1 Introduction

One of the most well explored composite materials is BaTiO3 (BT), that has an ABO3 perovskite structure with barium and titanium occupying the A-site and B-site, respectively, with titanium somewhat out of the core [1, 2]. Therefore, BT compounds are a great candidate for various applications like transducers, piezoelectric sensors, multilayer capacitors, thermistors, photovoltaic components, actuators, and so on [3,4,5]. Further advancement in BT is required, and recent research is pointing in that regard [6]. As a consequence, the incorporation of calcium (Ca) into the BT lattice to produce a barium calcium titanate (BCT) molecule has lately received interest due to its higher piezoelectric sensitivity than pure BT [1, 7]. Furthermore, the microstructure, dielectric, and functional characteristics modulators may be successfully adjusted by introducing a Ca+2 approach [8, 9]. As stated before, there are two techniques to enhance the dielectric characteristics. Reducing the grain size to boost the surface-to-volume ratio is one such technique. The alternate approach [6, 10], involves replacing some of the barium in composites to affect phase boundaries. As a consequence, a lot of research has concentrated on investigating multiferroic materials, like as Sn substance based on barium titanate employed as a dopant material to create high-dielectric BCST compositions [11, 12]. Whereas, the existence of a Sn+4 ions inside the BCT matrix influenced the intrinsic and extrinsic characteristics of the ceramic compounds, such as the coexistence of phases [13], and grain size [11]. Meanwhile, it affects the Curie temperature (Tc) whenever it is consumed within its solubility limit [14]. Tc drops almost linearly with increasing Sn content. This activity is usually attributed to the small ionic size of Sn+4. Increasing the Sn level reduces the unit cell size in the perovskite crystal structure, reducing the Tc in a manner similar to that of the hydrostatic pressure [15]. Moreover, earlier studies revealed that when the doping level in BaSnxTi1-xO3 was between 0.5 and 1.5, the ferroelectric and piezoelectric characteristics of BCST composites improved significantly [16, 17]. By making reference to it, these compositions provide a promising solution for a variety of fields, including diverse linked dielectric and piezoelectric characteristics, actuators, etc. [17].

In the current work, Ba0.95Ca0.05Ti1-xSnxO3 (BCST) compositions demonstrated a sequence of phase transitions followed by variations in Sn contents (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1). The as-produced perovskites’ ultimate microstructural, sintering, morphology, phase composition, and dielectric characteristics were examined. The optimal relaxation time and the Curie-Weiss constant are then presented. Owing to the developed BCST composites’ remarkable ferroelectric properties, the dielectric constant (Ɛ) was improved via introducing Sn+4 to reach an ideal value of 5557. Additionally, BCST-based ceramics are appropriate for memory and sensing applications due to their strong relaxation characteristic.

2 Sample preparation

Traditional solid phase reaction techniques were used to incorporate Sn+4 and Ca+2 ions into the BT perovskite-type structure seen in Fig. 1. Whereas Ca+2 was added at a constant ratio of 0.05 for all synthesized composites, Sn+4 ions were mixed in various concentrations within BCT crystals to create compositions of (Ba0.95Ca0.05)(SnxTi1-x)O3 (BCST), where x = 0, 0.02, 0.04, 0.08, and 0.1. High quality oxide powders from Sigma Aldrich, including BaCO3 (99%), TiO2 (98%), CaCO3 (99.9%), and SnO2 (99.9%), were used as source ingredients and weighted according to stoichiometric ratios. After homogenizing the combined powders in an agate ball mill for 6 h, the synthesized composites were fired in the following processes. In the first fire, the resulting mixture was calcined at 1200 °C for 8 h with a heating rate of 5 °C/min. Subsequently, sifted and pressed at 20 MPa into thin discs 2 cm in diameter and 1 ~ 1.5 cm in thickness. The calcined powder was then combined with a very little amount of distilled water and pressed into discs at 40 MPa once more. These discs were also put in a small silica crucible that was in the middle of a larger crucible and coated with Aluminum oxide. Following that, the tablets received a second burning (sintering) process that lasted 3 h at 1400 °C with a heating rate of 5 °C/min. The discs were also mechanically processed to create specimens that were 1.5 mm thick and 7.5 mm in diameter. Finally, a silver substance was applied to both sides of the top and bottom electrodes of the produced BSCT discs.

Fig. 1
figure 1

Block description of the solid-state reaction approach for BCST ceramic materials

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3 Characterization

In order to determine the phase purity and crystal symmetries of the calcined composites produced, X-ray diffraction measurement patterns (XRD, EMMAO 143, GBC, Australia) were used in the scanning range of Bragg’s angle (2θ) 20°–80°. The Archimedes’ principle was employed to estimate the bulk density (\({{{\mathrm{\rho }}}}_{{{\mathrm{m}}}}\)) of the ceramic composites. Following that, the theoretical density (\({{{\mathrm{\rho }}}}_{{{{\mathrm{th}}}}}\)) is determined using the composition molecular weight and the weighted cell volume from the XRD measurement. In addition, FTIR spectroscopy (Jasco 4100 Japan model) was employed to evaluate changes in BCST chemical compositions as the Sn concentration increased. At ambient temperature, FTIR spectra were performed on the calcined and sintered specimens with a resolution of 4 cm–1 in the range of 4000 to 400 cm–1. Further, through using scanning electron microscopy (SEM, JOUL, JSM – 5500, Japan), the morphology of the sintered BCST was discovered. Meanwhile, a computerized SPECORD 200 PLUS analytical spectrophotometer was used to test the optical characteristics of the calcined composites with a step of 1 nm at incident light in the 360–1100 nm wavelength range. Finally, an LCR meter (E4980A, Agilent, Palo Alto, USA) of the sintered compounds was used to examine the dielectric constant, loss tangent (tanδ), and Curie Weiss characteristics.

4 Results and discussion

4.1 X-ray diffraction (XRD) Analysis

Figure 2a and b depict XRD profile peaks of as-synthesized BCSxT composites (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) in the 20°–80° and 30°–35° ranges, respectively. All peaks in all compositions exhibit no splitting. Moreover, the detected peaks are well matched with JCPDS card no. 00-005-0626, showing that the substance crystallizes in a typical tetragonal form [16, 18]. The existence of a similar phase structure in all peaks after varying the amount of Sn+4 ions in the combinations suggests that these ions may combine with the perovskite BSCT system. Meanwhile, the deformation of the BCST lattice structure to rhombohedral phase is not observed [19]. Afterwards, to gain information into the structural evolution of the BCST compositions caused by the doping effect, the XRD data of all substances were explored using the Rietveld refinement process and analyzed using the Fullprof software [20]. Figure 3 depicts the Rietveld refinement of the BCS0.08T composite, as well as the structural model of the BCT and BCST produced by the “Vesta” software. Meanwhile, Table 1 lists the crystallographic parameters retrieved via the Rietveld refinement. It is clear that the level of Sn in the composites significantly affect the cell parameters, as ‘a’ and ‘b’ rise while ‘c’ declines as Sn concentration increases. Besides, all peaks displayed in Fig. 2a corresponding to the levels of (100), (110), (111), (200), (102), (211), (220), (212), (310) and (113) are shifted slightly towards a lower diffraction angle compared to the untreated BCST composite. Also, Fig. 2b clearly depicts this shifting behavior via utilizing a zoomed image of peak (110). The replacement of the larger ionic radius (I.R.) Sn+4 ion (I.R. = 0.69 Å) at the Ti+4-sites (I.R. = 0.605 Å) is mainly responsible for the shift of the peaks towards the lower angles [21]. However, the diffraction peaks shift slightly towards high angles with high concentrations BCS0.1T, which might be related to a structural phase transition.

Fig. 2
figure 2

X-ray diffraction profiles of BCST powder in the range of a 20°–80°, b 30°–5°, with x = 0, 0.02, 0.04, 0.06, 0.08 and 0.1, calcinated at 1200 °C for 8 h

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Fig. 3
figure 3

a The Rietveld refinement of BCS0.08T composite, b Schematic model of the BCT and BCST crystal structure

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Table 1 Crystallographic characteristic of obtained BCST composites at different contents of Sn ions
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According to the Bragg equation, lattice expansion causes diffraction peaks to shift to a lower angle [1]. Similarly, this trend is matched by an expansion in the lattice unit cell size of ceramics, with the level of Sn+4 ions rising to BCS0.08T as a result of ionic substitution between Sn+4 and Ti+4 in the unit cell. Whereas further increasing the dopant level causes a decline in cell volume as well as a limited shift of the diffraction peaks to higher angles, which may be attributed to partial substitution of Ba+2/Ca+2-sites by Sn+4 ions [22]. However, tetragonality (c/a) tends to decline till BCS0.04T before rising towards BCS0.08T specimen. These outcomes suggest that Sn+4 ions are well diffused inside the lattice structure and that variations in ion concentration have little effect on the composite structure. Similar behavior for BCST compounds has been reported in several previous studies [22,23,24].

On the other hand, the estimated crystallite size of BCST composites was impacted by various levels of incorporated ions [11, 17]. Scherrer’s formula was used to estimate the composites crystallite size [25]. According to Table 1, the pure compound had a size of 45.39 nm and subsequently reduced to 27.89 nm as the amount of incorporated ions increased up to Sn-0.04. Additionally, the crystallite size varied significantly with increasing Sn concentration in the perovskite lattice, with the Sn-0.08 specimen achieving an optimum value of 45.69 nm.

These differences can be explained via the energy exchange that exists between the loaded Sn+4 ions and the lattice locations of certain host atoms, consequently resulting in lattice deformation. This falls in line with the little peak-positioned shift at low Sn levels. Nevertheless, for perovskite titanate, the larger crystallites that were seen in the 0.04 ≤ Sn ≤ 0.08 samples—which also showed a similar tetragonality trend—are more desirable since they boost polarizability, thereby enhancing photoelectric characteristics [26]. Moreover, the noticed alterations suggest a greater propensity for phase transition in the BCST systems once a larger Sn dosage (e.g., 0.1-Sn) is added. As a result, a decline in the ferroelectric characteristics of the BCST matrix is predicted [27].

4.2 FTIR spectrum

FTIR spectrum was also performed in order to complete the investigation of crystal lattice vibration. As shown in Fig. 4, FTIR spectra were recorded on the as-synthesized compounds (BCSzeroT-BCS0.1T) before and after the sintering process. In the first stage before the sintering process, three major absorption bands are noticed. The first notable peak appeared between 1415–985 cm−1 and was assigned to the COO– bond. The second peak, however, is seen at 1415 cm−1, and it belongs to \({{{\mathrm{CO}}}}_3^{2 - }\) group, referring to the fact that either BaCO3 or CaCO3 or both overlap to form a single absorption peak with little shifting at high wavenumber [28]. The third one is associated to vibrations in the stretched Ti–O, Ba–O, Ca–O and Sn–O bonds and exhibits a high peak between 700–400 cm−1 [29]. Whereas, by increasing the Sn concentration, none of these oxide peaks changed in functional group locations. Following the sintering process of the second stage, some chemical bonds, like \({{{\mathrm{CO}}}}_3^{2 - }\) and COO–, disappeared [30]. The decomposition of these bonds suggests a partial carbonate ingredients reaction, which promotes the formation of the BCST phase [28]. Meanwhile, the composite’s colour gives support towards this impact via displaying colour fluctuation from grey to white with raising the processing temperature. However, a significant peak at 545 cm−1 was seen in the unaltered sample, which is attributable to Ti–O and Sn–O stretching mode [18]. The locations of these peak (545, 544, 540, 563, 574, and 580 cm−1) shifted with respect to Sn content (BCSzeroT–BCS0.1T). Whereas lower Sn concentrations caused them to move slightly in the direction of a lower wavenumber, compositions with higher concentrations exhibited the opposite trend, shifting in a higher wavenumber. Although Sn ions have a higher atomic mass than Ti ions (118.71 amu vs. 47.867 amu), less energy is needed to extend the vibration of TiO6 than SnO6 [31]. Therefore, the peak in the BCS0.02T and BCS0.04T compounds exhibited a slightly shift to a lower energy than that found for the BCSzeroT compound, indicating the existence of Sn+4 ions with a small concentrations will not be able to displace Ti+4 ions from its position, however will be able to penetrate the crystal at the same site [24]. Meanwhile, In concentrated BCST (x ≥ 0.06) compounds, the peak moves to higher energy compared to that reported in the BCSzeroT, indicates the presence of Sn+4 ion at the Ti+4 ion site of barium titanate [32, 33]. Also, the presence of minor peaks in the range of 3000–3500 cm−1 suggests the entrance of potassium bromide.

Fig. 4
figure 4

FTIR spectra of the modified BCST composites, a before, and b after Sintering (1400 °C for 3 h)

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4.3 Surface morphology and density measurement

The surface micrographs of the as-synthesized BCST composites are displayed in Fig. 5a–f. The pellets in the acquired micrographs have a mostly uniform structure and relatively irregular shapes [34]. Further, employing analytical methods, the statistics of the average grain size, bulk density, and relative densities of each were investigated, and the results are listed in Table 2. The small amounts of incorporated ions (0 ≤ x ≤ 0.04), will not be able to displace Ti+4 ions from its position, however will be able to penetrate the crystal at the same site [24]. This re-positioning action, along with increased lattice strain energy, leads to reduced grain growth in treated BCST. Additionally, the non-uniform distribution of added ions in grain boundary regions also restricts grains expansion, by preventing grain boundaries from moving and inhibiting mass transit [35]. However, further addition of Sn+4 (x ≥ 0.06) resulted in the opposite behavior, as the average grain size grew and reached an optimal value of 0.66 m at the 0.08-Sn dopant. These findings indicate that Sn+4 ions grow independently of Ti+4 ions inside their own lattice, resulting in a decline in the quantity of Bi+3 in the matrix and a rise in the average grain size of BCST [36, 37]. The typical grain sizes morphology observed in SEM investigations are larger than the estimated crystallite sizes derived from the Debye-Scherrer equation (XRD). Differences in crystallite and grain size are mostly due to the formation process; crystallite size is a measure of the material’s microstructure, while grain size is a measure of its macrostructure [38]. Whereas, crystallite size is the average size of individual crystalline domains in the material, whilst grain size represents the average size of grains or grains of similar crystalline size [38]. In general, grains are made up of crystallites, and both grain and crystallite are single crystals. Given the nucleation and growth kinetics, smaller crystallites get closer and expand to grow larger during processing [39]. The completely monocrystallized portion of a grain is called the grain size, while the size of the crystallites on a specific plane within a crystal structure is called the crystallite size [39]. Many variables influence this disparity, including the kind of material, temperature sensitivity, melting temperatures, crystalline domains, and grains. Therefore, in the most likely situation, the grain is larger than a crystallite size [40]. Our results indicate that every grain seen by SEM is composed of several crystallites.

Fig. 5
figure 5

SEM images and grain size distribution of the BCST composites sintered at 1400 °C for 3 h: a Sn = 0, b Sn = 0.02, c Sn = 0.04, d Sn = 0.06, e Sn = 0.08 and f Sn = 0.1

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Table 2 Microstructural characteristics of Ba0.95Ca0.05SnxTi1-xO3 compounds doped with varying levels of Sn ions
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On the other hand, the influence of Sn content on composites microstructure and average grain size resulted in a corresponding variation in the bulk and relative density of the BCST compositions, given in Table 2. Whereas, the values of bulk density (\({{{\mathrm{\rho }}}}_{{{\mathrm{m}}}}\)) and relative density (\({{{\mathrm{\rho }}}}_{{{{\mathrm{relative}}}}}\)) were evaluated using the theoretical density (\({{{\mathrm{\rho }}}}_{{{{\mathrm{th}}}}}\)). Meanwhile, \({{{\mathrm{\rho }}}}_{{{{\mathrm{th}}}}}\) was calculated from the composition molecular weight and weighted unit cell volume received from XRD measurement. The relative densities were estimated using the formula below [41]:

$${{{\mathbf{\rho }}}}_{{{{\mathbf{relative}}}}} = ({{{\mathbf{\rho }}}}_{{{\mathbf{m}}}}/{{{\mathbf{\rho }}}}_{{{{\mathbf{th}}}}})\;{{{\mathbf{x}}}}\;100{{{\mathrm{\% }}}}$$
(1)

For low dopant concentrations, the bulk density declines, extending from 5.61 g/cm3 (95.86% of its TD) for BCSzeroT to 5.45 g/cm3 (93.05% of its TD) for BCS0.04T composite. The relatively small concentrations of integrated Sn+2 ions may penetrate the crystal at the same location as the Ti+4 ions, but they cannot move them from their position [24]. Reduced density might result from this partially shifting action and greater lattice strain energy, leading in a slower Ion diffusion process during sintering and thus reducing the density expansion [35, 42]. However, there are rises in concentrated compounds (x ≥ 0.06), reaching an optimal value of 5.68 g/cm3 (97.77% of its TD) for BCS0.08T, as confirmed by the dense microstructure detected in the SEM image. These improvements in density are related to the formation of finer particles which in turn stimulate densification by growing atomic diffusion process rates, resulting in denser microstructures [43]. Since Ti+4 at B-sites was replaced by Sn+4 with smaller ionic radii and consequently there is a decline in Ti–O bonds, the bonding between Sn+4 and the TiO6 octahedral structure gets weaker, the bulk density of doped BCST ceramic material grows as the Sn ratio rises. Numerous studies have been carried out with this response [21, 42, 44]. The deviation of Sn+4 ions is simpler at B-sites due to the presence of additional space, thus enhancing ferroelectricity and elevating dielectric behavior [44]. Additionally, the Sn ion activation energies coupled to the ionization of oxygen vacancies complicate electric domain rotation, boosting the combinations’ dielectric performance [19]. Following that, a drop in density was reported to 5.59 g/cm3 (96.18% of its TD) for the heavily doped BCS0.1T system as a result to increase coarsening, which might be related to a structural phase transition.

The aforementioned transition from densification to coarsening is the result of the possibility of agglomerates forming when particle size is reduced. This agglomeration behaviour, according to a well-known fact, resulted from a reducing initial particle size that grew in its specific surface area and, therefore, its surface free energy which serves as the driving force for sintering [43, 45]. In addition, this behaviour grows increasingly prevalent as particle size falls due to the dominance of electrostatic and other surface forces that emerge with growing surface/volume ratio while particle size reduces [46, 47]. Besides, the tendency of these agglomerates to sinter together after heating causes the microstructure to become coarser.

4.4 Optical properties

A representative absorbance spectrum of the as-prepared BCST compounds is exposed in Fig. 6a. In the wavelength range of 400–900 nm, the unmodified composite exhibits a slight variation in absorption inclination, however lower 400 nm, a high absorption is seen. Additionally, Sn doped BCST compounds exhibit extra relative absorption inclination in the same region in their absorption pattern. The higher absorption of light in the visible spectrum is indicated by this inclination. This behavior is consistent with the literature findings [48]. Besides, Tauc’s relationship was employed to determine the energy band gap (Eg) for all compositions [49, 50]. According to Fig. 6b, the estimated Eg values were significantly impacted by the Sn incorporation process within the BCST matrix. Whereas, it begins at 4.66 eV for the unmodified compound and slowly reduces in respect to the Sn level, reaching its lowest value of 4.26 eV at Sn-0.08 specimen. These differences in the results for Eg could be explained by the existence of intermediary energy levels within the structure of Eg. These intermediary levels relate to the amount of structural order-disorder present on the lattice as a result of the symmetry break between the bonds of O–Sn–O and O–Ti–O and/or distortion on the groups of Sn+4 and Ti+4 [51, 52]. According to Cavalcante et al. [51], the presence of deep holes is related to low Eg values, whereas shallow holes are linked to high Eg values. Therefore, the broad band gap ~4.7 eV recorded at 0.1-Sn may be a result of the band gap’s shallow holes. Trend can draw the conclusion that adding Sn dopant ions to BCST systems can modify the optical characteristics of the material generating an active ceramic composite.

Fig. 6
figure 6

a Absorbance spectra vs wavelength, b Energy band gap of calcined (1200 °C, 8 h) BCST composites with varying Sn concentrations

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4.5 Dielectric properties of BCST composites

It is noteworthy that the dielectric constant (ℇ) value is significantly influenced by the dopant type, dopant ratio, and sintering temperatures. Additionally, there are other both intrinsic and external elements that can be affecting, including grain size, grain boundaries, density, pressure, and domain wall motion. Figure 7a depicts the temperature-dependent dielectric constant at various Sn concentrations. The ferroelectric–paraelectric phase transition, which establishes the Curie temperature, is thought to be responsible for a little hunch in the observed dielectric constant spectra in all compounds [53]. The dielectric characteristics of BCST systems are driven by this ferroelectric distortion. Since the electric dipoles freeze at relatively low temperature, ℇ appears small. In contrast, raising the temperature resulted in a rise in ℇ values, which is ascribed to the thermally stimulated relaxation process [2]. Based on a structural perspective, adding Sn+2 as a doping substance could impact the properties of the BCT matrix since Sn+2 and Ba+2 possess ionic radii of 146 pm and 118 pm, respectively. However, during the temperature rise, a portion of Sn+2 was oxidized to Sn+4, resulting in currently acting as an effective substituent for B-site [54]. At low Sn levels, the off-center movement of A and B location atoms rises dramatically when Sn+4 substitutes Ti+4 to generate deformed cubic BCST owing to a shift in cationic and anionic hybridization. In light of this, it is anticipated that the dielectric characteristics of BCST composites would significantly enhance in comparison to basic BCT [55]. As the Sn ratio grew, some flaws that resulted in oxygen vacancies were steered towards structural transitions. With respect to its high solubility, equal partial replacement of Ti+4 can lead to durable, dense binary BCST ceramic [56]. Sn doping produced oxygen vacancies, whereby could be thermally ionized at higher temperatures. Given of the vacancies’ mobility, greater vacancy level, and increased oxygen ion inertia, conduction of the vacancies would eventually gain dominant. Besides, it is most likely the nearby double ionized oxygen vacancy that provides the dipoles that supports the aliovalent Sn+4 material. The dipole orientations were influenced by the ions’ random arrangement inside the specific unit cells. Nevertheless, the arrangement the aforementioned dipoles resulted in the nucleation of polar nano-regions on a local level on the nanoscale. A higher dielectric permittivity value would ultimately follow from this [57]. Therefore, doping-induced crystal flaws are the cause of the doped specimen’s increased dielectric permittivity. Kim et al. [58] suggested that Sn-doping BT might be advantageous for outstanding stability and dependability in capacitor applications.

Fig. 7
figure 7

a Dielectric constant, b Debye relaxation time as a function of temperature (T °C) for BCST composites synthesized with various Sn concentrations, and sintered at 1400 °C for 3 h

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Notably, the presence of saturated Sn dopant levels revealed the ferroelectric phase of BCST. Nevertheless, more B-sublattice doping resulted in a decreased dipole movement owing to the phase shift that accompanied the electrical conductivity contribution. Additionally, the decreased net polarization values recorded for BCST matrix suggest that the growing structural disorder contributes to the ferroelectric ordering to decline and a relaxor-like state to arise. Moreover, the degree of t scale was identified to be in line with the tetragonal, cubic, and orthorhombic tendencies in the perovskite phase-stable series. As a consequence, a smaller t would indicate a tetragonal phase with less stability. Therefore, one would notice a reduction in the TC from the tetragonal ferroelectric phase to the cubic paraelectric phase [59]. Pavithra et al. [28], Srikanth et al. [60], and Sharma et al. [61] displayed similar transitions for BCT–BST, BCT–BSTO3, and BCT–VO3 compositions, respectively. Meanwhile, Fig. 7b depicts the Debye’s relaxation time (τ) behavior of the BCST systems as a function of temperature. The following formula was also used to estimate the values of the Debye relaxation time [62].

$$\varepsilon \prime - \varepsilon _\infty = (\varepsilon _{{{\bf{0}}}} - \varepsilon _\infty )/\left( {{{{\bf{1}}}} + {{{\boldsymbol{\omega }}}}^{{{\bf{2}}}}{{{\boldsymbol{\tau }}}}^{{{\bf{2}}}}} \right)$$
(2)

Where \(\varepsilon \prime\) represents the dielectric constant at angular frequency \(\omega\), \(\varepsilon _0\) and \(\varepsilon _\infty\) denotes the dielectric constants of low and infinite frequency, respectively, and \(\tau\) is the mean relaxation time [63]. For low Sn contents (x ≥ 0.04), the Debye relaxation time (τ) appears to be small (Fig. 7b). However, increasing the level of the Sn dopant (x = 0.06) resulted in a rise in values, which was attributed to increased defects, vacancy, and disordered sites within the composites’ structural system. Whereas, Substitution of Sn+4 ions at the Ti+4 site results in a substantially lower overlap between Sn+4 and O–2 than between Ti+4 and O–2. Therefore, dipole formation might be decreased, thus reducing interacting between continuous dipoles. Hence, the growth of polar nanoregions is induced by incorporating of Sn at a relatively high level, strengthening the relaxation behaviour [64, 65]. On the other hand, the highly concentrated BCST ceramic systems (x = 0.08, x = 0.1) achieved the lowest values of τ. Characteristics of the relaxation time distribution in the paraelectric—ferroelectric phase transition range suggested the existence of a “order-disorder” phenomenon during the phase transition [66]. This broader variation in temperature of the dielectric susceptibility at the phase transition vicinity—typical of structurally disordered materials—corresponded to the lower relaxation times. It is noteworthy that these outcomes match the Lyddane-Sachs-Teller relationship [67]. Given the strong concordance between this behavior of τ and its opposite behaviour for ℇ, the domain size is proportional to τ [28]. Therefore, this performance is essential for evaluating the findings of based on the critical C for ferroelectric substantial. The obtained data clearly reveal that the compound with an 0.08-Sn content had an optimal ℇ of 5557, whereas τ displayed a value of about 2.96 × 10−5 sec.

Accordingly, this pattern was supported by the measured dielectric loss values (tanδ, 1 kHz) at 27 °C. Depending on the Sn level in each composition, x = 0, 0.02, 0.04, 0.06, the reported values of tanδ were 0.0281, 0.0242, 0.0261, 0.0265, respectively, while yielding a minimum value of 0.0223 for the 0.08-Sn sample. Meanwhile, the additional quantity of filler x = 0.1 within the BCST system got 0.0253 of the tanδ value. Moreover, this dielectric loss sequence corresponds to the density improvement noticed with modifications to Sn concentration. Whenever a BCS0.08T specimen is sintered at a sufficient temperature, the microstructure coarsens, allowing agglomerates to grow. These agglomerates reduce the density of conduction paths for free mobile electrons, therefore raising the dielectric constant value.

In Table 3, the optimal dielectric value obtained for the BCS0.08T system were compared with that in previous literature research. With the same production technique and doping substances, the as-prepared BCST composition had a relatively high dielectric constant value of 5557, demonstrating excellent Sn/Ca concentration ratio performance in terms of dielectric constant.

Table 3 Comparison of dielectric properties of the as-synthesized BCST with other literature BCST based ceramic compositions
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Figure 8 depicts the variation in Curie temperature or Tc (ferroelectric transition temperature) and the maximum value of the dielectric constant (ℇm) as functions of the Sn levels inside the BCST structure. With an alteration rate of 10 °C every 2 mol%, the Tc variations almost clearly showed a linear trend. The principal reason of these variations in ℇm values is grain size, specifically differences in grain number and grain borders. This causes a large shift in Lorentz fields [68, 69]. For more details, adding a small amount of 0.02-Sn would be unable to displace the Ti+4 out of its position. As a consequence, it penetrates the crystal in the same location as it and increases the ℇm value relative to an untreated one. Despite the fact that the average grain size increases when the Sn+4 concentration at the Ti+4 position increases [70], BCTS-0.04 has both a smaller average grain size and a smaller unit cell. As a result, the dielectric constant similarly reduces until it reaches its lowest level at ℇm = 2479. This reduction in grain size when Sn+4 is replaced at the Ti+4 position is ascribed to the restriction of grain expansion caused by the presence of oxygen vacancies formed by charge compensation process linked with substitution of higher valence Sn+4 with lower valence Ba+2 is comparable to altering in (Ba1-xCax) TiO3 [71,72,73]. Following that, the Sn+4 level grows and exists independently of the Ti+4 ions within its own lattice. In this instance, compositional variations enhance the ℇm value, which reaches its greatest value at 0.08-Sn specimen. Additionally, at 0.08-Sn, the dielectric peak width seems to be broader, indicating the performance of the diffusion phase transition. This expansion of the dielectric peak is caused by changes in the ceramics’ polarization or spatial arrangement [74]. However, the continued increase in the Sn content has led to a reduce in the ℇm value, may be attributed to the following reasons:

  • Because of the growing disordered structure, lower temperatures were needed to overcome the barrier to domain shifting and domain wall motion [75].

  • lowering the connection between ferroelectric dipoles or the ferroelectric energy [76].

  • By increasing the site disorder and charge fluctuation at x > 0.08 Sn, the thermal induced current is reduced [77].

Fig. 8
figure 8

The peak value dielectric (ℇm) and temperature (Tc) vs a different Sn content into BCST compounds

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Figure 9 exposed how varying Sn concentrations affect the modified BCST compounds’ degree of relaxation (γ) values. Additionally, the dielectric performance with diffuse phase transitions was represented using the Curie-Weiss law (2) relation as follows:

$${{{\mathbf{1}}}}/\varepsilon - {{{\mathbf{1}}}}/\varepsilon _{{{\boldsymbol{m}}}} = \left( {{{{\mathbf{T}}}} - {{{\mathbf{T}}}}_{{{\mathbf{c}}}}} \right)^{{{\mathbf{\gamma }}}}/{{{\mathbf{C}}}}$$
(3)

Where ℇm is the highest dielectric constant at Tc, T is the absolute temperature, γ is the diffusion coefficient, and C is the Curie-Weiss constant [28, 78, 79]. When the value of γ is 1, the composite exhibits a typical ferroelectric phase transition and behaves normally according to the Curie-Weiss equation. Besides, compositions are said to be diffuse phase transition materials if γ is 2 [80]. Low dopant levels in the BCST lattice produced these high magnitudes of γ = 1.871 and γ = 1.79 with 0.02-Sn and 0.04-Sn, respectively, while the high Sn ratios induced a rapid decline. Given Ti sublattice doping, there was a shift in grain size (about one order of magnitude) and a gain in porosity, which suppressed ferroelectric characteristics and even induced a relaxor-like condition (Sn doping). In addition, the loading of Sn ions influenced the electric susceptibility anomaly associated with the presence of the low-temperature ferroelectric–paraelectric phase transition [59]. Subsequently was additionally evaluated for a tendency for relaxor-like qualities to arise. In light of the relaxor behavior for these ceramics, the transition temperature changes, the deviation from the irregular first-order ferroelectric–paraelectric transition, plus the additional propagation of this transition were modified. However, the lower γ values at concentrated ceramics revealed that the growing defects in structure contributed to the ferroelectric ordering to weaken along with a relaxor-like state to develop. Szeremeta et al. [59] highlighted similar behavior for modified-BCT ceramics. In this vein, the Curie-Weiss constant rose (3.14 × 104 °C – 5.8 × 104 °C) with increasing Sn ion concentration (x = 0–0.06), it declined to 1.13 × 104 °C in the 0.08-Sn sample. This drop in the aforementioned value could be ascribed to a reduced diffusiveness at the phase transition temperature. Additionally, this suggests that Sn ions are partially replacing at the Ti position, which assists in organizing the dipoles and reduces the variety of Curie points [61]. These finding suggest that BCST compounds exhibit clear diffuse phase transitions and high-grade ferroelectric ceramics [81].

Fig. 9
figure 9

Plot between ln(1/ℇ – 1/ℇm) and ln (T–Tc), degree of relaxation for BCST composites synthesized with various Sn concentrations and calcined at 1200 ˚C for 8 h

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5 Conclusion

Polycrystalline Ba0.95Ca0.05SnxTi1-xO3 (BCST) composites were successfully produced with different levels of Sn dopant (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) using a typical solid-state reaction approach. Herein, the work focused on the influence of Ca/Sn substitution on the optical, morphological and dielectric properties of pure BT. SEM micrographs revealed some fluctuation in average grain size as well as corresponding changes in the homogeneous structure and irregular shapes as Sn concentration increased. As a result, the average grain size and crystallite size were optimized to 45.69 nm and 0.66 µm, respectively. Where these results show the existence of Sn+4 ions at the Ti+4-site and the preservation of the perovskite tetragonal structure. The 0.08-Sn specimen, however, showed a relative reduction in the Curie-Weiss constant and a dielectric constant with an ideal value of 5557. These factors indicate that BCST ceramics exhibit clear diffuse phase transitions and high-grade dielectric materials. Therefore, it is obvious that varying the level of Sn doping applied considerably enhanced the dielectric characteristics of the BCST composites created in this study.