Enhancement of the Optical and Dielectric Properties at Low Frequency of (Sr1−xCax)5Ti4O13, (0 ≤ x ≤ 0.06) Structure Ceramics

Low loss Ruddlesden–Popper (RP) series, i.e., (Sr1−xCax)5Ti4O13, 0.0 ≤ x ≤ 0.06, has been synthesized by a mixed oxide route. In this work, the substitution of Ca2+ cation in Sr5Ti4O13 sintered ceramics was chosen to enhance the structural, optical, and dielectric properties of the product. It was found that the Ca2+ content has significant effects on enhancing the dielectric properties as compared to Mn and glass additions. It was observed that the relative density, band gap energy, and dielectric loss (tangent loss) increase while relative permittivity decreases along with Ca2+ content. High relative density (96.7%), low porosity, and high band gap energy (2.241 eV) values were obtained in (Sr1−xCax)5Ti4O13, 0.0 ≤ x ≤ 0.06 sintered ceramics. These results will play a key role in the application of dielectric resonators.

In the present work, the good results on structural, optical, and dielectric properties (at low frequency) of Sr 5 Ti 4 O 13 based structure ceramics will be studied. These results will be modified by making some doping elements at A-site cation in the base product.

Experimental Procedure
The solid solution of (Sr 1−x Ca x ) 5 Ti 4 O 13 , 0 ≤ x ≤ 0.06 ceramic was processed by using high grade pure carbonates and oxide powders, i.e., SrCO 3 (99.95%), CaCO 3 (99.9%), and TiO 2 (99.5%) as raw materials. The resultant stoichiometric ratio of the raw materials were mixed properly and then milled using horizontal ball milling with zirconia media in distilled water for 24 h and then calcined for 3 h in air at 980 • C. After re-milling, the calcined powder was mixed along with polyvinyl alcohol (PVA) solution at 4 wt.% and then made into green pellets of 2-3 mm thickness and 10-12 mm diameter. Then, the green pellets were kept in high energy furnace at 1200 • C sintering temperature for 3 h in air to dense the pellets. After sintering, the pellets were cooled to 600 • C at the rate of 10 • C/min and then cooled to room temperature inside the furnace further. The bulk density was calculated by using the Archimedes principle method for all the pellets. The phase analysis was identified by using X-ray diffraction (XRD, RIGAKU D/max 2550/PC, Rigaku Co-Tokyo Japan) with CuKα radiation. The surface morphology of the thermally etched and gold coated samples was studied using scanning electron microscopy (SEM, S3400; Hitachi, Tokyo, Japan). The relative permittivity (ε r ) and tangent loss were measured by the parallel plate capacitor method using vector-network Analyzer (E8363B, Agilent Technologies Inc., Santa Clara, CA, USA) [27]. At least four samples have been analyzed to ensure the accuracy of data. The reciprocal of Q-factor is the tangent loss (tanδ = 1/Q) [28]. Figure 1 shows the XRD patterns of RP series of (Sr n+1 Ti n O 3n+1 ) sintered ceramic for n = 4. The patterns revealed the tetragonal structure of RP series along with space-group (I4/mmm) matched to PDF card number 89-1383. The structure of the phase (at n = 4) was attained by put in a rock-salt type Sr-O layers, the strontium based titanates along with direction [001], resulting consecutive perovskite pieces due to shifting by direction 1 ÷ 2 [111], w.r.t the unit cell of RP series. The known RP structure has closely alike lattice parameters i.e., (a = b = 0.385 to 0.389 nm) but c = 2.812 nm for n = 4 [27][28][29][30][31][32]. The variation of lattice parameters and volume of the synthesized samples with Ca 2+ contents as shown in Table 1. The shifting of peaks to lowest Bragg's angles were due to the difference of ionic radii of Sr 2+ and Ca 2+ cations as shown in Figure 1b. No secondary phase has been observed and revealed the single phase of Sr n+1 Ti n O 3n+1 (n = 4) sintered ceramic. Figure 2 shows the variation of relative density with Ca 2+ contents of Sr n+1 Ti n O 3n+1 (n = 4) sintered ceramics. It has been noted that the relative density increases with the Ca 2+ content, which further modified the optical and dielectric properties. The highest values of relative density is (96.7%) of Sr n+1 Ti n O 3n+1 (n = 4) sintered ceramics was observed at x = 0.06 content.    Figure 3 shows the SEM images of the gold coated samples of (Sr 1−x Ca x ) 5 Ti 4 O 13 , 0.00 ≤ x ≤ 0.06 sintered ceramics. The variation of relative densities and grain size of all the samples has been investigated. The SEM micrographs of (Sr 5 Ti 4 O 13 ) green pellets with doping of Mn or glasses at different sintering temperature were studied by many scientific researchers [26]. It has been reported that the base product have small crystallite size and less porosity, which may be affected by the surface strain. However, new grains and porosity were produced by adding some dopant elements in (Sr 5 Ti 4 O 13 ) sintered ceramic [27][28][29][30]. Ca 2+ concentration has been observed to increase the porosity and grain size of all samples in (Sr 5 Ti 4 O 13 ) sintered ceramic. These factors will affect the structure, optical, and dielectric properties of the base product. In order to improve these properties, numerous studies have examined the synthesis settings used to create various dopants in the base product [31,32].  Figure 4 shows the FTIR spectra of (Sr 1-x Ca x ) 5 Ti 4 O 13 , 0 ≤ x ≤ 0.06 sintered ceramic. FTIR spectrometer plays a key role to characterize the vibrational stretching and un-stretching mode of the base sample synthesized by chemical reaction route [33,34]. The vibrational stretching mode (O-H) was observed with variable wave number (K = 2π/λ) i.e., 900.0 cm −1 , and 3200.0 cm −1 . This mode of vibration is produced by the absorption of vapors during synthesis process. Only asymmetric mode at wave number (3700.0 per cm) was recorded in the base product which shown carboxylates family [35]. In this characterization the normal stretching mode was observed at wave number (500.0 per cm).  Figure 5 shows the UV-spectra of (Sr 1−x Ca x ) 5 Ti 4 O 13 , 0.00 ≤ x ≤ 0.06 sintered ceramics. Many of the researchers reported that the Sr 5 Ti 4 O 13 base sample was found to be transparent for white light [36]. It is very important to note that the compound, i.e., Sr 5 Ti 4 O 13 , is translucent for visible light. The band structure and electronic transition were characterized using photon energy [37]. The electron needs to execute the inner shell transition in order to obtain the optical bandgap energy. This optical bandgap energy strongly depends upon the coefficient of absorption (α), which was calculated using Equation (1) [25].

UV Spectroscopy
where E g = bandgap energy, A = constant of proportionality, and hv = photon energy. The coefficient of absorption will be defined how distant light of specific wavelength can be penetrated into material before being absorbed. When light absorbed poorly by material have low coefficient of absorption looks to be thin at specific wavelength. The unit of absorption coefficient is (cm −1 ). The band gap energy of all the samples was calculated using the Tauc plots method. It was reported that the values of band gap energy increase with Ca 2+ content.  Emission at photoluminescence peak of the samples has been noted at the range of 400-550 nm. Multiple photonic processes such as PL have certain common uses, and PL is a multiple photonic process that has some typical applications, i.e., (i) determination of band gape energy, (ii) material quality, as well as (iii) molecular structure and crystallinity, reported by many researchers [38,39]. It has been observed that the broader emission spectra were located near to~2.48 eV (excitation energy) and wavelength (~500 nm) which is larger than bandgap energy of all the samples may be occurs due to the presence of impurities. In the photoluminescence spectrum, the cyan color may occur due to the oxygen vacancy [40].

Low Frequency Dielectric Properties
The low frequency dielectric properties of all the synthesized samples sintered at 1200 • C for 3 h in air were better due to their high relative densities. The variation of relative permittivity (εr) and tangent loss (tanδ) with varying temperature was measured at 100 Hz-1 MHz for (Sr 1−x Ca x ) 5 Ti 4 O 13 , 0.0 ≤ x ≤ 0.06 sintered ceramics using the vector network analyzer, as shown in Figure 7. Strong irregularity in relative permittivity (ε r ) and tangent loss (tanδ) were observed for the contents (at x = 0.0 and 0.02), which shows the transition of ferroelectric to Para electric phases. The same behavior was recorded in the values of 'ε r ' and 'tanδ' for Ba 5-x Sr x DyTi 3 V 7 O 30 (0 ≤ x ≤ 5) sintered ceramics at temperatures of 430 • C, 350 • C, 325 • C, 85 • C, and 42 • C, respectively [41,42]. The lowest value of ε r (~1400) was observed for (Sr 1−x Ca x ) 5 Ti 4 O 13 , (composition with x = 0.02) at 100 Hz frequency, and found to decrease with increasing operating frequency, which may be due to the interfacial polarizations. Moreover, the value of εr decreased with increasing Ca 2+ contents, which is due to the difference of ionic polarizebilities of Ca 2+ (3.16 Å 3 ) and Sr 2+ (4.24 Å 3 ) [43][44][45]. It has been revealed that the value of tangent loss increases with temperature due to the proces of conductivity and different types of polarizations at low frequency [21]. The lower value of the tangent loss was reported at 1 MHz operating frequency for the base sample. The variations in both the quantities may be due to the difference in the values of dielectric polarizabilities [46]. Generally, tanδ decreases when high cation ions are replaced by smaller cation ions [47].
The complex impedance spectroscopy mechanism is generally used to investigate the structural properties and bonding of the various types of materials, comprising the ferroelectric, ionic insulator, and linked ceramics under different experimental conditions [36]. The variation in real impedance Z and imaginary impedance Z of (Sr 1−x Ca x ) 5 Ti 4 O 13 , 0.0 ≤ x ≤ 0.06 sintered ceramics is shown in Figure 8. Initially, it was revealed that the magnitude of Z increases with Z and then decreases due to the release of space charge polarization [37]. It was observed that the magnitude of Z decreases by increasing the Z and Ca 2+ contents.

Conclusions
The solid solutions of (Sr 1−x Ca x ) 5 Ti 4 O 13 , 0.0 ≤ x ≤ 0.06 sintered ceramics was synthesized by conventional solid state method. The structural, microstructural, optical, and dielectric properties of all the samples have been investigated. The XRD patterns revealed the tetragonal phase with space group (I4/mmm). The SEM image revealed that the grain size and porosity increase with increasing Ca 2+ contents, which was due to the difference of ionic radii. The results of UV spectroscopy declared that the bandgap energy increases from 1.68 eV to 2.24 eV along with increasing Ca 2+ concentrations. The good values of dielectric properties (i.e., ε r~2 50, and tanδ = near to zero) in the frequency range from 100 Hz to 1 MHz was observed. It has been observed that the magnitude of Z in-creases with Z and Ca 2+ contents. The overall findings are suitable for the application of dielectric devices.