Energy Storage Capacity and Electrocaloric Effect on Sn Modified Barium Titanium Oxide (BaTi 0.8 Sn 0.2 O 3 )

: Modified Barium Titanium Oxide (BaTi 0.8 Sn 0.2 O 3 ) was prepared by using the solid-state reaction method. The crystal structure, energy storage behavior, and electrocaloric properties were studied. The phase purity and structural analysis were investigated using the X-ray diffraction technique and the Rietveld refinement of XRD pattern. The microstructure of the sample was recorded by using the Field Emission Scanning Electron Microscopy (FESEM). The temperature variation dielectric property shows that the ceramic exhibits diffuse phase transition behaviour. The ferroelectric nature in BaTi 0.8 Sn 0.2 O 3 has been depicted from P-E loops analysis. The energy storage behaviour and electrocaloric properties were estimated from the temperature variation P-E loops at 40kV/cm. The electrocaloric effect was studied by an indirect method using Maxwell relation, and the electrocaloric value has been estimated to with 94% of energy storage efficiency.

temperature change ( ) and isothermal entropy change ( ), as well as electrocaloric strength, are very small to be used for technological applications. So, it cannot be used in the potential application. Generally, this electrocaloric effect study has two benefits such as its efficiency and Environment-friendly nature [11] [12]. At first, the study of electrocaloric effect was done on Rochelle salt, and the adiabatic temperature change value was found to be which is very small and cannot be used as a potential material [13]. After that, a lot of research works were carried out on the material aspects. The large electrocaloric effect with and at 226 o C was found on PZT thin film [14]. studied the energy density behaviour on lead-free ceramic and found the recovered energy density(W rec ), total energy density(W tot ), and energy storage efficiency(η%) of 84.4mJ/cm 3 , 92.7mJ/cm 3 and 91.04%, respectively [19].
Among all the lead-free materials, Barium Titanium Oxide-based ferroelectric materials have gained much attention by the researchers due to their high dielectric constant, piezoelectric, pyroelectric, ferroelectric behaviour, high energy storage capacity, etc. [20][21][22]. As per the electrocaloric phenomena, the material should exhibit large temperature-dependent entropy variation. Basically, this happens near phase transition from ferroelectric to paraelectric (i.e., Curie temperature) which is observed above the room temperature. [18,23,24].
In the present work, the prime objective is to explore material with para to ferroelectric transition temperature close to room temperature and leads to technological applications. Premier II) at a frequency 50Hz. The relaxor behaviour of the sample was confirmed by using temperature-dependent ferroelectric hysteresis loops.

Results and Discussions:
Structural Properties: The room temperature XRD pattern is shown in Figure-1 structural stability of the perovskite materials can be estimated by using the Tolerance factor which is given by [26], Where 〈 〉 and 〈 〉 are the average ionic radius of A and B position cations such as; Ba 2+ and Sn 4+ /Ti 4+ and is the ionic radius of the anion (O 2-). It is found to be 1.02, which is near to 1 and confirms the stable perovskite compound. Furthermore, the Rietveld refinement of the XRD pattern has been carried out to estimate the various crystal structure parameters. It confirms that the sample exhibits a tetragonal phase with P4mm space group. During the refinement, space group and fractional atomic coordinates are taken as a fixed parameter, whereas background, lattice parameters, and scale parameters are kept as free variables [27]. The refinement pattern is shown in Figure- Table 1.
The crystallite size is calculated by using the Williamson-Hall formula, as shown in Figure- Where K is the shape factor, which is generally taken as 0.9 by assuming the circular grain. λ = 0.1540nm is the wavelength of CuK radiation, is the Bragg's diffraction angle, is the full width at half maximum of the XRD peak and is the strain effect.
The average crystallite size of BaTi 0.8 Sn 0.2 O 3 is calculated by the Williamson -Hall method is tabulated in Table-1.      As a result, dipoles can be formed by the combination of ion and oxygen vacancies. This can affect the dielectric relaxation of the system. So that dielectric loss is of the system is more at higher temperature [31][32].
To describe the relaxor ferroelectric behaviour, the reciprocal of dielectric constant versus temperature was plotted at frequency 1kHz and fitted with the Curie Weiss law as shown in equation (3)[19]; ……… (3) Here is the real part of the dielectric constant, is Curie-Weiss temperature. Figure 4-(c) shows that sample's transition temperature is at around 80 but the sample follows Curie-Weiss law above 100 . This difference between the transition temperature and Curie-Weiss temperature is due to the Relaxor behaviour of the sample. So that the modified Curie-Weiss law has been used to derive the diffuse phase transition(DPT) and can be expressed as ; Here, are the real dielectric constant and maximum dielectric constant, is the exponent, which denotes the degree of diffuseness, and C is Curie constant.
In the case of normal ferroelectrics, the degree of diffuseness, i.e., =1 and which obeys Curie Weiss law. But, in the case of Ideal Relaxor ferroelectrics, the degree of diffuseness is =2 and obeys the modified Curie Weiss law. The degree of diffuseness can be obtained by the linear fitting of and as shown in Figure-4(d). Here, the degree of diffuseness of BaTi 0.8 Sn 0.2 O 3 is found to be 1.749(6) at 1kHz, which confirms sample's relaxor behaviour.

Ferroelectric properties:
The ferroelectric properties of the sample can be analyzed from the ferroelectric hysteresis loops. Room temperature ferroelectric hysteresis loops of the sample are taken at different fields are shown in Figure-5(a). It is observed that the remnant polarization (P r ), maximum polarization (P max ), coercive field (E c ) increases with the increase of the applied electric field as shown in  The continuous increment of characteristic parameters suggests that the dipole moments along with ferroelectric domains align in the field direction.

Energy storage properties:
Previously, Researchers on Energy storage performance under a high electric field have done a lot of works. The generally high electric field produces high polarization so that the energy storage performance of the sample increases. But according to an application basis, it cannot be used as energy storage devices [33]. The main parameters of energy storage performance are energy storage density(W rec ) and efficiency(η). These parameters directly depend on the difference between maximum polarization and remnant polarization ( ). Hence Energy storage performance is more if the value of is more and vice versa. Here, the major aim is to get a slim hysteresis loop to use as energy storage devices. In the view of energy storage performance, the temperature variation ferroelectric hysteresis loops are taken at a constant field 40kV/cm with a frequency 50Hz, as shown in Figure-6.  Generally, high dielectric constant, maximum polarization, small remnant polarization are needed to get high energy storage performance in ferroelectric materials [37,35]. Another parameter that is needed for the energy storage capacity is efficiency, which is given as [32]; ……………….. (7) The W rec , W tot, and of the sample BaTi 0.8 Sn 0.2 O 3 at room temperature were found to be 126mJ/cm 3 , 119.2mJ/cm 3, and 94%, respectively. The comparison of the present result with several lead-free materials is shown in Table-2. It is observed that the maximum polarization(P max ) of the sample decreases with the increase of temperature, which is evidenced from figure 7 (a). Also, there is the slimness of the loop occurs, which indicates this sample is suitable for energy storage performance. At 50 o C, the P max , W rec and W tot were 3.14 , 118.8mJ/cm 3 and 125.6 mJ/cm, 3 respectively. It is conclude that up to 90 o C, the efficiency increases due to a decrease in energy loss density and, after 90 o C, the efficiency decreases due to the broadening of loop thickness, which increases the energy loss density. Also, Figure-7(c), indicates that at room temperature, the material shows the highest value of P max , W rec and W tot . It is due to that with an increase in temperature microdomain of the materials convert into nanodomain, which creates distortion in the material and decreases its polarization with increasing the efficiency of the material.
Hence, the above studies indicate that BaTi 0.8 Sn 0.2 O 3 is a promising material for energy storage applications.

Electrocaloric effect Properties-:
As per the literature, the ceramic's electrocaloric effect strongly depends on the order parameter's randomness with respect to the temperature. Hence, the relaxor ferroelectrics are the best candidates who exhibit electrocaloric effect due to the presence of short-range polar island known as polar nanoregions (PNRs). Therefore, the electrocaloric effect of the present sample has been analyzed with the help of temperature-dependent ferroelectric hysteresis loops. Figure-8 and Figure-6  ∫ ( ) ………………………… (8) Here stands for the density of the sample. The pyroelectric ratio ( ) is calculated by the polynomial fitting of P-T curves [42][43][44]. The adiabatic temperature change and isothermal entropy change of lead free ferroelectric materials are shown in Table-3.   This behaviour of the sample is generally due to the presence of 20% of Sn, which reduces the transition temperature towards a lower value. The present sample behaves as a Normal Ferroelectric material at lower temperatures, as shown in Figures 6 and 8. As the temperature increases, the macrodomain cannot withstand and form small-small Iceland-like domains known as nanodomain. Again with the increase of temperature, these nanodomains cannot sustain and breakdown takes place. As a result, polarisation decreases, and the sample behaves as a paraelectric material. The Pyroelectric Ratio(( ) ) was calculated by an indirect method using the Polarization vs Temperature curve. The electrocaloric value was found to be 0.174kJ/kgK at 88 o C with field 40kV/cm.

Conclusions:
In summary, the present ceramic (BaTi 0.8 Sn 0.2 O 3 ) was successfully synthesized by using the solidstate reaction method. The Rietveld refinement of the XRD pattern reveals the formation of perovskite structure (P4mm) without the presence of any other secondary phase. The FESEM micrograph shows homogeneous distribution of particle. The sample's degree of diffuseness is found to 1.749 (6) and confirmed the relaxor ferroelectric behaviour. The electrocaloric value is found to at 88 o C. In addition, the sample exhibits total energy density (W tot )