3.1 Structural and Microstructural Analysis
Fig. 1 shows the diffraction pattern of Mg substituted barium strontium titanate with magnesium i.e. Ba0.95-xSr0.05MgxTiO3 with x having values 0.005, 0.010 and 0.015. It is revealed that Mg substituted barium strontium titanate exhibit ABO3 cubic perovskite structure with high orientation at (110). The substitution of magnesium in 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 peak of other material or phase which indicates that addition of magnesium has not formed any new phase such as MgCO3, 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 higher angle on adding Mg. This implies that magnesium has successfully substituted Barium on A site of ABO3 on surface. Since the atomic radius of Mg (0.72Å) is smaller than Ba (1.35Å) 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 doping magnesium [17,19]. The average crystallite size (t) of magnesium substituted barium strontium titanate powder samples is calculated using Scherrer’s formula as mention below:

Where, λ is the wavelength of Cu-Kα radiation source (λ=0.15418nm), β (in radians) is the full width at half maxima and θ is the Bragg’s angle. The calculated values of average crystallite size are shown in Table 1. Crystallite size is in the range of nano scale and pure BST have smaller crystallite size than Mg substituted BST. The lattice constant and density seems to decrease with increasing Mg+2 concentrations. This emphasises that Magnesium is successfully substituted on Ba (A) site in barium strontium titanate lattice.
Table 1
Change of lattice constant, volume of unit cell, density crystallite size of Ba0.95-xSr0.05MgxTiO3 with Mg content:
Mg Content
|
Lattice constant
a(Å)
|
Volume
V(Å)3
|
Density
ρ(g/cc)
|
Crystallite Size
t(nm)
|
x=0
|
3.9838
|
63.688
|
6.014
|
32.89
|
x=0.005
|
3.9926
|
63.647
|
6.003
|
35.24
|
x=0.010
|
3.9916
|
63.599
|
5.989
|
40.11
|
x=0.015
|
3.9893
|
63.488
|
5.988
|
40.36
|
3.1.2 Microstructural Analysis
Fig. 2 shows the microstructure of fractured surfaces of pure and Magnesium consisting Barium Strontium Titanate (Ba0.95-xSr0.05MgxTiO3) with Mg = 0, 0.005, 0.010 and 0.015. The micrographs were analysed using Image J Software and the size of grain was calculated with the help of line intercept method. The average grain size of pure BST samples was measured to be 2.185 µm, and the average grain size was observed to decrease with the increasing concentration of Magnesium. The grain size reduced from 2.315 to 1.789 µm in Magnesium substituted samples. The grains are distributed evenly and seem to scatter all around. The increased scattering of the grains show 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.
3.2 Dielectric Properties
The modification of dielectric constant (ε) as a function of frequency at room temperature for pure and Mg+2 substituted barium strontium titanate (Ba0.95-xSr0.05MgxTiO3: x=0, 0.005, 0.010 and 0.015) ceramic system is plotted in Fig. 3a. As we had limited experiment range of frequency so we have taken frequency from 100Hz – 800 kHz. The dielectric permittivity decreases with the increase in frequency confirming 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 is due to the presence of dipolar & electronic polarization that may be justified by Maxwell-Wagner space charge polarization model [20]. The values of dielectric constant were found to decrease with increase in Mg content and 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 sudden increase in value of dielectric constant at maximum concentration of magnesium. The Curie temperature also shows a non-linear and decreasing behaviour with magnesium concentration. Similarly, the effect of Mg+2 substituted BST on dielectric loss (D) as a function of frequency at room temperature was also monitored as shown in Fig. 3b. The value of dielectric loss is very much less than 1 for all the samples which makes the samples applicable for dielectric applications. Moreover, these low values of tanδ accounts for the impurity and defect free sample.
Table 2
Dielectric and ferroelectric parameters for Mg substituted BST.
Mg Content
|
Curie’s Temperature
(TC)
|
Dielectric Constant
at TC
|
Dielectric Constant
at RT
|
Tangent Loss
tanδ
|
Remnant
Polarization
(Pr)
|
Coercive
Field
(Ec)
|
Saturation Polarization
(Ps)
|
x=0
|
96
|
5307
|
2946
|
0.023
|
3.69
|
3.55
|
18.23
|
x=0.005
|
85
|
8509
|
2662
|
0.041
|
6.71
|
2.77
|
20.36
|
x=0.010
|
66
|
2589
|
2527
|
0.044
|
1.87
|
1.87
|
13.64
|
x=0.015
|
63
|
3840
|
3301
|
0.028
|
1.05
|
1.35
|
12.987
|
The plot of temperature dependent dielectric constant and tangent loss (tanδ) at frequencies (1kHz, 10kHz 50kHz and 100kHz) for pure and Mg substituted BST is shown in Fig. 4a and 4b. The dielectric peaks for Mg+2 substituted Barium Strontium Titanate (BST) ceramics are not dispersed which encouraged that all prepared ceramic samples behaves like normal ferroelectrics. Sample with x=0 and x=0.005 shows sharp maxima and the other two compositions have broad peaks with increasing Mg content. The broadening of peaks may be due to compositional variation which lead to different transition temperatures and decrease in dielectric constant. There is no shift observed in dielectric maxima with increment in frequency. All the samples show the increase in dielectric constant with increasing temperature up to transition temperature and then decrease with increase in temperature showing typical ferroelectric behaviour. The tangent loss factor also decreases with increasing concentration of Mg and values in every sample are less than 1. A noticeable improvement has been seen in all samples with varying Mg content. The dielectric effects of Mg doped barium strontium titanate are reported in earlier literature also. Liu et al. in 2012 also reported value of dielectric constant of Mg substituted BST thick films around 385 [21] and similar value was reported by Venkata et al. in 2009 [22]. Shujuan from his work reported the value of dielectric constant as a function of frequency in the range of 3.1-3.8 [17]. When the same material is synthesized by sol- gel technique the value of dielectric constant was reported around 14000 [19]. 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 present study is about 20°C to 23°C less than reported earlier in the literature [23] and nearly same as mentioned for samarium substituted BST [24]. This may be attributed to internal stress that has caused reduction in Curie temperature. The shift of TC toward lower temperature is due to the decrease in grain size that is because of increase in internal stress [25].
3.3 Ferroelectric Properties
The polarization (P) versus electric field (E) behaviour was measured using an electric field at 10kV/cm at 50Hz. The P-E loop characteristics for Mg substituted barium strontium titanate are shown in Fig. 5. The sharpness of the loops indicates the crystallinity and homogeneity. The values of Remanence (Pr), Saturation polarization (Ps) and Coercive field (Ec) of all the samples observed from the graphs are listed in table 2. All samples shows well saturated typical ferroelectric hysteresis loop. The remnant polarization (Pr) and Coercive field (Ec) of pure BST are 3.69µC/cm2 and 3.55kV/cm respectively at maximum field 21.753kV/cm and with substitution of Mg, x=0.005, coercive field decreases to 2.77kV/cm while remnant polarization increases, in fact has highest value of remanence among all compositions. For other two compositions the values of both remnant polarization and coercive field has decreased. The composition with Mg=0.005 (lowest Mg concentration among all concentration) has maximum value of Pr 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 decrease in Pr with increasing Mg concentration may be due to presence of pores in addition to the increase in grain size. The reduction in the value of Ec indicates the softening of the material on increasing Mg content. The sample having x= 0.005, Pr/Ps (0.329) ratio have stable value and then it decreases with Mg concentration i.e. for x=0.010, Pr/Ps is 0.137 and for x=0.015, Pr/Ps is 0.081.