Functional properties of Sr1-xGdxTiO3 ceramics synthesized by solid state reaction method

Sr1-xGdxTiO3 (0.00 ≤ x ≤ 0.11) ceramics by the addition of Gd3+ on A-sites, were processed and fabricated through the conventional sintering method and optimised the sintering temperature at 1390 °C–1470 °C for 2 h. The experimentation shows that all ceramics are possessed by cubic structure, in which the increment of Gd3+ content, in terms of x, decreases the lattice parameters of the ceramics, as well as affects the grain size. Comparing the results to pure SrTiO3 (STO) compounds, the addition of Gd3+ lowers the mass loss, increased the conductivity as well as permittivity by decreasing the dielectric losses at lower frequencies. The x = 0.03 and 0.01 evidenced the maximum Seeback Coefficient, ∼281 μV K−1 at ∼370 K confirming the higher carrier concentration. Also, the magnetic properties as a function of Gd3+ ions doped with STO showed gradual improvement, showing maximum saturation with the maximum concentration of x.


Introduction
Materials having the potential of transforming either thermal to electrical gradient or from electrical to refrigeration are favourable as alternate means of energy transformation [1][2][3]. This alternate mechanism of direct conversion of thermal gradients into electricity was proposed by Thomas Johnson Seebeck in 1821 [4,5] and proved a solution to existing challenges of energy transformations [6]. Semiconductor materials such as AgPb 18 SbTe 20, Bi 2 Te 3 , and skutterudite-thermoelectrics are known to fulfil the requirements of functional properties, as these structures are non-centrosymmetric. Since then, oxide materials have gained greater attention among Thermo-electrics (TE) research society [7]. Among research for non-toxic and lead-free ceramic materials, the perovskite (ABX 3 ) structure with excellent electric and magnetic properties is gaining attention by varying induction of metallic transitions on A or B positions [7][8][9]. Strontium titanate (STO), is a well-known perovskite quantum paraelectric compound having a high melting point (2,080°C), a wider range of defect chemistry, and exhibiting semiconducting behaviour on donor/acceptor doping [4,10,11]. STO possess a perovskite (cubic) structure at the ambient temperature ranges while having an energy band gap of 3.2 eV, which makes it an insulator. Furthermore, the STO compound has the lattice parameters of 0.3905 nm and the density reported is to be 5.12 g cm −3 [12]. The doping (or impurity) can enhance their electrical conductivity to a greater extent [13,14] and rapidly generates thermoelectric materials for commercial uses [14,15]. The electrical properties of Perovskite structured ceramic oxide STO can be enhanced by the induction of heavy metal ions on A or B site substitution [16][17][18][19][20]. For example, Sr 2+ is substituted by Ca 2+ , Ba 2+ , Bi 3+ , and Pb 2+ on A-sites, similarly, Al 3+ and Zr 4+ are substituted on B-sites by modifying the dielectric and energy storage properties of STO compound [21][22][23][24][25][26][27][28][29][30][31][32][33].
Oxygen vacancy as a self-doping element may be generated during the sintering process, which has the potential to modify the dielectric relaxation behaviour of oxide materials, which are typically sintered at high temperatures [34][35][36][37][38][39]. In pure STO ceramics, oxygen vacancies have been linked to a recently reported dielectric relaxation behaviour at high temperatures. Researchers found that oxygen post-annealing in Bi-doped STO ceramics had a significant effect on a series of low-temperature dielectric relaxation peaks, providing additional evidence that the relaxing species were related to oxygen vacancies [13]. Since RE-doped STO may be comparable to Bi-doped STO, we postulate that oxygen vacancies may also spontaneously form [39,40].
Recently, the researchers worked on the trivalent rare Earth materials (Dy 3+ , Sm 3+ , Pr 3+ , Nd 3+ , etc) to modify the characteristics of STO ceramics by altering the oxygen vacancies [41][42][43][44][45][46][47][48][49][50][51][52]. Despite reports of lowtemperature dielectric relaxation in RE-doped STO systems as a result of lattice distortion, one may question whether this phenomenon is related to oxygen vacancies. Unfortunately, the results of such investigations are rarely published. However, based on the literature review, Gd 3+ is chosen mainly in this work because of its excellent conductive properties [53][54][55][56] which will enhance the electrical, magnetic, and properties of STO. Moreover, the A-site doping of Gd 3+ in STO is studied in this work by producing it through a conventional approach of solid-state reaction/sintering. The ionic radius of Gd, which is significantly smaller than that of Sr 2+ , is the primary rationale for its selection [37,51]. In addition, the high sintering temperature's effect, and the modification of STO's properties with Gd 3+ addition have been investigated thoroughly by analyzing structural, thermal, magnetic, and electrical properties.

Experimental procedures
Sr 1-x Gd x TiO 3 ceramics, where 0.00 x 0.11, were fabricated and optimized by the Conventional approach of the solid-state reaction method. The precursors with high purity Strontium carbonate (SrCO 3 ) (Chemo savers, 99.8%, fine powder), Titanium dioxide (TiO 2 ) (Loudwolf, 99.9%, 44-micron size), and Gadolinium oxide (Gd 2 O 3 ) (Chivine, 99.97%) were dried for 3h at 300°C to remove impurities. The batches were prepared as per the stoichiometric composition and then mixed for 2h using Yttrium Stabilized Zirconia balls of different diameters as grinding media in the ball milling machine. After drying the powder was calcinated at 900°C for 4h in the air to remove volatile impurities. The post-ball milling for 2h was done after the calcination of powder, dried, and then granulated by using the binder, Poly Vinyl Acetate (PVA), and then pressed into pellets of 12.7 mm in diameter and 2.0 mm in thickness by applying the 200 MPa on the uniaxial pressing machine. The pellets were burned to 600°C for 1h 20 min to burn the PVA and then sintered in the air at optimized-sintering temperatures (by optimising the bulk density), 1390°C-1470°C with 2h of holding time.
The characteristics x-ray Diffraction (XRD) peaks analysis on XPert-Pro diffractometer system equipped with Cu-Kα radiation (λ = 1.5406 Å) in the range of 2θ = 10°to 80°, at 40 kV and 30 mA and the continuous scan was carried out for sintered pellets. Scanning Electron Microscopy (SEM) analysis was carried out (by using the JEOL JSM 6380L model) on sintered surfaces by coating carbon through sputtering. Fourier Transmission Infrared (FTIR) analysis of calcinated powder compositions was obtained by using the FTIR machine, model no. NICOLET IS50. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) analysis (by using the model SDT/Q600, USA) were performed on calcinated powder samples. Electrical conductivity and relative permittivity were calculated using corresponding formulations on the measurements collected through Inductance, Capacitance, and Resistance (LCR) meter (Tonghui, TH 2826) at the frequency range from 25 kHz to 1 MHz, by coating the sintered pellets with the silver electrode. Seebeck-coefficient measurements of compositions were performed using a Keithley nano-voltmeter. The stack of four cylindrical (diameter = 11.4 mm, thickness = 1.7 mm) sintered pellets was placed in a locally prepared ceramic jig with one end at a hot plate for smooth heat flow at a temperature range from 300 K to 380 K. K-type thermocouples were used to measure temperatures of the different temperature sides (hot and cold) respectively as shown in figure 1. The hysteresis loops for magnetic properties were generated at room temperature (T RT ) using Vibrating Sample Magnetometer (MicroSense EZ-9) at 2.5 tesla of the magnetic field.

Results and discussions
Figure 2(a) shows the XRD patterns of STO ceramics with the doping of Gd 3+ at different percentages of x at room temperature. Observation reveals that each sample has a cubic structure, without any secondary phases. In contrast, when x concentration increases, all the diffraction peaks move to wider angles, as evidenced by the enlarged (200/210) peak in figure 2(b), showing that the crystal lattice of the doped samples has been stretched. In table 1, the computed structural parameters with their densities along with the bulk density calculated by Archimedes principle, as per the card matched from PDF 4+ (2022) library are shown, which is stimulated by the XRD patterns. For x = 0.00 in Sr 1-x Gd x TiO 3 , the relative density achieved is ∼97.9% when compared to the theoretical density calculated through the XRD analysis. However, the lattice parameters decrease on the addition of Gd 3+ in x amounts by providing the maximum parameter in x = 0.03, which also reports the highest relative density (∼99%) having a crystallite size of ∼60 nm. This is because the ionic radii (1.0-1.2 Å) of Gd 3+ are   small as compared to the Sr 2+ (1.45 Å) and larger than that of Ti 4+ (0.65 Å), so Gd 3+ preoccupies the sites of Sr 2+ causing the unit cell volume to shrink. Figure 3 shows the SEM micrographs of Sr 1-x Gd x TiO 3 ceramics which is revealing that the addition of Gd 3+ created the tiny voids observed as shown in figures 3(b)-(c). [1], showing a cuboid form include submicronsized voids inside the grains in the composition of x = 0.03 and x = 0.11 [1]. The microstructure of prepared compositions showed stable structural composition at very high sintering temperatures [10,24]. Figure 4 shows the infrared spectral analysis to identify the bonding between the metal-oxygen in the postcalcined products, FT-IR spectra recorded the transmittance (%) concerning the wave number (range 3000-400 cm −1 ). The IR peaks generated at ∼1445 cm −1 can be termed with the hydroxyl (OH) group from the absorption of water. The Ti-O 6 phase's presence, either the stretching of bonds or the bending vibrations, at around the ∼800 cm −1 and ∼525 cm −1 . However, the band located at 525 cm −1 attributes Sr-O bond stretching within the cubic structure. The absorption peak band shifts towards the lower wave number value for a structure having lower symmetry with decreasing Sr ion contents. Although the absence of impurity peaks at x = 0.00 confirms the formation of STO, similarly, when the Gd 3+ is doped, the peaks show that the morphology does not transform the host structure and is embedded into it.    figure 5(b), one endothermic peak, showing the absorption of energy on the loss of C-content, at 100°C and another endothermic peak at 950°C is observed. The hump at 100°C is due to the presence of moisture and the peak at 950°C is due to the thermal decomposition of the compound in compositions [2,57]. However, the x = 0.03 ceramic, having maximum lattice parameters in Gd 3+ doped ceramics, shows minimum heat flow up to 620°C reporting the absorption of heat, and endothermic reaction, because of the putrefaction in the composition as well as the vaporization of water content. Figure 6 shows the electrical conductivity in Sr 1-x Gd x TiO 3 ceramics concerning the frequencies (25 kHz to 1 MHz). The frequency spectrum ranging from 25 kHz to 1 MHz shows an abrupt increase in electrical conductivity in x = 0.03 (1.35 × 10 -1 μS) and then in x = 0.11. Whereas, all the other compositions stayed unaffected by the addition of Gd 3+ . The x = 0.03 composition having the highest conductivity confirms the presence of charge carriers (free electrons), shows that up to x 0.03 and then in x = 0.11 of Gd 3+ , which verifies that the semiconducting mechanism in the STO ceramics [5,32], either doped or not, is supported by the contribution of grains and its boundaries. However, the oxygen vacancies in Sr-based composition are compensated by the Gd 3+ addition, which is causing the conductivity to increase significantly as compared to x = 0.00. Figure 7(a) shows that the electrical permittivity (ε r ) values were calculated by LCR meter at room temperature from 25 kHz to 1 MHz. For Sr 1-x Gd x TiO 3 ceramics, the permittivity fluctuates concerning the  values of x. However, x = 0.00 has low values for ε r (∼280) throughout the selected range on the frequency spectrum. However, at the lowest ranges of frequency, x = 0.03 tends to give higher values of permittivity (∼340), whereas the maximum values are observed for the x = 0.07 and x = 0.11, which are ∼600 at 100 kHz, but eventually, they decrease to ε r ∼ 450 at higher ranges of frequencies due to ionic polarization [58,59]. Similarly, figure 7(b) presents the dielectric losses (tan δ) analysed at room temperature, showing similar trends that for lowest frequencies (<100 kHz) the losses are <10%, however, on increasing the range of frequency, the dielectric loss value fluctuate and increased to <28% (for all compositions) and >73% (for x = 0.03).

mass loss). As shown in
The Seebeck coefficient (figure 8) is measured as a function of the thermal gradient from 310 K to 380 K temperature of Sr 1-x Gd x TiO 3 ceramics with silver coating on both sides is obtained by using K-type thermocouple evaluates the potential gradient. It has been evaluated using the following formula.

= -D
Where S is the Seebeck Coefficient (μV/K), V Th is the thermal voltages applied measured in volts, and T shows the temperature difference in Kelvins.  When the electrical conductivity and the Seebeck coefficient both go from increase or decrease at the same period it's clear that the carrier concentration has been enhanced by Gd substitution. To verify the significance of the Gd 3+ substitution in SrTiO 3 , Sr 1-x Gd x TiO 3 has been produced. Sr 1-x Gd x TiO 3 has better electrical conductivity and lower absolute values of the Seebeck coefficient compared to SrTiO 3 , presumably because of the presence of Gd 3+ . In comparison to undoped SrTiO3-, the thermoelectric performance of the n-type series material Gd 3+ is superior. Adding Gd 3+ to SrTiO 3 has the potential to increase its electrical conductivity and decrease its Seebeck coefficient in absolute values. However, with increasing the temperature, the Seebeck coefficient also increases confirming the n-type behaviour of the ceramics [1,4,5,44,51,53,60]. The x = 0.00 ceramic showed the maximum coefficient at ∼144 μV K −1 ; while x = 0.03 shows significantly higher than the x = 0.00, the maximum coefficient of ∼251 μV K −1 at ∼370 K which confirms the presence of higher carrier concentration.
All the compositions of Sr 1-x Gd x TiO 3 ceramics are characterized for magnetic properties at room temperature showing the magnetic hysteresis (M-H) loop in figure 9. Saturation magnetization and magnetic moments respond strongly to an increase in Gd doping concentration. A prominent effect of magnetic properties on Gd 3+ , when doped at the A-site of STO-based ceramics has been shown in table 2. As Gd 3+ ions have a much larger spin magnetic moment (7.9 μ B ) than Fe 3+ ions (5 μ B ). As a result, the total magnetic moment might be increased by magnetization by substituting a tiny amount of Gd 3+ ions. Therefore, the total magnetic moment may be increased to improve magnetization with a minor portion replacement of Gd 3+ ions as shown in the calcined samples. However, the magnetic dipoles relax out and the values decrease in sintered samples. Furthermore, the sintered x < 0.09 samples exhibit no distinguishable spontaneous magnetization with nearly linear behaviours [16,17,19,33]

Conclusion
High-quality Sr 1-x Gd x TiO 3 ceramics containing the inclusions of Gd 3+ was prepared under the solid-state reaction method at optimised sintering temperatures (1390°C-1470°C). The presence of Gd 3+ increases the density up to ∼99% comparing the theoretical density and reducing the grain size by minimizing the lattice parameters and reducing the porosities. Micrographs taken with a scanning electron microscope (SEM) revealed a dense structure caused by metallic inclusions (submicron to micron size) along the grain boundaries of the predominantly cuboid matrix. A very small mass loss (∼8%-14%) and heat flow at 1000°C are the evidence of the thermal stability of compositions at high temperatures. FT-IR spectroscopy proved the presence of the Ti-O bond in STO by exhibiting their stretching frequency at 525 cm −1 and 800 cm −1 wavelengths, also the stretching of Sr-O within cubic structure confirmed the no big change in structure on the addition of Gd 3+ . Seebeck coefficient, electrical properties, and dielectric constant are the function of doping concentration, increasing the carrier mobility and concentration up to x = 0.03 by introducing the semiconducting mechanism in the ceramic. Magnetic properties in calcined samples showed prominent effects of A-site dopings, while in the case of sintering, the magnetic properties decreased due to the high-temperature sintering effect.