Amm2 symmetry does not showing the satisfactory agreement with the observed and calculated profiles. However, the composition x=0.03, y=0.015 reveals the best Rietveld refinement fit to the combination of orthorhombic (Amm2-ICSD#186460) (7.71%) and tetragonal (P4mm ICSD#168783) lattice symmetries. Therefore, the composition x = 0.03, y 0.015 is showing the phase coexistence of P4mm and Amm2 lattice symmetries of tetragonal and orthorhombic crystal structure. The similar results of phase coexistence of P4mm-Amm2 near room temperature for Ca2+ and Sn4+ modified BaTiO3 ceramics by using neutron and XRD diffraction profiles are reported earlier in the literature [7, 21-23]. The Rietveld refined structural parameters are shown in Table 1. In the present work, the simultaneous substitution of Ca2+ and Sn4+ in BaTiO3 responsible to shift the orthorhombic-tetragonal TO-T phase transition near room temperature for BCxTSy ceramic with x=0.03, y =0.015 content [18, 21]. The TO-T phase coexistence near room temperature has clearly observed from enlarged x-ray diffraction patterns in the 2θ range 73o-77o as shown in Figure S1 [7,18]. As shown in figure S1 it is clearly seen that the compositions with x=0.01, y =0.01; x=0.05, y=0.02; and x=0.07, y=0.025 reveals the tetragonal lattice symmetry with higher intensity (103) reflection than the (310) reflection at 2θ~75o [7, 15, 24-25]. However, the composition x=0.03, y=0.015 depicts the orthorhombic-tetragonal lattice mix phase symmetry having higher intensity (311) reflection than the (133) reflection at 2θ~75o [7, 18]. The lattice constants of BCxTSy ceramics were calculated and tabulated in Table 1. The minute variations observed in c/a ratio between 1.0086 to 1.0074 because of distortion and shrinkage due to the simultaneous substitution of Ca2+ and Sn4+ cations in pure BaTiO3 unit cell [26-28]. The theoretical X-ray density has calculated, which varying in between 6.01 g/cm3 for the composition x=0.01, y= 0.01 to 5.94 g/cm3 for the composition x = 0.07, y =

0.025. The observed actual densities calculated by using Archimedes principle, are in the range of 5.82 g/cm3 to 5.51 g/cm3 for BCxTSy ceramics. The bulk density in accordance with the theoretical and actual density for all the samples lies in between 92.77 % to 96.84 %. The values of theoretical, actual and bulk densities of BCxTSy ceramics are tabulated in Table 2. Figure 2 (a) shows the room temperature Raman spectra for the BCxTSy ceramics. The spectrum recorded at room temperature of BCxTSy ceramics show the presence of υ4(LO), υ2(LO, TO), υ1(TO), and υ1(LO) Raman-active modes situated at around 256 cm-1, 306 cm-1, 517 cm-1, and 715 cm-1 respectively which corresponds to tetragonal (T) phase characteristics [29-30]. Because of the non-uniform grain size distribution in BCxTSy ceramics, the directions of the phonon wave vectors are randomly distributed from one grain to another with respect to the crystallographic axes. As a result, the Raman lines in ceramics resulting from mode mixing and long-range electrostatic force effects are responsible for a broadening of the lines [31]. Figure 2(b-e) shows the Raman spectra of BCxTSy ceramic for composition x = 0.03, y = 0.015, was recorded in the temperature range -100 oC to 150 oC, showing the various phase changes. Figure 2(f) shows the position of observed modes and the ferroelectric transition temperatures. The spectrum measured at -100 oC shows the presence of Raman-active modes υ3(LO), υ3(TO), υ4(LO), υ2(LO, TO), υ1(TO), and υ1(LO) situated at around 176 cm-1, 189 cm-1, 252 cm-1, 306 cm-1, 519 cm-1, and 717 cm-1, respectively, which correspond to rhombohedral (R)-phase characteristics [29]. With increasing test temperature (Tt) up to -50 ⁰C the υ3(LO) peak of wavenumber 176 cm-1 decreases its sharpness and vanishes above -50 ⁰C, which indicates that the crystal structure changes from rhombohedral to orthorhombic phase (O). However, near room temperature 25 oC the υ3(TO) peak disappears and the spectra shows only υ4(LO), υ2(LO, TO), υ1(TO), and υ1(LO) situated at around 256 cm-1, 306 cm-1,

517 cm-1, and 715 cm-1 respectively, these four Raman active modes which clearly suggest that there is a phase transition from orthorhombic phase (O) to tetragonal phase (T). This result is supported by Rietveld refined XRD study which also shows orthorhombic-tetragonal phase coexistence near room temperature for composition x = 0.03, y = 0.015. In paraelectric cubic phase normal modes are not Raman active and decrease the sharpness with increasing the broadness of the peak. Above Tt 125 ⁰C, sharpness of υ2(LO, TO) peak at 306 cm-1 decreases and observed two broad peaks of υ1(TO), υ4(LO) at 519 cm-1 and 252 cm-1 respectively. This reveals that for BCxTSy ceramic at Tt ≥ 125 ⁰C phase changes from Tphase to cubic (C) phase i.e. ferroelectric to paraelectric phase transition. The modes of phonon vibrations in Raman spectra strongly depend on temperature and this dependency of phonon vibration modes is useful to interpret the change in structural properties of ferroelectric material [29-32]. Figure 3 shows the micrographs of BCxTSy electroceramics sintered at 1300 ˚C. All the samples reveal the pores free dense microstructure with definite grain boundaries having average grain size in the range of 10.7 µm to 22.4 µm and are given in Table 2. It is observed that simultaneous substitution of Ca2+ and Sn4+ make significant changes in microstructure. It is well recognized and understood that the grain size and its variation affect the piezoelectric properties of BT based ceramics [33-34]. Ceramics with fine average grain size ≤ 10µm possess regular laminar domain structure which enhances the piezoelectric properties of material [35]. For BCxTSy ceramics the composition x=0.03 and y=0.015 possesses the average grain size of ~10.72 µm which can be favorable for enhancing the ferroelectric and piezoelectric properties.

3.2. Dielectric properties

Figure 4(a-d) shows dielectric constant as a function of temperature for BCxTSy ceramics measured at a frequency of 10 kHz in the temperature range of -180 ºC to 180 ºC. All the BCTS ceramics reveal polymorphic phase transition behavior having three-phase transitions namely rhombohedral to orthorhombic (TR-O), orthorhombic to tetragonal (TO-T) and tetragonal to cubic (TC). The Curie temperature (TC) values of BCxTSy ceramics are varying between 120 ºC to 126 ºC, which does not show a significant increase or decrease, indicates the control of TC. The closer look in ferroelectrics reveals the dependency of Curie temperature on tolerance factor (t). Tolerance factor (t) values are shown in Table 2. A decrease in tolerance factor value improves packing in the unit cell that supports to increase in interaction between ions. At the same time it increases the bond strength between ions, which results large energy for giving rise to ferroelectric to paraelectric structural changes and thus the Curie temperature (TC) is stabilized [36-44]. In the present work we observed the decrease of tolerance factor with increase of Ca2+ and Sn4+ substitution with control of Tc between 120 ºC to 126 ºC. The results of temperature-dependent dielectric properties for 10 kHz are tabulated in Table 2. The phase diagram was developed on the basis of observed dielectric results and shown in Figure 4(e). The phase diagram consists of three ferroelectric phases R, O, and T and one cubic paraelectric phase. From the phase diagram it is seen that initially T(O-T) phase transition temperature approaches near to room temperature up to the composition x=0.03, y=0.015 and then decreases, while the T(R-O) phase transition temperature suppress to lower temperature indicates the dominating effect of Ca2+ than Sn4+ due to the higher concentration of Ca2+ than Sn4+ in BCxTSy ceramics. From the temperature-dependent dielectric study and developed phase diagram of BCxTSy ceramics observed that the composition x=0.01, y=0.01; and x=0.03, y=0.015 have the orthorhombic to tetragonal phase transition temperature at 16ºC and 19ºC respectively. This confirms the mixed phase of P4mm and Amm2 crystal

symmetry also has additional evidence from Rietveld refinement of x-ray diffraction pattern and Raman study as shown in supporting Figure S2. 3.3. Ferroelectric properties Figure 5(a-d)

shows the room temperature polarization-electric field i.e. P-E

hysteresis curves and Figure 5(e-h) shows the displacement current density for different applied electric voltages at 0.1 Hz for BCxTSy ceramics sintered at 1300 ºC.. With an applied electric field, the ferroelectric domain orientation occurs along the applied field direction resulting in electric field-induced polarization hysteresis loop [45-46]. The ferroelectric parameters of BCxTSy ceramics are calculated from P-E hysteresis loops and tabulated in Table 2. Among all the compositions, x=0.03 and y = 0.015 exhibits higher remnant polarization (Pr = 11.80 µC/cm2), lower coercive electric field (Ec = 3.5 kV/cm), and maximum polarization (Pmax = 21.92 µC/cm2), due to the phase co-existence of noncentrosymmetric orthorhombic-tetragonal (O-T) lattice symmetries near room temperature as evidenced from XRD, Raman and dielectric studies. For the composition x=0.03, y=0.015 tetragonal structure gives 6 axes of polarization and the orthorhombic structure gives 12 axes of polarization. Thus, there are total 18 axes of polarization along polarization axis results in easy switching of dipoles which enhances the effective polarization [1, 6, 45]. Lower Ec values reveal lower energy barriers are needed for polarization rotation which effectively improves the piezoelectric properties. The uniform grain size with a highly dense microstructure observed for sample with x = 0.03, y = 0.015 also contributes to the enhancement of the polarization. Because the uniform grain size and dense microstructure reduces leakage current this effectively increases polarization values [32, 47]. Displacement current density (J) as a function of applied electric field gives a sharp peak in forward and reverse cycles which indicates cyclic and uniform nature of polarization along both directions. The variation and nature of J-E loop with an applied field gives an idea

about the saturation state of the ceramics by revealing the sharp peaking behavior [7]. From Figure 5(e-h) it is observed that displacement current density with respect to applied electric field for all BCxTSy electroceramics shows well saturated J-E loop. The observed values of current density for BCxTSy ceramics are tabulated in Table 2. The composition, x= 0.03 and y= 0.015 shows maximum average current density having value, J = 8.835 × 10-5 A/cm2. The maximum value of current density (J) for the composition x= 0.03 and y= 0.015 indicates that a maximum number of charges accumulated on surface which effectively increases the current and thus current density. Such a high current density is a sign of low leakage current during measurement [7, 46]. 3.4. Piezoelectric and Electrostrictive properties The bipolar stain versus electric field nature of all BCxTSy electroceramics measured for various applied electric voltages and at 0.1 Hz is shown in Figure 6. All the composition reveals a typical butterfly loop, a well-known effect of dual-polarization (bipolar) for ceramics material is due to the switching and movement of domain walls [41]. Negative strain observes in all BCxTSy compositions which is significant for ferroelectric material to be a piezoelectric for actuator application. A careful observation of S-E plots indicates that the butterfly shape is not symmetric for the positive and negative cycle of electric field. Thus, it is important to take average value of strain for determining converse piezoelectric constant ∗ ൭݀ଷଷ =

ܵ௠௔௫ ൗ‫ܧ‬ ൱ from positive and negative cycle of electric field [7, 32]. ௠௔௫

∗ The observed values of ݀ଷଷ for BCxTSy ceramics are tabulated in Table 2. Initially the

converse piezoelectric constant increases from composition x= 0.01, y= 0.01 up to the composition x= 0.03, y= 0.015 and then start to decrease with increasing concentration of ∗ Ca2+ and Sn4+. The maximum value of ݀ଷଷ ~ 505.5 pm/V observed for the composition x=

0.03, y= 0.015 is due to orthorhombic and tetragonal mixed-phase near to room temperature,

which is suitable for displacement of the central Ti4+/Sn4+ atom along the 18 polarization axes. Thus, this composition shows good ferroelectric and piezoelectric properties as ∗ compare to the other compositions [1, 6, 48]. The maximum ݀ଷଷ value and lower coercive

electric field (3.50 kV/cm) of composition x=0.03, y=0.015 reveals that low electric field is requires generating strain in the material and this because of switching maximum number of dipole in the direction of applied electric field is possible at low electric field [32, 47]. The observed values of direct piezoelectric coefficient ሺ݀ଷଷ ሻfor BCxTSy electroceramics are

shown in Table 2. Initially the ݀ଷଷ value increases up to 287pC/N for the composition x= 0.03, y= 0.015, after this composition while increasing concentration up to x= 0.07, y= 0.025 the piezoelectric constant (d33) decreases linearly. The maximum value of piezoelectric constant (d33 = 287pC/N) for the composition x= 0.03 and y= 0.015 is due to phase coexistence as well as highly dense microstructure with uniform grain size as compare to other electroceramics [3, 32, 47].

Figure 7 shows the strain vs polarization i.e. S-P curves for BCxTSy polycrystalline ceramics measured under different electric voltages at 0.1 Hz. The domain wall motion intensively contributes to the polarization at lower electric field, but not all domain wall rotation can benefit the polarization. The 180º domain wall motion is unfavorable for elastic deformation (strain) while the domains wall with non-180º orientation takes part in the polarization. Thus, there are six and twelve polarization axes for tetragonal and orthorhombic crystal symmetry respectively, which contribute the strain and thus strain-induced polarization [6, 45, 49]. The observed results for electrostrictive coefficient (Q33) for the electric voltage 2.5 kV at a frequency of 0.1 Hz are tabulated in Table 2. The strain versus polarization gives typical ferroelectric and piezoelectric nature of ceramics shows double parabola shape and the area under the parabola indicates the hysteresis. The double parabola shaped strain-polarization, hysteresis loop implies that polarization and strain are not in phase

as shown in Figure 7. The electrostrictive coefficient is calculated using formula Q33 = ∆S / ∆ P2, gives value 0.041m4/C2, 0.036 m4/C2, 0.038 m4/C2, 0.042 m4/C2, for BCxTSy electroceramics and tabulated in Table 1. The enhancement in the Q33 value indicates that for an applied electric field of 35kV/cm the BCxTSy electroceramics generates maximum strain [49]. Faruta A. (in 1993) and S. Zheng et al. (in 2014) states that the electrostrictive coefficient (Q33) for lead-based material (PMN-PT) is about 0.017 m4/C2, also Wangfeng Bai et al. states that electrostrictive coefficient (Q33) for BNT-BT-(Sr0.7Bi0.2)TiO3 (BNT-BT-SBT) relaxor ferroelectrics is about 0.029 m4/C2, thus it is clear that the observed Q33 values of BCxTSy evidently larger than that of lead-based electroceramics materials [50-52]. Thus, with this improved maximum electrostrictive coefficient be a sign of replacement of lead-based actuators by Ca2+ and Sn4+ modified BaTiO3 electroceramics. The ferroelectric and piezoelectric properties of recently reported BaTiO3-based piezoelectric materials are compared in Table 3 [32, 36, 53-59]. It is observed that BCxTSy ceramics show the high electrostrictive coefficient in the range of Q33 = 0.036-0.042 m4/C2 with good values of direct piezoelectric coefficient d 33 = 216 − 287 pC / N and converse piezoelectric coefficient

d *33 = 371 − 505 pm / V as compare to lead based as well as lead free piezoelectric ceramics. However, here it can be noted that the sintering temperature of our BCxTSy electroceramic system is 1300 oC which is lower than the other high performance BT based piezoelectric materials [32, 36, 53-59].

4. Conclusions The Ba1-xCaxTi1-ySnyO3 (BCxTSy) system was successfully synthesized by solid-state reaction method. The Rietveld refinement of X-ray diffraction, temperature-dependent micro Raman spectroscopy, and dielectric measurement study reveal the phase co-existence of tetragonal (P4mm) and orthorhombic (Amm2) non-centrosymmetric lattice symmetries for x = 0.03, y = 0.015 composition near room temperature. Temperature-dependent dielectric measurements

suggested that the incorporation of Ca2+ and Sn4+ simultaneously at A and B sites of BT respectively results in the stabilization of TC in between 120 oC to 126 oC with improved ferroelectric and piezoelectric properties than pure BaTiO3. The composition x = 0.03 and y = 0.015 showed the remarkable properties having T(R-O) ~ -60 oC , T(O-T) ~ 19 oC and TC ~ 126 ∗ C with higher Pr = 11.80 µC/cm2, Ec = 3.5 kV/cm, Pmax = 21.92 µC/cm2, ݀ଷଷ = 505.5 pm/V,

o

݀ଷଷ = 287pC/N and electrostrictive coefficient (Q33) = 0.036m4/C2 which could be useful for piezoelectric AC device application. Thus, BCxTSy ceramics can be potential candidate with TC above 100 oC and having reliable ferroelectric, piezoelectric and electrostrictive properties due to the phase co-existence near room temperature to replace hazardous lead-based piezoelectric materials. Acknowledgment RCK thankfully acknowledge the Science and Engineering Research Board (SERB)DST, Government of India (File No. EMR/2016/001750) for providing the research funds under the Extra Mural Research Funding (Individual Centric) scheme. Compliance with ethical standards Conflict of interest: The authors declare that they have no conflict of interest.

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Figure Captions Figure 1 (a-d): (Color Online) Rietveld fitted x-ray diffraction patterns of BCxTSy ceramics for

(a) x = 0.01, y = 0.01 (c) x = 0.05, y = 0.02 (d) x = 0.07, y = 0.025 using the P4mm

model and (b) x = 0.03, y = 0.015 using the P4mm+Amm2 model. Figure 2: (a) Room temperature Raman spectra of BCxTSy ceramics. (b-e) Temperature dependent Raman spectra of BCxTSy ceramic for composition x =0.03, y = 0.015. (f) Position of observed modes and the ferroelectric transition temperatures. Figure 3(a-d): SEM micrographs of BCxTSy ceramics sintered at 1300oC. Figure 4: (a-d) Temperature dependence dielectric constant (εr) for the BCxTSy ceramics measured in the temperature range of -180 oC to 180 oC at frequency 10 kHz. (e) Phase diagram for Ba1-xCaxTi1-ySnyO3 ceramics. Figure 5: (a-d) Variation of polarization and (e-h) Variation of displacement current density with different applied electric voltages at 0.1 Hz for BCxTSy ceramics. Figure 6: Bipolar electric field induced strain hysteresis loops measured at various applied electric voltages and at 0.1 Hz for BCxTSy ceramics. Figure 7: S-P curves under different electric voltages at 0.1 Hz for BCxTSy ceramics.

Table 1: Rietveld refined parameters of Ba1-xCaxTi1-ySnyO3 ceramics X=0.01,

X=0.03,

X=0.05,

X=0.07,

y=0.01

y=0.015

y=0.02

y=0.025

Parameter

Crystal system

Tetragonal

Orthorhombic

Tetragonal

Orthorhombic

Tetragonal

Tetragonal

Space group

P4mm

Amm2

P4mm

Amm2

P4mm

P4mm

Lattice constant a (Å)

3.9954

3.9714

3.9917

4.0028

3.912

3.9925

Lattice constant b (Å)

3.9954

5.6419

3.9917

5.7012

3.9912

3.9925

Lattice constant c (Å)

4.0288

5.6613

4.0261

5.7890

4.0256

4.0266

Unit cell volume ( Å)3

64.3126

126.8484

64.1507

132.1075

64.1290

64.1850

Tetragonality (c/a)

1.0083

1.4255

1.0086

1.4462

1.0086

1.0085

Content ( %)

97.11

2.29

92.29

7.71

100

100

1.66

1.74

χ2

3.81

Atomic position Ba/Ca

Ti/Sn

O1

O2

2.25

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

y

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

z

0.0000

0.0000

0.0000

0.0000

0.0000

0.0000

x

0.5000

0.5000

0.5000

0.5000

0.5000

0.5000

y

0.5000

0.0000

0.5000

0.0000

0.5000

0.5000

z

0.5359

0.5093

0.5370

0.5093

0.5370

0.5370

x

0.5000

0.0000

0.5000

0.0000

0.5000

0.5000

y

0.5000

0.0000

0.5000

0.0000

0.5000

0.5000

z

0.0000

0.4915

0.0370

0.4915

0.0370

0.0370

x

0.5000

0.5000

0.5000

0.5000

0.5000

0.5000

y

0.0000

0.2422

0.0000

0.2422

0.0000

0.0000

z

0.5094

0.2370

0.5180

0.2370

0.5180

0.5180

x

Table 2: Density, microstructural, dielectric, ferroelectric, piezoelectric, and electrostrictive parameters of Ba1-xCaxTi1-ySnyO3 ceramics x = 0.01

x = 0.03

x = 0.05

x = 0.07

y = 0.01

y = 0.015

y = 0.02

y = 0.025

Theoretical density (g/cm3)

6.01

5.98

5.94

5.94

Apparent density (g/cm3)

5.82

5.72

5.69

5.51

Porosity (%)

3.16

4.34

4.02

7.23

Relative density (%)

96.84

95.66

95.98

92.77

Average grain size (µm)

12.34

10.72

14.75

22.42

Tolerance factor (Tetragonal)

1.2169

1.2135

1.2101

1.2067

εr (10 kHz, at Tc)

10246

9093

9699

10653

Parameter

T

(R-O)

˚C

-56

-60

-66

-80

T

(O-T)

˚C

16

19

6

-2

T

(T-C)

˚C

120

126

126

122

Pr (µC/cm2)

8.38

11.80

9.03

10.74

Ec (kV/cm)

3.74

3.50

3.74

5.62

Pmax (µC/cm2)

19.1

21.92

18.84

17.87

J (*10-5 A/cm2)

3.96

8.835

5.185

7.265

(d33 )ave(pC/N)

269

287

250

216

(d ⃰33 )ave(pm/V)

437.22

505.5

385.75

371.42

(Q33)ave (m4/c2)

0.041

0.036

0.038

0.042

Table 3: The electrical properties of recently reported BaTiO3- base piezoelectric ceramics Tc

d33

d33*

Q33

(kV/cm)

(pC/N)

(pm/V)

(m4/C2)

2.60

360

Structural phase

Ba0.984Ca0.016Ti0.976Sn0.024O3

1480

O-T

112

8

Ba0.97Ca0.03Ti0.94Sn0.06O3

1500

O-T

65

12.2

BZT-BCT-xBST

1430

R

60

6

1

BaTi0.95Zr0.05O3

1200

O

120

2.92

3.63

Ba0.98Ca0.02Ti0.96Sn0.04O3

1500

O-T

80

13.2

Ba0.98Ca0.02Ti0.95Sn0.05O3

1450

O-T

75

8.2

4.50

BaTi0.9Zr0.1O3

1400

95

10

2.50

288

0.045

57

BaTi0.96Zr0.04O3

1400

O

119

10

3

253

0.038

57

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

1420

O-T

120

9

8.70

195

Ba0.88Ca0.12Ti0.94Sn0.06O3

1450

O-T

68

5

2.62

220

650

0.041

59

Ba0.97Ca0.03Ti0.985Sn0.015O3

1300

O-T

126

11.80

3.50

287

505

0.036

This work

Ba0.93Ca0.07Ti0.975Sn0.025O3

1300

T

122

10.74

5.62

216

371

0.042

This work

(oC)

Pr (µC/cm2)

Ec

Sintering Temperature (oC)

BaTiO3 based Composition

300

Ref.

32

440

53

540 248

0.0366

54

0.0312

55

510

36

464

56

58

Research highlights

•

The phase co-existence of Orthorhombic (Amm2)-tetragonal (P4mm) lattice symmetries near room temperature is observed for Ba0.97Ca0.03Ti0.985Sn0.015O3 ceramic.

•

Temperature-dependent Raman and dielectric studies also supports the phase coexistence.

•

Relatively

higher

Curie

temperature

(Tc)

~

126

o

C

is

observed

for

Ba0.97Ca0.03Ti0.985Sn0.015O3 ceramic with d33 = 287 pC/N. •

Ferroelectric and piezoelectric response of BCxTSy ceramics seems to be improved in the vicinity of phase co-existence.

Ferroelectric and piezoelectric properties of Ca2+ and Sn4+ substituted BaTiO3 lead-free electroceramics on the emphasis of phase coexistence Pravin S. Kadhane1, Bharat G. Baraskar1, Tulshidas C. Darvade1, Ajit R. James2, and Rahul C. Kambale1* 1

Department of Physics, Savitribai Phule Pune University, Pune 411007, Maharashtra, India 2

Defence Metallurgical Research Laboratory, Hyderabad 500058, Telangana, India *

E-mail: [email protected]; [email protected]

Compliance with ethical standards Conflict of interest: The authors declare that they have no conflict of interest.