Mono and co-substitution of Sr2+ and Ca2+ on the structural, electrical and optical properties of barium titanate ceramics

Ceramics International 45 (2019) 10154–10162

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Mono and co-substitution of Sr2+ and Ca2+ on the structural, electrical and optical properties of barium titanate ceramics

T

Tasmia Zamana,∗, Md Khairul Islama, Md Abdur Rahmana, Arman Hussainb, Md Abdul Matinb, Md Shamimur Rahmana a b

Department of Glass & Ceramic Engineering, Rajshahi University of Engineering & Technology (RUET), Rajshahi, 6204, Bangladesh Department of Glass and Ceramic Engineering, Bangladesh University of Engineering & Technology (BUET), Dhaka, 1000, Bangladesh

A R T I C LE I N FO

A B S T R A C T

Keywords: Sintering X-ray diffraction Grain size Dielectric properties Optical properties

In this work, Ba0.9Sr0.1TiO3, Ba0.7Sr0.3TiO3, Ba0.5Sr0.5TiO3, Ba0.5Ca0.25Sr0.25TiO3 and Ba0.5Ca0.5TiO3 have been synthesized to evaluate the influence of mono and co-substitution of A-site dopants (Sr2+ and Ca2+) on the structural, electrical and optical properties of BaTiO3 ceramics. Sr2+ added samples showed a tetragonal structure which became slightly distorted with increasing Sr2+ concentration and finally achieved a cubic structure for x = 0.50. Ba0.5Ca0.5TiO3 also retained their tetragonality with limited solubility. Presence of second phase, CaTiO3 demonstrated the fact of restricted solubility. The concurrent effect of Sr2+ and Ca2+ didn't alter the tetragonal structure. Sr2+ substitution enhanced the apparent density as well as grain size which stimulated the domain wall motion and improved dielectric properties. However, the ferroelectric nature of Ba1-xSrxTiO3 was poor due to the redistribution of point defect at grain boundary. The optical band gap was found to be reduced from 3.48 eV to 3.28 eV with increasing Sr2+ content. Co-substitution of cations improved the electrical property significantly. The highest value of dielectric constant was found to be ∼547 for Ba0.5Ca0.25Sr0.25TiO3 ceramics. Both Ba0.5Ca0.25Sr0.25TiO3 and Ba0.5Ca0.5TiO3 had developed P-E loop having lower coercive field and moderate optical band gap energy. Co-doping with Sr2+ and Ca2+ was a good approach enhancing materials electrical as well as optical property.

1. Introduction

ferroelectrics is a very effective way to improve its ferroelectric and piezoelectric properties. BT forms solid solutions with other perovskites, such as BaTiO3eKNbO3 [6], BaTiO3eBi0.5Na0.5TiO3eKNbO3 [7], BaTiO3eCaTiO3eBaHfO3 [8], BaTiO3eBiYbO3 [9], BaTiO3eCaTiO3 [4], BaTiO3eCaTiO3eBaZrO3 [1,10] BaTiO3eSrTiO3 [11] etc. These BT-ABO3 type solid solutions show high chemical and mechanical stability, exhibits high dielectric constant with low loss and follows easy preparation method. Recently, barium strontium titanate or BST (Ba1-xSrxTiO3) has attained extensive attention in the field of electroceramics. It is considered as the most propitious electronic material for tunable microwave devices due to the strong dielectric non-linearity, low dielectric loss, high tunability and sensible dielectric thermal stability [12–17]. Its high dielectric constant combined with low dissipation factor and chemical stability makes it suitable for applications such as capacitor, piezoelectric transducers, wireless communication devices, high speed random access memories (FRAM) and dynamic random access memory (DRAM) applications [14,16,18–21]. Another prominent depressor being CaTiO3 (CT) which reduces the

Lead-based piezoelectric materials such as lead zirconate titanate (PZT) are most widely used in various electronic applications such as piezoelectric and pyroelectric sensors, actuators, transducers, surface acoustic wave (SAW) devices, transformers and in piezoelectric energy harvesting, due to their excellent piezoelectric and ferroelectric properties [1,2]. However, these materials contain more than 65% lead oxide which is highly toxic in nature. Synthesis and processing of these materials require high temperature at which, these toxic lead oxides become volatile (above 880 °C) and can cause a serious health and environment hazard [1–3]. Due to the increasing demand of global environmental protection, lead-free materials have attained much attention recent days and BaTiO3 (BT)-based ceramics shows much promising performance replacing lead-based piezoelectric materials. BT is not only the first discovered perovskite (ABO3) type ferroelectric and but also most intensely studied ferroelectric for last 70 years since its discovery [4,5]. Although, pure BT shows low dielectric constant and piezoelectric coefficient, alloying of BT with other ABO3 type ∗

Corresponding author. E-mail address: [email protected] (T. Zaman).

https://doi.org/10.1016/j.ceramint.2019.02.064 Received 22 November 2018; Received in revised form 29 January 2019; Accepted 10 February 2019 Available online 11 February 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

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Fig. 1. (a) XRD patterns of BST (10, 30 and 50) ceramics at RT and (b) magnified pattern in the range of 44.5°∼47.5°.

Fig. 2. (a) XRD spectrum of BST50, BCT50 and BCST25 ceramics sintered at 1200 °C and (b) magnified portion of (110) peak in the range of 31°∼33° showing the pattern of peak shift.

dielectric loss of BT system [22]. A tetragonal phase of Ba1-xCaxTiO3 (BCT) was reported for Ca2+ substitution (up to x = 0.25 due to limited solubility) which slightly changes Curie Temperature (TC), however has significant effect on dielectric properties [23,24]. Ca2+ being able to substitute for both A and B-site atoms, might provide some interesting features [25,26]. Hence, BCT is known as one of the potential candidates replacing hazardous electro-optic modulator PZT system. Both BST and BCT is a congruent solid solution of BaTiO3eSrTiO3 [27–30] and BaTiO3eCaTiO3 [4,31] respectively where BT being a ferroelectric material having temperature dependent dielectric properties. On the other hand, ST and CT are paraelectric in nature which never shows such ferroelectric phase transition [12,27,28]. Doping with Sr2+ is a good approach to attain diffused peak along with lower value of TC [28]. The linear relationship between TC and x (0.15 ≤ x ≤ 1.0) for Sr1-xBaxTiO3 ceramic was found to be TC = 360x+45 which signify the decreasing trend in TC with increasing concentration of Sr2+ [32] while addition of Ca2+ introduces greater atomic polarizability as well as intensified interaction with Ti4+ ions [33]. According to R.K. Roeder et al. [34], the role of (Ba + Sr)/Ti also need to be considered maintaining the stoichiometry of BST. Unreacted Ba2+ in solution could result in stoichiometric imbalance and Ti4+ rich phases [30]. K.A. Razak et al. reported the threshold value of (Ba + Sr)/Ti as 2.4 above which a two phase structure appeared [35]. Generation of second phases like Ba6Ti17O40 and Ba4Ti13O30 in a nonstoichiometric system of BST was also investigated by Lee et al. [36]. Moreover, limited solubility of CT restricts the use of it in micro-electromechanical systems (MEMS) [31]. In the recent years, many researchers have reported the significant effect of co-substitution of Ca2+ and Sr2+

in BT system. They observed the intensified dielectric and ferroelectric properties [12,37]. Co-substitution of Ca2+ and Sr2+ showed satisfactory correspondence both theoretically and practically [38]. In this work, we attempted to investigate the individual as well as combined effects of Sr2+ and Ca2+ ions substitutions on the structural, dielectric, ferroelectric and optical properties of Ba1-xSrxTiO3 (for x = 0.10, 0.30 and 0.50), Ba1-x-ySrxCayTiO3 (x = 0.25, y = 0.25) and Ba1-yCayTiO3 (y = 0.50) ceramics. 2. Methodology Conventional solid-state sintering route was adopted preparing Ba0.9Sr0.1TiO3 (BST10), Ba0.7Sr0.3TiO3 (BST30) & Ba0.5Sr0.5TiO3 (BST50), Ba0.5Ca0.25Sr0.25TiO3 (BCST25) and Ba0.5Ca0.5TiO3 (BCT50) ceramics. Highly pure staring raw materials of BaCO3 (purity > 99%, Merck Specialties, India), CaO (purity > 99%, Qualikems Fine Chem. Pvt. Ltd., China), SrCO3 (purity > 99%, Merck, India) and TiO2 (purity > 99%, Merck, India) were used in proper stoichiometric ratio. The weighted powders were ball-milled in ethanol (purity > 99.8%, Merck, Germany) media for 20 h. After milling the powder was dried in an oven (100°C-24 h) and calcined at 1000 °C for 2 h. The calcined powder was again ball-milled, dried and pressed into pellet (13 mm in dia and 1 mm in thickness). PVA was used as binder. Sintering was carried out in ambient pressure at 1150–1250 °C for 2 h. The apparent density of the sintered samples was measured by Archimedes method. X-ray diffraction (XRD) analysis was carried out for phase identification using 40 kV-40 mA (step size of 0.02°) and Cu-Kα radiation of wavelength Kα1 = 1.54060 Å and Kα2 = 1.54439 Å (Bruker Advanced D8, Germany). Structural refinement was done with the help of FULLPROF

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Fig. 3. Rietveld analysis of (a) BST10, (b) BST30, (c) BST50, (d) BCST25 and (e) BCT50 ceramics.

software. Morphological characterization was accomplished by SEM (ZEISS-EVO 18, UK). Grain size and distribution was analyzed by linear intercept method and IMAGE J software respectively. Room temperature (RT) dielectric constant was measured at 500 mV by varying frequency (20 Hz-5 MHz) with impedance analyzer (Wayne Kerr 6500B series, UK). Voltage dependence (200 V-2 kV at 1 Hz) P-E loop was obtained using multiferroic tester (Radiant Tech. Inc., USA). Finally, optical band gap energy was measured using UV–Vis spectroscopy (SHIMADZU UV/Vis-1650 PC, Japan) in the range of 200–800 nm. 3. Results and discussions

ceramics sintered at 1250 °C is shown in Fig. 1(a). Formation of polycrystalline BST structure devoid of any secondary phases was observed for all the three samples. Unreacted Ba2+, which is thought to be the prime reason of stoichiometric imbalance creating Ti4+ rich second phases was not found in our case [30]. The characteristic peaks (100), (110), (111), (200), (201), (211), (220) and (310) with high diffraction intensity and acute shape demonstrated the fact of complete homogeneous solid-state reaction [39]. Debye-Scherrer formula [40] was used for the most intense peak (110) calculating the crystallite size (τ). The equation can be written as:

τ = kλ β cos θ

3.1. XRD analysis Room temperature (RT) XRD spectrum of BST (10, 30 and 50)

(1)

Here, the Full Width at Half Maxima (FWHM) is β, k being the dimensionless factor having a constant value of 0.9, while λ and θ is the

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Table 1 Structural parameters of BST (10, 30 and 50), BCST25 and BCT50 ceramics. Composition

Space Group (%)

Cell Parameter (Å)

Volume (Å3)

Crystal Density (gcm−3)

R factors (%)

GoF

Bragg R Factor

BST10

P 4mm

63.76

6.074

8.14

P 4mm

61.77

5.269

1.0

12.1

BST50

P m −3 m

59.86

5.090

1.0

6.05

BCST25

P 4mm

61.07

6.342

1.1

10.4

BCT50

P 4 m m (61.79)

63.43

6.11

Rp = 36.7 Rwp = 43.1 Rexp = 40.1 Rp = 33.4 Rwp = 39.8 Rexp = 38.5 Rp = 27.6 Rwp = 36.9 Rexp = 33.3 Rp = 33.7 Rwp = 40.1 Rexp = 37.4 Rp = 35.0 Rwp = 41.2 Rexp = 39.3

1.1

BST30

a = 3.9868 b = 3.9868 c = 4.0115 a = 3.9513 b = 3.9513 c = 3.9563 a = 3.9119 b = 3.9119 c = 3.9119 a = 3.9346 b = 3.9346 c = 3.9445 a = 3.9784 b = 3.9784 c = 4.0068 a = 5.3930 b = 5.4339 c = 7.6670

1.0

8.33

224.68

4.019

P b n m (38.21)

29.5

Table 2 Structural, electrical and optical properties of BST, BCST and BCT ceramics. Composition

Crystallite size, τ (nm)

Tolerance factor (t)

Dielectric Constant (κ) at 1000 kHz

Dielectric Loss (tanδ)

Optical Band Gap Energy, Eg (eV)

BST10 BST30 BST50 BCST25 BCT50

48.45 73.94 76.85 44.03 34.17

1.0788 1.0660 1.0531 1.0446 1.0361

70 260 450 543 158

1.83 1.85 2.08 1.85 1.61

3.41 3.27 3.23 3.36 3.38

Table 3 Effect of sintering temperature on structural properties. Composition

BST10 BST30 BST50 BCST25 BCT50

Apparent Density, ρexp (gcm−3)

Average Grain Size, d ± 0.05 (μm)

Percent Porosity (%P)

1150 °C

1200 °C

1250 °C

1150 °C

1200 °C

1250 °C

1250 °C

3.12 3.25 3.76 3.19 3.10

4.14 4.39 4.51 3.77 3.22

4.35 4.68 4.89 3.97 3.34

0.93 0.95 0.96 0.80 0.88

0.96 0.98 1.02 0.81 0.95

1.25 1.33 1.43 0.94 1.02

28 12 4 37 37

wavelength of Cu K α radiation and Bragg angle correspondingly. The development of more and more intense peaks with the addition of Sr2+ indicate the progressive increment of crystallinity as well as crystallite size (Fig. 1(a)) [39]. It is to be noted that, incorporation of Sr2+ also shifted the diffraction peaks to higher angle (Fig. 1(b)). The occurrence could be attributed to the substitution of large size Ba2+ (1.61 Å) by small Sr2+ (1.44 Å) ions which distorted the structure remarkably. The shifting of peaks towards the higher angle also directs towards the fact of diminution in the lattice parameter. As shown in Fig. 2(a and b), the XRD patterns of BCST25 and BCT50 were also analyzed and compared with BST50. The addition of Ca2+ ions in BCT50 shifted the peaks to lower angles (Fig. 2 (b)). Further, the structural refinement was evaluated by Rietveld analysis (Fig. 3). A tetragonal to cubic phase transformation was reported to occur when a large fraction of Sr2+ is added in BT [39]. The obtained patterns of BST10 and BST30 showed a satisfactory correspondence with the tetragonal structure having P4mm space group. While BST50 exhibited the cubic symmetry (Pm-3m). Both BCST25 and BCT50 seemed to be well-fitted with P4mm pattern. However, an additional secondary phase of CaTiO3 (marked as * in Fig. 2 (a)) was detected for BCT50 samples. The diffusion mechanism of Ca2+ in BT lattice is still controversial as mentioned by Chen et al. [28]. Since (1-x)BT-xCT is reported to have a limited solid solubility over x ≥ 0.25, hence, during

the formation of BCT50, CaTiO3 was precipitated out. Moreover, a small fraction of Ca2+ (1.35 Å) could substitute for Ti4+ (0.61 Å) when the molar ratio of (Ba + Ca)/Ti is exactly 1, therefore, the existence of Ca rich second phase was expected in case of BCT50 [24,27]. The typical values of structural parameters of all the sintered samples along with the values of the Bragg factor, Structure factor and GOF are tabulated in Table 1. The structural transformation could also be assessed using R. D. Shannon's ionic radii table [41] and Goldschmidt's rule [42]. According to the formula, the tolerance factor (t) could be calculated by:

t=

rA + rO 2 [rB + rO]

(2)

Where, rA, rB and rO are the ionic radii of A-site, B-site cations and Oion correspondingly. The decreasing trend in t value could justify the structural transformation of the doped ceramics. The proposition is that, for a stable perovskite structure, the value of t varies in between 0.88 and 1.09 [43]. For a cubic symmetry, t is supposed to be exactly 1.0. Addition of Sr2+ slightly decreased the tolerance factor (Table 2) which ratifies structural modification of the tetragonal phase and chronological transformation towards cubic. The low tolerance factor of BCST25 and BCT50 could be due to the lower ionic radii of Ca2+ ions. Nevertheless, the values of tolerance factor imply the ferroelectric nature of the

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Fig. 4. Surface microstructures of (a) BST10, (b) BST30, (c) BST50, (d) BCST25 and (e) BCT50 at a magnification of × 20,000.

fabricated ceramic samples. 3.2. Structural analysis The effect of sintering temperature on apparent density (ρ), grain size (d) and percent porosity (%P) of the synthesized samples could be predicted by the data presented in Table 3. The percent porosity of the sintered samples was calculated using following formula:

ρexp %P = ⎡1 − × 100%⎤ ⎥ ⎢ ρxrd ⎦ ⎣

(3)

Here, ρexp and ρxrd are the apparent density and crystal density respectively. As can be seen from Table 3, almost a linear enhancement in

apparent density with sintering temperature was perceived for all BST (10, 30 and 50), BCST25 and BCT50 samples which were quite expected. For BST samples, percent porosity was found to decrease with the increment of apparent density (Table 3). Augmented sintering temperature leads to an increase in the grain size which in turns enhanced the density. However, BCST25 and BCT50 showed comparatively lower density than BST. The occurrence could be attributed to a number of factors like lower atomic weight and ionic radius of Ca atom, impurity phases etc. [44]. It is believed that, in this case, the manifestation of a high percentage of porosity reduced the overall apparent density of BCST25. Whereas, precipitation of secondary phase having lower crystal density than the parent phase might have an adverse effect on densification of BCT50. Fig. 4 represents the SEM micrographs of BST, BCST and BCT ceramics sintered at 1250 °C. The reduction in porosity with the Sr2+ concentration in BST ceramics can clearly be visualized in Fig. 4(a–c).

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Fig. 5. (a)Variation of dielectric constant (Inset of (a) shows dielectric constant vs frequency curves above 1000 kHz) and (b) loss tangent with frequency of synthesized BST (10, 30, 50), BCST25 and BCT50 ceramics.

In case of Ca2+ added samples (BCST25 and BCT50), relatively high proportion of porosity was observed Fig. 4(d and e). Vigilant observation of the micrograph also revealed the existence of liquid phase for BCT50 which is believed to play a critical role during the process of densification. Presence of some rod-shape grains were also detectable in the micrograph which might be due to the generation of second phase as mentioned earlier (Fig. 4(e)).

3.3. Dielectric properties Fig. 5(a and b) represents the frequency dependent dielectric constant (κ) and dielectric loss (tanδ) curves of BST (10, 30 and 50), BCST25 and BCT50 ceramics. All the samples exhibited a high dielectric constant at starting frequency which drastically fell apart and attained a satisfactory level of stability with moderate κ above 1000 kHz (Table 2). An intuitive justification of this incident could be given by MaxwellWagner (M-W) space charge polarization model. Basically, any polycrystalline material having heterogeneous permittivity or resistivity might follow the M-W model. According to the model, due to the

differences in conductivity between grains and grain boundaries (GBs), diversified permittivity might be witnessed at low and high frequencies [45]. Qualitatively, accumulation of space charges at GBs require a definite time which can only be possible at low frequencies. With increasing frequencies, it becomes rather difficult for the space charges to follow the field direction. Finally, when at some high frequency the space charges become faulty, only the intrinsic property of that material will be revealed. The drastic fall in κ at a definite frequency for all samples also indicates the manifestation of Debye like relaxation [46]. Hence, it is vindicated that, the relaxation of all the space charges happened at or near 1000 kHz and above this relaxation frequency space charge induced polarization was inoperative. The obtained values of κ for BST, BCST and BCT ceramics measured as 1000 kHz are tabulated in Table 2. The maximum value of dielectric constant was found ∼1352, ∼1408 and ∼1503 for BST10, BST30 and BST50 correspondingly at 20 Hz. The increasing trend in κ values of BST ceramics clearly show that, by increasing Sr2+ content a significant improvement in dielectric constant is possible. The incident of improved κ could be elucidated by the concept of grain size effect. Fundamentally, partial substitution of Ba2+ by Sr2+ generates lattice deficiency which stimulates the mechanism of grain growth. It was found that, the process of grain growth becomes more diligent for 50% substitution of Ba2+ [22]. The measured grain size for BST (10, 30 and 50) ceramics indicates the significant impact of Sr2+ promoting grain size. Since the domain wall mobility is much easier and regular for large grains, hence an improved dielectric property was obtained for Sr2+ added BT ceramics. This observation is in good agreement with previous works [47,48]. Moreover, variation of dielectric property could also be attributed to the generated stress by means of structural distortion. On the other hand, both Ca2+ and Sr2+ occupying A-site (Ba2+ position) in BCST25 ceramics possibly reduced the concentration of oxygen vacancies. As a result, the oxygen vacancy induced grain growth was restrained (Table 3). On the top of that, the simultaneous effect of Ca2+ and Sr2+, both having smaller ionic radii than Ba2+ had greater polarizability and exaggerated ionic interaction. Hence, the highest value of κ was found for BCST25 ceramics (κ ∼1783). However, the circumstances were not similar for BCT50. Presence of Ba2+ in A-site generates local polar regions [28]. While when a cation like Ca2+ substitutes for Ba2+ at A-site, it creates non-polar region. The combined effect of these polar and non-polar regimes is thought to be responsible for the characteristic relaxor behavior of BCT ceramics. Since, the diffusion mechanism of Ca2+ is rather contentious; Ca2+ ions could occupy both A- and B-site of a perovskite structure. The hypothesis is that, when the concentration of Ca2+ is essentially x ≤ 0.25 in BCT solid solution, substitution of Ba2+ results. But for higher concentration range (0.90 ≥ x ≥ 0.25), Ca2+ (1.00 Å) readily enters into the B-site and substitutes Ti4+ (0.61 Å) [24]. Kröger-Vink [49] notion could be adopted expressing the equilibrium equation during B-site substitution: ●● CaO ↔ Ca″Ti + OO + V O

(4) 2+

in B-sites, generation of Due to the partial substitution of Ca oxygen vacancies promoting grain growth was observed as validated in '' Table 3. The newly formed dipole V ●● O − Ca Ti can make the polar region inactive by means of locking the non-polar region. Hence, oxygen vacancies lead to generate domain wall clamping and thus reduce dielectric properties [50,51]. Henceforth, BCT50 gave a poor κ compared to others. Moreover, presence of second phase having different crystal structure than the parent phase might have an inauspicious effect on permittivity. The loss tangent vs. frequency curves (Fig. 5) attained a broad peak at ∼1000 kHz and then sloping down gradually. The generation of high loss was certainly due to the relaxation associated with GBs as mentioned earlier. The value of tanδ vacillated in between 1.8 and 2.1 which was within the tolerance limit.

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Fig. 6. P-E curves of (a) BST50, (b) BCT50 and (c) BCST25 ceramics at RT.

As conspicuous from Table 3, the percentage of porosity was very high for almost all the samples. Hence, it will not be unusual for one to think that, the contribution of porosity might play a pivotal role in changing dielectric properties. Usually, materials behave more like compact powders rather than sintered ceramics when the percentage of porosity exceeds the value 10 and for those ceramics, the dielectric property does not remain as a sole function of porosity. Other parameters like domain density, internal stress imposed by surrounding grains etc. need to be considered as well [52,53]. Several theoretical models like M-W relationship, Niesel's equation stand for ceramics having low porosity (< 10%) [54,55]. However, the experimental value of porous ceramics shows inconsistency with the theoretical values. Besides that, the nature of change in percent porosity curve and internal strain curve as observed by Hsiang et al. [52] indicates that pore morphology, depolarization etc. should also be count into account. 3.4. Ferroelectric property Further, to probe into the ferroelectric behavior of BST50, BCST25 and BCT50 ceramics, the electric field induced polarization (P-E) characteristics (Fig. 6) were observed under a maximum electric field of ± 12 kV/cm. The development of unsaturated hysteresis loops with discontinuity at zero voltage indicates the occurrence of leakage current [56]. Nearly round shape hysteresis loops were observed for BST samples (Fig. 6(a)). Presence of SrTiO3 in the solid solution are responsible for redistributing the point defect concentrations in the vicinity of GBs and creates a space charge layer across the grain boundary barrier which could possibly explain the round shaped nature of P-E loop for BST50 samples [57–59]. Comparatively well-developed P-E loops were found for BCST25 and BCT50 measured at RT. However, the

value of remnant polarization, Pr was relatively low (Fig. 6(b and c)). Accretion of space charges at GBs creating internal bias field could show distinct response with the direction of external applied field [60]. Thus, the values of polarization with filed might be reduced as observed in this case. 3.5. Optical property Fig. 7 (a) illustrates the absorbance vs. wavelength plot of BST (10, 30 and 50), BCST25 and BCT50 ceramics. Kubelka-Munk (K-M) function [61] and Tauc linearization method [62] was adopted constructing the [hυF(R)]2 vs. hυ plot (Fig. 7(b)). According to the formalism, the intersection between the tangent line and x axis shows the value of optical band gap energy of that particular material. Incorporation of Sr2+ reduced the band gap significantly from 3.41 to 3.23 eV. The obtained values are in good agreement with previous work as reported by Tian et al. [63]. Generally, the optical property of a perovskite structure is influenced by the TieO octahedral coordination [64]. However, the influence of oxygen vacancies on optical absorption was also explained by previous authors [65,66]. The density functional theory states that, for a perovskite ceramics, optical transition occur between 2p O (as valence band) and 3d Ti (as conduction band). Hence, it is believed that, due to the structural modification by the substitution of smaller size Sr2+ ions, shrinkage of oxygen-octahedral occurred which is the possible explanation of the red shift. Moreover, it is well-known that, crystallite size influences the optical band gap energy significantly. Increment in crystallite size with Sr2+ content could also illuminate the phenomenon of reduced band gap. The measured value of optical band gap energy for BCST25 and BCT50 was 3.36 and 3.38 respectively.

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Engineering & Technology (RUET) and Bangladesh University of Engineering & Technology (BUET), Bangladesh for providing testing facilities. Special thanks to Mr. Nabil Haque, Department of Glass & Ceramic Engineering, RUET for doing Rietveld Refinement. References

Fig. 7. (a) UV–Vis absorption spectra of BST(10, 30 and 50), BCST25 and BCT50 ceramics; (b) [αhϑ)]2 vs photon energy, hϑ plot (insight showing band gap energy of the corresponding samples).

4. Conclusions In summary, BST(10, 30 and 50) along with BCT50 and BCST25 were synthesized using traditional solid-state sintering route. All samples except BST50 retained the tetragonal structure. BST50 formed a cubic structure. The presence of CaTiO3 as second phase in BCT50 had detrimental effect on electrical properties. The dielectric response of BST samples improved with Sr2+ concentration. Due to the greater ionic interaction, highest dielectric constant was perceived for BCST25. The optical band gap energy of all prepared ceramics indicates its potentiality in optoelectronic devices. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Acknowledgement The authors would like to thank the Rajshahi University of

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