Relaxor behavior and electrothermal properties of Sn- and Nb-modified (Ba,Ca)TiO3 Pb-free ferroelectric

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Introduction
Relaxor ferroelectrics (or relaxors in short) are among the most successful electronic ceramics because of their highly attractive dielectric, piezoelectric, pyroelectric, and electrocaloric properties [1,2,3,4,5]. In addition, fundamental understanding of their physicochemical nature has drawn much attention because of various fascinating properties, such as a strong frequency-dependent maximum permittivity temperature (T M ),

Abhijit Pramanick
Abhijit Pramanick is an Assistant Professor at the City University of Hong Kong. He obtained his Bachelors in Engineering from the National Institute of Technology, India, and his Masters in Engineering from the Indian Institute of Science, Bangalore. He received his Ph.D. in Materials Science and Engineering in 2009 from the University of Florida, Gainesville, working under the mentorship of Prof. Jacob L Jones. After his Ph.D., he held a postdoctoral appointment in the research group of Prof. Scott Misture at the Alfred University in New York, USA. Subsequently, he moved to the Oak Ridge National Laboratory, where he spent three and half years working on the applications of different X-ray and neutron scattering techniques to understand the microscopic origins of functional responses in ferroelectric and ferromagnetic materials. He has coauthored more than 40 publications. For his work on in situ structural characterization of ferroelectric ceramics using X-ray diffraction, he was awarded the prestigious Edward C. Henry Award by the American Ceramic Society in the years 2010 and 2012. His current research group at City University of Hong Kong focuses on synthesis and characterization of structure-property correlations of advanced ceramic and polymeric materials for application in smart and energy technologies.
ª Materials Research Society 2020. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. deviation from Curie-Weiss law for ferroelectric phase transition, relatively large @P/@T over a broad temperature range, and a large electromechanical response [5,6,7,8]. The behavior of relaxors can be generally attributed to the presence of nanoscale polar clusters with correlated atomic disorder [9,10], and in this regard, relaxors can be broadly compared with spin glasses and dipolar glasses [11,12]. Lead (Pb)-based ABO 3 perovskites, with the general formula of Pb B 0 2þ 1=3 B 00 5þ 2=3 O 3 (B9 21 5 Mg, Co, Ni, Zn; B0 51 5 Nb, Ta) and their solid solutions with prototypical ferroelectric PbTiO 3 , are the most studied and technologically important relaxor ferroelectrics [13]. However, due to the negative environmental concerns surrounding the presence of Pb, design of new Pb-free relaxors has developed into an important research endeavor in materials science [14]. In this regard, investigation of Pb-free compounds with similar structural characteristics to Pb-based relaxors is a promising approach toward developing new functional materials.
One salient feature of Pb-based relaxors is the existence of cooperative multisite atomic displacements, which are correlated over only nanometer length scales. The cooperative atomic displacements can be understood based on the classical picture provided for Pb B 0 2þ 1=3 B 00 5þ 2=3 O 3 relaxors [15,16]. In this model, the octahedral B-site is occupied by two sets of ions: B9 21 ions which are larger and ferroelectrically inactive, and B0 51 ions which are smaller and ferroelectrically active. For certain arrangements of the B9 21 and B0 51 ions, the intermediate oxygen ions can be forced to move toward B0 51 , which consequently induces a movement of the A-site cation (Pb 21 in this case) toward B9 21 to form three shorter Pb-O-B9 21 and three longer Pb-O-B0 51 bonds. In this manner, the offcentered displacements of the A-site and B-site ions toward or away from the intermediate oxygen ions can lead to large electrical dipoles. At the same time, a disordered arrangement of B9 21 and B0 51 ensures that the dipoles are correlated over only short distances. The type of cooperative atomic displacements in Pb-based ferroelectrics described above is aided by two factors: (a) hybridization between the empty B0 51 dorbitals and the O 2À p-orbitals, which makes the B0-O bond ferroelectrically active in contrast to the inactive B9-O bond, and (b) the presence of lone pair electrons on Pb 21 , which favors the formation of shorter Pb-O bonds and thus accommodates the A-site displacements [15,17]. Replication of these mechanisms for Pb-free chemistries can be a rational step for replacement of Pb in many electronic ceramics; nevertheless, such a possibility has not been systematically investigated.
In the present work, we have synthesized and characterized ceramics of a new composition, namely, Nb-and Sn-doped (Ba,Ca)TiO 3 . BaTiO 3 is similar in structure to the prototype Pb-based ferroelectric PbTiO 3 near room temperature (RT), i.e., both compounds exhibit a tetragonal crystal structure with relatively large c/a ratio. However, there are significant differences between BaTiO 3 and the Pb-based ferroelectrics in terms of chemical bond formations [17,18]. In Pb-based ABO 3 compounds, the lone-pair 6s electrons on the A-site Pb atoms hybridize strongly with the O-2p orbitals, which reinforces cooperative atomic displacements. By contrast, the Ba-O bond in BaTiO 3 is more ionic in nature, which hinders the formation of shorter and longer A-O bonds. Nevertheless, A-site displacements in solid solutions of BaTiO 3 could be induced through various mechanisms. One such mechanism is quantum fluctuations, which can be introduced by forming a solid solution with ions of smaller radii. In this case, by substituting Ba 21 with Ca 21 , Ca 21 become off-centered along the h113i direction [19]. Another possibility is the stereochemical effect, which is introduced by incorporating small amounts of Sn 21 at the A site of BaTiO 3 . Similar to Pb 21 , Sn 21 has a lone-pair of electrons, and thus it can be expected to increase A-O bond covalency and consequently lead to off-centered atomic displacements [20,21]. Exclusive inclusion of Sn 21 onto the A site is however difficult because of disproportion of Sn 21 and Sn 41 .
Here, it has to be noted that a natural distribution between the two oxidation states has been reported earlier [22,23], and such a distribution can also be expected for Sn-doped (Ba,Ca) TiO 3 [24].
To induce cooperative atomic displacements as indicated in Fig. 1, we need to introduce different ions at the B site of BaTiO 3 , which form bonds to oxygen with different degrees of covalency [15,16]. As shown earlier by Cohen, hybridization between the empty d-orbitals of the transition metal ions and oxygen 2p states can form strong covalent bonds and stabilize ferroelectricity [17]. Here, Nb 51 is introduced at the B site to form stronger covalent bonds with oxygen. At the same time, Sn 41 at the B site is expected to form weakly covalent bonds because of lack of d and p orbitals' hybridization. In such a scenario, the larger O anions can be expected to move closer to Nb 51 ions and away from the Sn 41 ions, thereby facilitating the off-centered displacements of the A-site cations. The proposed mechanism is illustrated in Fig. 1.
Following the ideas outlined earlier, we successfully designed a solid solution composition of Nb-and Sn-doped (Ba,Ca)TiO 3 that exhibits relaxor-like properties with temperature for maximum permittivity (T m ) near RT. The relaxor-like properties of the material are confirmed from dielectric spectroscopy, specific heat capacity, and piezo force microscopy (PFM) measurements. The dielectric permittivity spectrum showed two anomalies, as evident from a peak at T m ; 300 K and a broad shoulder at a lower temperature T s ; 270 K. These two anomalies can be correlated to temperature dependent transition to a tetragonal polar phase and further increase in tetragonality in the polar phase, respectively, which are characterized from synchrotron X-ray diffraction. The pyroelectric coefficient (@P/@T) and electrocaloric properties for this material are furthermore examined. Interestingly, the material exhibits two peaks for temperature dependence of pyroelectric coefficient with values of -(@P/@T) ; 563 lC/(m 2 K) at T ; 270 K and -(@P/@T) ; 1021 lC/(m 2 K) at T ; 320 K, which correlates to the temperature-dependent structural changes. In addition, a broad peak for electrocaloric temperature change near 320 K is predicted from Maxwell relations, which gives rise to a high relative cooling power of ;17 J/kg. The study shows that length scale-specific structural changes can be induced at different temperature ranges in this material, which can be selectively used for pyroelectric energy harvesting or electrocaloric cooling.

Dielectric properties
The real part of dielectric permittivity (e9) and dissipation factor (tan d) of the BCST-Nb 0.04 samples were measured in the frequency (f) range of 100 Hz # f # 100 kHz and for temperature (T) range of 173-373 K. The results are shown in Fig. 2(a). The e9 versus T curve exhibits a main peak at T m ; 300 K, whereas an additional shoulder (T s ) is noticed between 225 and 270 K. We also observe that T m shifts up by 2.5 K, and the maximum permittivity at ;T m decreases in magnitude, when the frequency is increased from 0.1 to 100 kHz. This behavior provides evidence of the relaxor-like behavior for the BCST-Nb 0.04 ceramics [13,25,26,27].
In the case of normal ferroelectrics, the temperaturedependent dielectric permittivity follows the Curie-Weiss law; 1/e9 5 (T À T C )/C, where C is the Curie constant and T C is the Curie-Weiss temperature [28]. By contrast, the relaxation of the electric dipoles can lead to significant deviation from Curie-Weiss law above T m [29]. In such cases, e9 (T) can be modeled following the Uchino and Nomura formula, also known as the modified Curie-Weiss law [30].
where e m is the maximum of e9 at T m , and the coefficient c represents the degree of diffuseness for the transition across T m . c is equal to 1 for normal ferroelectric behavior, and it varies between 1 and 2 for relaxor ferroelectrics [30]. Accordingly, the c value has been evaluated from the slope of the log(1/e9 À 1/e9 m ) versus log(T À T m ) graph, (see Supplementary material Fig. S1).
The obtained value of c for the BCST-Nb 0.04 ceramics is 1.89 6 0.01 (f 5 1 kHz) (c is close to 2 at higher frequencies), which is comparable to that of other Pb-free ferroelectric relaxors listed in Table I [29,31,32,33,34,35]. The critical exponent c depends on the underlying ordering mechanism; c ; 1 for pure Curie-Weiss behavior or mean field theory, c ; 1.3 for 3D random bond Ising model, whereas c is higher for random field Ising models and approaches values close to 2 [36,37,38]. The relatively high values of c for the current material and others listed in Table I might be related to the strong random field interactions in these materials, although it needs to be carefully evaluated with high-precision dielectric measurements over broader frequency ranges.
To further examine the relaxor behavior in this material, we analyze the frequency dispersive dielectric permittivity using Arrhenius model and Vogel-Fulcher model [39,40]. The unphysical parameters obtained from the fitting analysis   x , E a is the activation energy, and k B are the Boltzman constant. Figure 2 Table I. The attempt jump frequency obtained in the present case are similar those observed for relaxor ferroelectrics, which lies between 10 9 Hz and 10 14 Hz [13,39,40,41]. The value of E a of BCST-Nb 0.04 is quite low compared with other Pb-free relaxor materials, e.g., 0.95BaTiO 3 -0.05Bi(Zn 2/3 Nb 1/3 )O 3 [29] and 0.8K 0.5 Na 0.5 NbO 3 -0.2Ba 0.5 Sr 0.5 TiO 3 [34], and signifies easier reorientation of local dipoles in the BCST-Nb 0.04 ceramics. The microscopic phase below T m can be furthermore ascertained from specific heat measurements. For transition below T m from polar nano regions (PNR) to a ferroelectric state with nanoscale domains, one can expect heat capacity C p in excess of what is predicted from classical thermodynamic laws [42,43]. By contrast, if no excess heat capacity is observed near T m , the nanoscale ferroelectric model can be dismissed in favor of a transition to a dipolar glass state. For the present system, a broad anomaly in the form of excess C p is observed in the range of 305-328 K [ Fig. 2(c)], which indicates transition from PNRs at higher temperatures to ferroelectric domains below T m . Nevertheless, the rather weak and broad nature of the peak for excess specific heat point to the lack of establishment of a true long-range order [44]. Piezoresponse force microscopy corroborates the findings above as only nanoscale domains can be observed at RT (see Fig. S3 in Supplementary material).

Analysis of average and local atomic structures
The structural transitions accompanying the different anomalies observed in the dielectric spectrum are analyzed using Xray diffraction and total scattering data, as presented below.

Average atomic structure
The average atomic structure was modeled from the X-ray diffraction patterns in the temperature range of 220 K # T # 380 K using Rietveld refinement implemented in the FULL-PROF suite software [45]. First, to determine if Sn disproportionate into Sn 21 and Sn 41 oxidation states, we performed the X-ray photoelectron spectroscopy (XPS) measurements. The XPS spectra of BCST-Nb 0.04 along with the pure SnO (Sn 21 ) and SnO 2 (Sn 41 ) are shown in the Supplementary material Fig. S4. Two peaks are observed for Sn in BCST-Nb 0.04 ceramics, indicated as Sn-3 d 5/2 and 3 d 3/2 . A high energy tail is present on the 3 d 3/2 peak, indicating that both oxidation states are present in the compound. Both peaks are deconvoluted into two peaks centered at 485.48 eV (P 1 ) and 485.68 eV (P 2 ), and 493.93 eV (P 3 ) and 495.15 eV (P 4 ), respectively. Peaks P 1 and P 3 correspond to Sn 21 , and peaks P 2 and P 4 correspond to Sn 41 . All of these closely match the reference energies obtained from SnO and SnO 2 [46,47]. From peak fitting analysis of XPS data, we estimate close to equal concentration of Sn 21 and Sn 41 (Sn 21 :Sn 41 5 0.57:0.43). These values are therefore used as initial parameters for concentration of Sn 21 and Sn 41 in Rietveld models. Figure 3(a) shows a representative fit for the diffraction pattern measured at 300 K. The peaks were fit using a Pseudo-Voigt profile. The background was fitted with a Chebychev polynomial. The scale factor, zero-point correction, isotropic thermal parameters, lattice parameters, and positional coordinates were refined. It was observed that from 220 to 360 K, the best fit was achieved for a tetragonal phase with P4mm space group, whereas for 380 K, the best fit was achieved for a cubic phase with the Pm 3m space group. All the structural parameters obtained from the final Rietveld refinement analysis are listed in Table II. Figure 3(b) shows "tetragonality" or "c/a" ratio of the structure as a function of temperature. The c/a ratio increases with decreasing temperature. However, the increase in not linear and a sharper increase is observed below T ; 270 K. The nonlinear deviation in the c/a ratio of the average structure below 270 K is most likely due to an increase in the mediumrange correlations arising as a result of growth of polar nanodomains. The onset of nonlinear increase in tetragonality coincides with T s in Fig. 2(a).
Above 320 K, we use dotted line to show the variation in c/ a ratio because no diffraction data were measured in the intervening temperatures. Nevertheless, the temperature- Local structure analysis from X-ray total scattering The temperature-dependent evolution of the local atomic structure within the polar nanodomains are analyzed using the X-ray pair distribution function (PDF) method [48]. Figure 4(a) shows the PDFs in the region 1.5 , r , 9.5 Å (full range shown in Fig. S5), in the temperature range of 80-400 K. Each of the different bond lengths contained within a single unit cell is labeled. The bond between the A-site and the 2nd nearest B-site atoms is also included because of its close proximity to the M-M 3 bond. The peaks between 8.3 , r , 9.5 Å correspond to contributions from all possible ionic pairs and relate to the medium range order [49]. At 80 K, each peak is well defined, and small peaks corresponding to split in A-O bond lengths are visible. A split in the A-O PDF peak demonstrates that substitution of Ca 21 and Sn 21 at the A site breaks the degeneracy of the A-O bond distances. At higher temperatures, the thermal motion increases, causing this detail to "smooth" out. Rietveld-type fits of the whole PDF data across the region 1.0 , r , 60.0 Å were carried out using the software TOPAS [50]. Representative fits are shown in Fig. 4(b) at 300 K for models of orthorhombic (Amm2), tetragonal (P4mm), and cubic Pm 3m ð Þ symmetries [51]. The residuals show that the orthorhombic structure produces the best fit. This indicates that although the long-range average structure (from Rietveld refinement of X-ray diffraction data) shows tetragonal symmetry, the local structure appears to have orthorhombic distortions. Similar behavior was also reported earlier by Laurita et al. for inclusion of Sn 21 at the A site, even at very small concentrations [21]. To further assess the length scales of the distortions, present in the structure, orthorhombic box car refinements were performed, and the results are shown in Fig. 4(c). Here, fitting boxes with r-ranges of 6.5 Å starting from 1 Å and overlapped by 0.5 Å were used. To have a clear comparison with the pseudotetragonal perovskite structure, the monoclinic equivalent lattice parameters were calculated for the orthorhombic unit cell [52]. Interestingly, the short-range lattice parameters are smaller than the long-range average, which then tends toward average values with the increase in r. An   (13) 3.983 (2) 3.984 (2) 3.985 (3) 3.986 (4) 3.987 (6) 3.989 (7) c (Å) 3.992 (2) 3.992 (4) 3.992 (4) 3.990 (6) 3.990 (4) 3.989 (12) 3.989 (7)  cambridge.org/JMR 5 explanation could be that the A-and B-site atoms have both positive and negative local correlations that result in the nearest-neighbor A-B and M-M peaks to display an overall cubic local structure, see Fig. 4(a). This indicates that the shortrange atomic correlations in this material are significantly more complicated than an orthorhombic model, which is rather unusual compared with other box-car studies based on relaxors [53] and warrants further investigation via other techniques, e.g., neutron PDF, and EXAFS or big box modeling.

Hysteresis loops and electrothermal properties
Relaxor ferroelectrics exhibit attractive electrothermal properties, viz. pyroelectric and electrocaloric effects. The pyroelectric effect is defined as a change in spontaneous polarization as function of temperature. The resultant short-circuit pyroelectric current i p is given by where A is a constant, @P/@T is the pyroelectric coefficient P(T) and dT/dt is the temporal rate of change in temperature. In recent years, pyroelectric effect has received much attention for potential application to recover electrical energy from waste heat [5]. The converse of pyroelectric effect is the electrocaloric effect or reversible change in temperature as a function of applied electric field. It is well known that the electrocaloric effect and pyroelectricity are related by the Maxwell relation in thermodynamics, and it can be expressed as: where S is the entropy, E is the applied electric field, and P is the total macroscopic polarization. Using the above relation, we can calculate the adiabatic temperature change DT and isothermal entropy change DS from the following equations: where P is the polarization, q is the mass density, T is the absolute temperature, E 1 and E 2 are the starting and final values of the electric fields, respectively, and C p is the heat capacity [54]. It can be seen that a large (@P/@T) over a broad temperature range is desirable for both pyroelectric energy recovery as well as electrocaloric applications. In this regard, the attractiveness of relaxor ferroelectrics becomes clear, as they exhibit a broad permittivity peak and relatively large slope of (@P/@T) over a broad temperature range. To characterize the electrothermal properties of BCST-Nb 0.04 ceramics, the polarization versus electric field (P-E) loops were recorded under ac fields of frequency 10 Hz and field amplitude of 50 kV [ Fig. 5(a)]. To avoid electrothermal history, after measuring the P-E loop at each temperature, the sample was again heated to the paraelectric state (.50°C) and then cooled down to the next desired measuring temperature. The remnant polarization (P max ) and maximum polarization (P r ) were extracted from the P-E loops. The results are shown in Figs. 5(b) and 5(c), respectively. Several interesting features can be noted from the temperature-dependent P max (T) and P r (T) curves. P max first increases and then decreases with increasing temperature, with the inflection point located at T s ; 270 K. The P r (T) continuously decreases with increasing temperature up to 320 K except for a deviation at T s ; 270 K. A small increase in P r appears above 320 K, which is an experimental artifact due to increase in leakage current. Above T m , decaying trends for both P max and P r are observed, which indicates deviation from longrange ordered ferroelectric state [55].
The pyroelectric coefficient (@P/@T) calculated from the P max (T) curve is shown in Fig. 6. A broad peak for pyroelectric coefficient near T s ; 270 K with -(@P/@T) ; 563 lC/(m 2 K) essentially results from an inflection in the P max versus T curve at this temperature. We note that this also correlates with nonlinear increase in c/a ratio near T s ; 270 K, which is likely due to transition to longer range domains. Similar equivalence between temperature-dependent and field-dependent transitions to longer range correlations was noted earlier for the prototypical relaxor PMN [45]. Therefore, we propose that the broad peak for DT at 270 K is likely due to enhanced susceptibility for electric field-induced growth of polar nanodomains. We observed a second peak for pyroelectric coefficient at T m ; 320 K with -(@P/@T) E ; 1021 lC/(m 2 K). The second peak can be correlated to the excess C p observed at ;320 K, where the system transforms from a cubic to a tetragonal structure as evident from analysis of the average crystallographic structure.
The two broad peaks for pyroelectric coefficient observed in the range of 270-325 K can be potentially useful for energy harvesting by utilizing the induced pyroelectric current from large thermal fluctuations, as indicated by Eq. (3). A useful figure of merit (FOM) for pyroelectric energy harvesting is given by [55] Using Eq. (7), we obtain F E ; 6.5 J/(m 3 K 2 ) for the peak at T ; 270 K and F E ; 17.5 J/(m 3 K 2 ) for the peak at T ; 320 K, which are comparable to the values for Pb-based ferroelectric materials [55].
The electrocaloric temperature change (DT) and isothermal entropy change (DS) for BCST-Nb 0.04 ceramics are calculated using Eqs. (5) and (6) and using data shown in Fig. 6. The values for DT and DS obtained using Maxwell equations are shown in Figs. 7(a) and 7(b). A peak value of DT ; 0.6 K is predicted at 320 K from Maxwell relations, which is comparable to several Pb-based and Pb-free relaxors [56,57,58,59]. Furthermore, direct measurements of electrocaloric temperature changes were also obtained for comparison with those  Fig. 7(c). The values of DT obtained from direct measurements are slightly smaller than those obtained from indirect measurements, which could be related to limitations in reaching equilibrium conditions as necessary for application of Maxwell relations. For temperatures above 300 K, it was not possible to reliably determine DT from direct measurements due to Joule heating.
One interesting aspect to note is that application of Maxwell relations below 270 K results in a negative DT. Nevertheless, no negative electrocaloric temperature change was observed in direct measurements. Similar discrepancies between direct and indirect measurements were noted earlier, such as Refs. 56 and 60. In most cases, this discrepancy originates from peculiar polarization behavior (related to change in domain structure), which results in a different P(T) relations. In the present case, the negative DT below 270 K could be related to the growth of the polar nanodomain structure below this temperature, which is indicated from Xray structural analysis as presented in section "Analysis of average and local atomic structures".
A broader temperature range for large (@P/@T) is desirable for cooling applications. Classical ferroelectrics with sharp firstorder transitions typically result in a higher electrocaloric temperature change due to higher contribution from latent heat, although the temperature span of high properties is typically limited only to about 10 K [61,62]. On the other hand, in relaxor ferroelectrics, the temperature span of higher electrocaloric properties is wider (30 K or more) [61,63,65]. This is because of the ergodic temperature range of relaxors, where transition between relaxor and ferroelectric phase is reversible. However, the ECE temperature change in relaxor system is typically smaller than that in ferroelectrics because of smaller latent heat contribution when the ferroelectric phase is induced. Therefore, for a fair comparison of usefulness of electrocaloric materials for refrigeration applications, it is imperative to calculate the relative cooling power or refrigeration capacity (RC), which is given by the following equation: To calculate RC, we integrated the area under the DS(T) peak at T ; 320 K [ Fig. 7(a)] over full-width at half-maxima (FWHM) (T 1 5 303.8 K and T 2 5 321.6 K). A value of RC ; 17 J/kg is obtained for the current material using Eq. (8), which is higher than the value of RC ; 11.87 J/kg reported for Pbbased relaxors [59].

Conclusion
A new Pb-free relaxor system was obtained by adopting a combined A-and B-site substitution scheme in the prototypical ferroelectric BaTiO 3 . The dielectric spectrum of this material shows a peak permittivity at T m ; 298 K (f 5 1 kHz) along with a broad shoulder at T s ; 270 K. The frequency dispersive behavior in the e9(T) follows the Uchino and Nomura and Vogel-Fulcher relations, which further informs about the relaxor ferroelectric behavior. Existence of an anomaly in the temperature dependence of C p , as well as Rietveld analysis of high-resolution X-ray diffraction data, indicates that below T m , the material transforms from a cubic to an average tetragonal phase. Analysis of local structure from X-Ray PDF shows that in short-range scale ,9 Å, the local structure is close to orthorhombic and remains unchanged in the temperature range 100-300 K. In addition, a strong nonlinear deviation in the temperature dependence of the c/a ratio of the average tetragonal phase is observed below T s ; 270 K, which is likely associated with an increase in the size of polar nanodomains. The temperature-dependent structural transitions can be correlated to the electrothermal properties of the material. Based on temperature-dependent polarization measurements, we observed two peaks in the (@P/@T) curve in the temperature range of 250-320 K: (i) -(@P/@T) ; 563 lC/(m 2 K) at T ; 270 K, and (ii) -(@P/@T) ; 1021 lC/(m 2 K) at T ; 320 K. In addition, the broad peak in (@P/@T) near T ; 320 K gives rise to a large electrocaloric temperature change with relative cooling power of ;17 J/kg. These properties make BCST-Nb 0.04 , a potential candidate for pyroelectric energy harvesting and electrocaloric cooling over broad temperature range.

Material synthesis
Polycrystalline Ba 0.77 Ca 0.21 Sn 0.02 (Ti 0.94 Nb 0.04 Sn 0.02 )O 3 (BCST-Nb 0.04 ) ceramics were prepared using solid-state synthesis from high-purity BaCO 3 (99.5%), CaCO 3 (99%), SnO 2 (99.8%), TiO 2 (99%), and Nb 2 O 5 (99.9%) as precursors. Appropriate amounts of all these raw materials were weighed and mixed thoroughly for 24 h using zirconia balls in a plastic jar. The balls to powder weight ratio was maintained at 5:1. Ethanol was used as a milling medium. The mixed powder slurry was then dried at 100°C for 12 h in an oven. The dried powder mixture was calcined at 1350°C for 6 h in a box furnace. After that, the calcined powder was ground, sieved, and pressed into 10-mm diameter pellets by applying a uniaxial pressure ;140 MPa. Finally, all the pellets were sintered at ;1450°C for 4 h in air to obtain the desired ceramics. The relative density of the final sintered samples was found to be ;92.8% by Archimedes method.

Electrical and electrothermal measurements
The temperature-dependent (173 K # T # 373 K) as well as frequency-dependent (100 Hz # f # 100 kHz) dielectric properties were measured using a LCR meter (Agilent-4284A) connected to a heating and cooling (;liquid nitrogen temperature) setup. A Radiant Precision LC-II Ferroelectric Test System was used to measure the electric-field-dependent polarization (P-E loops) at various temperatures using applied fields of frequency 10 Hz.
The heat capacity measurements (C p ) were performed in a computerized homemade ac-calorimeter. A detailed description of the ac operation mode can be found elsewhere [64]. The electrocaloric temperature change was measured in a homebuilt measurement chamber with a stable surrounding temperature of 62 mK. The temperature change of the sample was recorded by a small bead thermistor (NTC GLS9, TE Connectivity Measurement Specialists Ltd.) attached to the sample surface.

Structural characterization
The structural characterization of all the samples were performed at various temperatures (220 K # T # 380 K) using Rietveld refinement of X-ray diffraction and X-ray PDF analysis. High-resolution diffraction patterns were measured at the SPring-8 using X-ray radiation of wavelength k 5 0.49988 Å. X-ray total scattering patterns were measured at the Sector 11-ID-B of the Advanced Photon Source using X-rays of energy E 5 86.72 keV and wavelength k 5 0.143 Å. The sample to detector distance was kept at 251.1 mm. The maximum scattering vector (Q max ) was limited to 29 Å À1 . The PDF patterns were obtained from the X-ray total scattering patterns using PDFgetX2 [65]. Quantitative elemental composition of all the samples was analyzed by means of XPS with Physical electronics (PHI 5802) system using Al-K a source (1486.6 eV). The XPS data were collected in the range of 0-1200 eV.

Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.