Observation of high dielectric properties of Mg-substituted BST ceramic synthesized by conventional solid-state route

Barium strontium titanate exhibits ferroelectric or paraelectric behavior that depends upon the composition and other factors, making it suitable for several applications. We have adopted the conventional solid-state reaction route to synthesize magnesium-substituted barium strontium titanate with magnesium concentration (Mg = 0, 0.005, 0.010, and 0.015). X-ray diffraction has confirmed the perovskite cubic single phase. The dielectric studies were carried out as a function of temperature at different fixed frequencies (1 kHz, 10 kHz, 50 kHz, and 100 kHz) and as a function of frequency at room temperature. Curie’s temperature of pure BST was 96 °C, and for Mg-substituted BST samples, it dropped from 85 to 63 °C. The dielectric permittivity is firstly suppressed and then the maximum value observed for Mg content 0.015 and dielectric loss is also less than one. The remanence (Pr) and coercivity (Ec) are obtained and analyzed for ceramics samples. Improvement in dielectric properties due to substitution of magnesium is observed.


Introduction
Lead-free oxide type ceramics with perovskite structure (ABO 3 ) have attracted researchers towards itself because of their wide applications in the piezoelectric, capacitor, pyroelectric, microwave devices, memory storage, solar cells, etc. [1,2]. The most studied ferroelectric material of this group is BaTiO 3 (BT) having excellent properties like a high dielectric constant and low dielectric loss. However, high permittivity and high loss have some limitations also but that can be adjusted by the doping of softeners (donors) and hardeners (acceptors) on either A site or B site [3][4][5][6]. The addition of donors like La, Sr on Ba, or A site enhances the value of dielectric constant, mechanical losses and reduces the strength of the coercive field and elastic modulus. Similarly, the addition of acceptors like Fe for Ti enhances conductivity, increases mechanical quality factor, reduces dielectric constant, hinders grain growth, and reduces dielectric constant [7,8]. Barium strontium titanate (Ba 1-x Sr x TiO 3 ) has become the most promising candidate and being studied for years for many electronic applications [9,10]. These materials have various applications because of having suitable permittivity, good chemical stability, higher tunability, sensitivity to the electric field, and low tangent loss, but sometimes these factors restrict their application [11].
The high dielectric permittivity of pure ferroelectric BST ceramic also limits its application in microwave devices which require a low dielectric constant with low dielectric loss [12]. The various properties can be modified by substituting various selected ions on either A site or B site [13,14]. The high tangent loss and permittivity of BST have hindered its application [15]. To meet the requirements, elements of different groups are added or their composites are synthesized to minimize the value of the dielectric constant and put down the tangent loss [16]. The dielectric tunability is lessened, by attaining a higher sintering temperature. Recently, composites consisting of BSTO/MgO are awaited to show a good response as tunable devices because of their low permittivity and low tangent loss [17][18][19]. In the present work, we have selected (Ba 0.95 Sr 0.05 )TiO 3 composition due to high crystallinity than other composition with increasing strontium content, high transition temperature (Tc & 120°C), high dielectric constant (in range of several 1000 s), relatively low dielectric loss (tan d very less than 1), and high dielectric breakdown strength as well as good dielectric stability under dc bias field [20,21] being potential for storage [22] and many other applications.
It has been reported by various research groups that magnesium oxide is an important non-toxic insulator material and the addition of Mg or MgO in BST can put down the dielectric permittivity and losses [23]. Few reports are available on the effect of Mg-doped barium strontium titanate on different properties [24]. For serving the purpose of enhancing the applications of materials, magnesium compounds like MgO, MgTi 2 O 4 , MgAl 2 O 4 , MgB 2 O 6 , and MgTiO 3 are being used these days as dopants to be added into BST materials for increasing their suitability for many applications [9]. Among all the compounds, magnesium oxide (MgO) has been reported as an important material for altering dielectric properties and the sintering process of barium strontium titanate materials [2]. It was analyzed from the literature that densification could not be promoted, but it also stops the growth of grains in the sintering process. Even the Curie temperature could also be shifted towards lower temperature on the addition of Magnesium Oxide in BST. Therefore, it was concluded from the available reports that MgO could reduce permittivity dependence and tangent loss when added to BST. Recently, improved dielectric properties, good tunability, and low leakage current have been revealed in 0 to 10 mol% of magnesium-doped BST. Chou et al. revealed from their work that ferroelectricity is diluted, diffused phase transition was shown, and Curie temperature of composite ceramics is decreased in comparison to pure Ba 0.5 Sr 0.5 TiO 3 due to magnesium (Mg 2? ) substitution [25]. It can be observed from the reports that magnesium-added barium strontium titanate materials can be used as a promising material for processing capacitors, resonators, phase shifters, filters, etc. in microwave applications [9,26,27].
In the present paper, magnesium-substituted barium strontium titanate (Ba 0.95-x Sr 0.05 Mg x TiO 3 ) is synthesized using the conventional ceramic method.
To check the impact of Mg substitution, we have carried out X-ray diffraction, dielectric performance as a function of temperature (at fixed frequencies) and frequency (at room temperature), and PE loop measurements at room temperature.

Experimental details
The pure and Mg-substituted BST powder samples were prepared using the conventional solid-state reaction method. AR (Analytical Reagent) grade BaCO 3 (purity [ 99%, Sigma Aldrich, India), SrCO 3 (purity [ 99%, Sigma Aldrich, India), MgO (purity [ 99%, Qualikems Fine Chem. Pvt. Ltd., China), and TiO 2 (purity [ 99%, Sigma Aldrich, India) were used as raw materials for the synthesis process. These powders were taken and weighed according to stoichiometric composition of Ba 0.95-x Sr 0.05 Mg x TiO 3 with x = 0, 0.005, 0.010, and 0.015. Once the powder was weighed, it was ball milled with acetone as medium and zirconia balls as grinder or mixer for 5 h to form a slurry. The slurry was poured into a beaker and dried in a hot air oven for 8 h. The dried powder was then kept in a high-temperature programmable muffle furnace at 1000°C for 4 h with a heating rate of 5°C/min. The process of ball milling and calcination was repeated once again. Finally formed double calcined powder was pressed into pellets of diameter 15 mm and thickness about 1-2 mm in a uniaxial hydraulic press. The pellets were then sintered in high-temperature programmable muffle furnace at 1325°C for 4 h.
The structural and microstructural analysis of sintered pellets was done using X-ray diffractometer (XPERT-PRO Philips PW 3064) having Cu-Ka radiation (k = 0.15418 nm) and scanning electron microscope (JSM 6490). Dielectric studies were carried out with the help of the HIOKI IM 3536 LCR meter. For carrying out this study, the sintered pellet was coated with silver paste and then dried in an oven at 400°C to ensure good ohmic contacts. The data were recorded as a function of temperature at four different frequencies (1 kHz, 10 kHz, 50 kHz, and 100 kHz) and as a function of frequency at room temperature. The ferroelectric analysis of the material was done with PE Loop Tracer (Marine India) at an applied electric field of 10 kV/cm at room temperature.
3 Results and discussions 3.1 Structural and microstructural analysis Figure 1 shows the diffraction pattern of Mg-substituted barium strontium titanate with magnesium, i.e., Ba 0.95-x Sr 0.05 Mg x TiO 3 with x having values of 0, 0.005, 0.010, and 0.015. It is revealed that Mg-substituted barium strontium titanate exhibits ABO 3 cubic perovskite structure with a high orientation at (110). The substitution of magnesium in the BST lattice is confirmed by X-ray diffraction. The peaks 100, 110, 111, 200, 210, 211, and 220 are observed around angles 22°, 32°, 39°, 45°, 51°, 56°, and 66°. X-ray diffraction patterns revealed the absence of additional peaks of other material or phase which indicates that the addition of magnesium has not formed any new phase such as MgCO 3 , MgO, etc. in the samples. The well-defined crystallinity of material is increasing on increasing the magnesium concentration as indicated by the sharp peaks. On referring to pure BST, it has been observed from Fig. 1 that diffraction peak shift towards a higher angle on adding Mg. This implies that magnesium has successfully substituted Barium on the A site of ABO 3 on the surface. Since the atomic radius of Mg (0.72 Å ) is smaller than Ba (1.35 Å ) and when a smaller element has substituted a larger ion, on whole, there is lattice contraction. Wang et al. in 2010 and Gao et al. in 2020 have also reported the lattice contraction and cubic structure of barium strontium titanate on magnesium doping [24,28]. The average crystallite size (t) of magnesium-substituted barium strontium titanate powder samples is calculated using Scherrer's formula as follows: where k is the wavelength of Cu-Ka radiation source (k = 0.15418 nm), b (in radians) is the full width at half maxima, and h is the Bragg angle. The calculated values of average crystallite size are shown in Table 1. Crystallite size is in the range of nanoscale, and pure BST has a smaller crystallite size than Mgsubstituted BST. The lattice constant and density seem to decrease with increasing Mg ?2 concentrations. This emphasizes that Magnesium is successfully substituted on Ba (A) site in barium strontium titanate lattice. The phase structure of all the samples was found to maintain the same pattern at different magnesium concentrations, and no secondary phase was observed. The splitting of (200) peaks corresponding to the tetragonality was not observed in all the samples, revealing that all samples exhibit cubic perovskite structure. The splitting was observed only at x = 0 and x = 0.005 samples and the absence of and seem to scatter all around. The trend of decrease in bulk density of the samples with increasing magnesium content is given in Table 1. The decrease in density is due to an increase in the lattice volume of the samples. Also, the lattice parameters are found to decrease with the increasing magnesium content. All samples show dense microstructure as shown in Fig. 2 which can be attributed to the concentration of magnesium. When the magnesium content is increased in the samples, there is a reduction in the grain size of the samples. This indicates that magnesium content can suppress the growth of grains which further reduces the density [28,31,32]. This is because Mg ?2 ions diffuse into BST lattice to occupy Ba site and formed oxygen vacancy leads to lattice distortion requiring some energy. Therefore, Mg ?2 can separate on grain boundary perplexing the motion of grain boundary that further hinders the grain growth of barium strontium titanate ceramics   [33]. The increased scattering of the grains shows the increasing porosity in Mg-substituted samples. The agglomeration of the grains is reducing with Mg content. All samples have uneven shape and size of the grains as can be seen from the micrographs.

Dielectric properties
The modification of dielectric constant (e) as a function of frequency at room temperature for pure and Mg ?2 substituted barium strontium titanate (Ba 0.95-- x Sr 0.05 Mg x TiO 3 : x = 0, 0.005, 0.010, and 0.015) ceramic system is plotted in Fig. 3a. As we had a limited experiment range of frequency, so we have taken frequency from 100 Hz to 800 kHz. The dielectric permittivity decreases with the increase in frequency confirming the general characteristic of ferroelectric materials. The high value of dielectric constant for all samples measured at low frequency and decrease in the dielectric constant with increasing frequency are due to the presence of dipolar & electronic polarization that may be justified by the Maxwell-Wagner space charge polarization model [34]. The values of the dielectric constant were found to decrease with an increase in Mg content, and the maximum value was observed for Mg concentration x = 0.015, which refers to better crystallinity, and from Table 2, it is observed that BST with Mg concentration 0.015 is most crystalline among all the compositions that have also been confirmed by XRD study. It is observed that there is a decrease and then a sudden increase in the value of the dielectric constant at the maximum concentration of magnesium. We have reported earlier [35] that the dielectric constant and tangent loss of the Ba 0.95-x Sr 0.05 Mg x TiO 3 ceramic system at composition x = 0.02 and the results of this report gave us the motivation to study the ceramic system at low composition. The value of the dielectric constant for x = 0.02 is less than the composition at x = 0.015 and observed that the increase in Mg content leads to deterioration in the dielectric constant. The Curie temperature also shows a non-linear and decreasing behavior with magnesium concentration. Similarly, the effect of Mg ?2 -substituted BST on tan d as a function of frequency at room temperature was also monitored as shown in Fig. 3b. The value of the dielectric loss is very much less than 1 for all the samples which make the samples applicable for dielectric applications. Moreover, these low values of tan d account for the impurity and defect-free sample.
The plot of temperature-dependent dielectric constant and tangent loss (tan d) at frequencies (1 kHz, 10 kHz, 50 kHz, and 100 kHz) for pure and Mgsubstituted BST is shown in Fig. 4a and b. The dielectric peaks for Mg ? 2 -substituted barium strontium titanate (BST) ceramics are not dispersed which  [36] and a similar value was reported by Venkata et al. [37]. Shujuan from his work reported the value of dielectric constant as a function of frequency in the range of 3.1-3.8 [24]. When the same material is synthesized by the sol-gel technique, the value of the dielectric constant was reported around 14,000 [28]. The peaks observed from Fig. 4a help in determining ferroelectric to paraelectric transition at Curie temperature. The Curie temperature for pure BST ceramics system observed in the present study is about 20 to 23°C less than reported earlier in the literature [20] and nearly the same as mentioned for samarium-substituted BST [29]. This may be attributed to internal stress that has caused a reduction in Curie temperature. The shift of T C toward lower temperature is due to the decrease in grain size that is because of an increase in internal stress [38].

Ferroelectric properties
The polarization (P) versus electric field (E) behavior was measured using an electric field of 10 kV/cm at 50 Hz. The P-E loop characteristics for Mg-substituted barium strontium titanate are shown in Fig. 5. The sharpness of the loops indicates crystallinity and homogeneity. The values of remanence (P r ), saturation polarization (P s ), and coercive field (E c ) of all the samples observed from the graphs are listed in Table 2. All samples show a well-saturated typical ferroelectric hysteresis loop. The remnant polarization (P r ) and coercive field (E c ) of pure BST are 3.69 lC/cm 2 and 3.55 kV/cm, respectively, at maximum field 21.753 kV/cm, and with substitution of Mg, x = 0.005, coercive field decreases to 2.77 kV/cm while remnant polarization increases and has the highest value of remanence among all compositions. For the other two compositions, the values of both remnant polarization and coercive field have decreased. The composition with Mg = 0.005 (lowest Mg concentration among all concentrations) has a maximum value of P r which shows that in this composition, internal energy is more than other compositions referring to greater dipole alignment [7]. The trend observed from Table 2 showing a decrease in P r with increasing Mg concentration may be due to the presence of pores in addition to the increase in grain size. The reduction in the value of E c indicates the softening of the material on increasing Mg content. The sample having x = 0.005, P r /P s (0.329) ratio has stable value, and then it decreases with Mg concentration, i.e., for x = 0.010, P r /P s is 0.137 and for x = 0.015, P r /P s is 0.081.

Conclusions
The ceramic samples were prepared by the conventional solid-state reaction route. All samples with crystalline perovskite structure having cubic single phase have shown the increase in crystallinity of the samples with increasing Mg content in the XRD pattern. The density was found to decrease with the magnesium content. The dielectric constant decreases with Mg concentration and the maximum value of the dielectric constant observed for Ba 0.935 Sr 0.05-Mg 0.015 TiO 3 . The T C peaks of ceramics prepared are suppressed, broadened, and shifted to lower temperature with the increase of Mg content. The value of remnant polarization (P r ) and coercive field (E c ) decreases as Mg concentration increases. It is concluded from the results that the temperature stability of dielectric properties of Mg-substituted BST is of great practical use. The results indicate that the material Ba 0.935 Sr 0.05 Mg 0.015 TiO 3 is the most promising candidate for capacitive and other engineering applications.