Effect of strontium doping on structural and dielectric behaviour of barium titanate nanoceramics

Abstract

Barium strontium titanate (Ba_(1−x)Sr _x TiO₃ where, x = 0.0, 0.2, 0.3, 0.4 and 0.5) nanoparticles have been successfully synthesised by sol–gel method and characterised thoroughly. With the increase in Sr concentration (x > 0.3), the symmetry of the crystal structure changed from tetragonal to cubic phase. The dielectric behaviour of the ceramic nanoparticles was evaluated using impedance analyser with an operating frequency of 1 Hz to 1 MHz. A very high dielectric constant 4915 was obtained for Ba_0.7Sr_0.3TiO₃ at 1 Hz frequency with low dielectric loss of 1.91, which showed very good value than the one previously reported. The density of the material was found to be >98% of theoretical density. Dielectric constants were measured by varying the temperature from 35 to 150°C and the Curie temperatures were also evaluated. The small amount of Sr doping was beneficial for obtaining high dielectric constant material which can be used in various electronic applications.

Keywords

Barium strontium titanate nanoparticle sol–gel method crystal structure dielectric constant Curie temperature

Introduction

Barium titanate and metal-doped barium titanate ceramics have been widely used in the preparation of high dielectric capacitors, transducers, positive temperature coefficient resistors, piezoelectric sensors, ferroelectric memories and dynamic random access memories (DRAM) [1]. Among these ceramics, strontium-doped barium titanate has been commonly used for various device applications due to its high dielectric constant, high breakdown strength, good thermal stability and small dielectric loss [2]. All these properties of Barium strontium titanate are strongly related to its microstructure. Nowadays strontium-doped barium titanate thin films are expected to replace silicone dioxide, which is a conventional dielectric material and cannot give the required charge storage density [3]. Barium strontium titanate films have effectively contributed to reducing the size of microwave devices. In recent years, composite films have also been used to minimise the size of electronic components; therefore, a variety of polymer/ceramic composites have been prepared to enhance certain properties, including dielectric properties [4,5]. The properties of ceramics are strongly dependent on their crystal structure. Barium titanate ceramic belongs to the perovskite family having a simple crystal structure with ABO₃ stoichiometry, where the A-site cation is large (usually rare earth element), present at the centre, and the B-site cation is smaller (a transition metal ion), present at the corners of the crystal and Oxygen ion in the centre of the face edge. Barium titanate is a polymorphic ceramic, which occurs in different crystal structures – rhombohedral, orthorhombic, tetragonal and cubic with the symmetry R3m, Amm2, P4mm and Pm3m, respectively. The structure interchanges from one form to another and the changes are temperature-dependent [6 -11]. Any small distortion in a cubic unit cell is responsible for the different symmetry of ceramic. Tetragonal structure of barium titanate is a result of distortion along an edge (001) of the unit cell, the orthorhombic crystal structure is due to elongation along a face diagonal (011) or along a body diagonal (111) forms rhombohedral barium titanate. Crystal structure of Barium strontium titanate is a primitive cube like where Ba and Sr present in the corner of the cube, Ti in the middle of the cube and Oxygen in the centre of the face edge.

The method of preparation has an important effect on the structure and nature of the ceramic. Traditionally, BaTiO₃ and doped BaTiO₃ are synthesised by solid-state reaction method under high temperature (above 900°C) [12]. However, the solid-state reaction process is not very useful for the preparation of high-performance ceramics, because the materials synthesised by this method provide large particle size, non-homogeneous mixture of the components and high impurity contents [13 -15]. To overcome these problems, different chemical methods are used for the synthesis of barium titanate-based ceramics, for example sol–gel, hydrothermal, spray pyrolysis and Pechini's method [16 19]. Among these, sol–gel is one of the most important techniques used for the synthesis of barium titanate and metal-doped barium titanate. In this method, mild reaction conditions are required; along with this, it provides excellent particle distribution, high purity product, compositional control and good homogeneity of the components [20]. Also, it is a simple, economic and effective method to synthesise a high-quality product. Barium titanate-based compounds with tetragonal perovskite structure are an interesting class of materials, which show a variety of unique electronic, magnetic and optical properties [21 25].

The present work deals with the synthesis, characterisation and dielectric properties of strontium-doped barium titanate ceramic. A series of five Barium strontium titanate nanoparticles (Ba_(1–x)Sr _x TiO₃ where, x = 0.0, 0.2, 0.3, 0.4 and 0.5) of different stoichiometric ratios were synthesised by the low-temperature sol–gel method. The synthesised ceramic samples were characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM), which confirmed the formation of the single-phase structure of nanoparticles. The effects of strontium doping in the crystal structure and dielectric properties of barium titanate are also discussed. The effects of both frequency and temperature on the dielectric properties were reported. We have observed that tetragonal (x = 0.0, 0.2 and 0.3) to cubic (x = 0.4 and 0.5) phase transition occurred on increasing the concentration of Sr in barium titanate. The calculated dielectric constant values for BaTiO₃, Ba_0.8Sr_0.2TiO₃, Ba_0.7Sr_0.3TiO₃, Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃ were 1175, 2653, 4915, 1631 and 1230, respectively (at 1 Hz frequency). A very high dielectric constant value was found for the small concentration of Sr-doped barium titanate as compare to the previously reported values. Also, a significantly improved density (>98% of theoretical density) for Ba_0.7Sr_0.3TiO₃ (sintered at 1000°C) was observed. The Curie temperatures (Tc) were also detected for the synthesised ceramic samples and were found to be 120, 50 and 40°C for BaTiO₃, Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃, respectively.

Experimental

Materials

Barium acetate (99.5%, Anala R, A.R.), strontium acetate (99.55%, Himedia, A.R.) and titanium (IV) isopropoxide (98%, ACROS organics) were used as the precursor materials for the Barium strontium titanate. 2-Methoxyethanol (99%, Spectrochem, A.R) was used to avoid precipitation of titanium (IV) isopropoxide. For the complete dissolution of the precursor materials into the acetic acid (99.5%, Fisher Scientific) solvent, ethylene glycol (98%, Qualigens Fine Chemicals) was added to it.

Synthesis procedure

Ba_(1–x)Sr _x TiO₃ of different stoichiometric ratios were synthesised by the sol–gel process. Figure 1 represents the steps for the synthesis of Ba_(1–x)Sr _x TiO₃ nanoparticles. The measured amount of barium acetate and strontium acetate were dissolved in acetic acid solvent and refluxed at 60°C for 3 h. After that, ethylene glycol was added to the precursor solution during this process to ensure the complete dissolution of the reactants. Thereafter, titanium isopropoxide was added to 2-methoxyethanol to avoid any precipitation of titanium compound, and this mixture was then added to the precursor solution to obtain the sol phase. The sol was further refluxed for 2 h to get the gel phase. The gel was evaporated at 100°C with continuous stirring to form precursor powder. The obtained powder was further dried in an oven at 60°C for 12 h. The powder was grinded well in a mortar and pestle. The obtained barium strontium titanate powder was calcined at 700°C for 2 h in a muffle furnace for crystallisation.

Characterisation

FTIR spectra of the synthesised Ba_(1–x)Sr _x TiO₃ powder were measured on IR-Prestige-21 (Shimadzu) made in Japan, in the range of 4000–400 cm⁻¹ to understand the chemical bonding in the ceramics. The XRD patterns of the ceramic powder were observed by using Brucker D8 diffractometer with CuKα (λ = 1.541 Å) radiation, at a scanning rate of 2° min⁻¹ in the range of 20–70° Bragg's angle. Raman spectroscopy of the samples was studied by Renishaw InVia Raman microscope (made in UK), in the range of wavenumber 100–800 cm⁻¹. Structure and surface morphology was analysed by SEM using JEOL JSM 6390LV instrument. The density of the sample was measure by applying the Archimedes concept. Evaluation of dielectric constant of the synthesised ceramic samples with respect to temperature over the range of 35–150°C was done by using LCR meter (HIOKI 3532–50 CR Hi Tester). The frequency-dependent dielectric measurements were also performed using impedance analyser (Novocontrol ALPHA ATB) in the frequency range of 1 Hz to 1 MHz. The impedance analyser provides capacitance values in a different frequency range. To obtain capacitance values, pellets (measuring 10 mm in diameter and 1 mm in thickness) of calcinated Ba_(1–x)Sr _x TiO₃ powder were prepared by using a stainless steel die and then sintered at 1000°C for 2 h in order to increase the mechanical strength. For capacitance measurements, both the surfaces of the sintered pellets were coated with silver paste. The dielectric constant was calculated by using the formula, k = Ct/ϵ_oA [26], where, k is the dielectric constant, C is the capacitance, t is the thickness, ϵ_o is the permittivity of free space (8.85 × 10⁻¹² F m⁻¹) and A is the area of the sintered pellet.

Result and Discussion

Balachandran et al. synthesised Sr-doped barium titanate, Ba_0.5Sr_0.5TiO₃, by sol–gel method. They reported the dielectric constant value 1164 and dielectric loss value 0.063. They have compared the simulated dielectric properties with the experimental values of the ceramic [27]. Patil et al. observed that the dielectric constant increased with increasing Sr content in Ba_(1–x)Sr _x TiO₃ (x = 0.1–0.3) ceramic. They reported the dielectric constant values 750, 830 and 1056 at room temperature for Ba_0.9Sr_0.1TiO₃, Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃, respectively. Dielectric properties were reported as a function of both frequency and temperature. Curie temperature was also reported as 60, 45 and 35°C for Ba_(1–x)Sr _x TiO₃ with x = 0.1, 0.2 and 0.3, respectively, which means the Tc decreased with the increase in Sr concentration. All the samples were synthesised by using the double sintering ceramic technique. They observed the tetragonal structure for all the three Sr-doped barium titanate [28]. Electrical properties of Ba_0.8Sr_0.2TiO₃ were studied by Barik et.al. They used the solid-state reaction technique for the synthesis of ceramic and characterised thoroughly. XRD pattern of the ceramic powder sample confirmed the tetragonal structure of the sample. Electrical properties related to the microstructure of the synthesised ceramics were also described by them [29]. Zhang et al. prepared Ba_0.6Sr_0.₄TiO₃-thick films by the sol–gel method. The cubic perovskite structure of the material was reported. The observed dielectric constant value for the same material was 1200 with the dielectric loss value 0.01 at 10°C [30]. Mahani et al. studied the dielectric behaviour of barium strontium titanate but did not notice a remarkable change in dielectric properties. They synthesised only two different compositions of strontium-doped barium titanate (x = 0.1 and 0.5) with the tetrahedral phase. They used modified sol–gel method for the synthesis procedure [31]. Wodecka-Duś et al. synthesised Ba_0.6Sr_0.4TiO₃ ceramic by the sol–gel method. They observed the cubic perovskite structure of the ceramic, after sintering at 1450°C (high temperature) for 4 h. A temperature-dependent dielectric constant was reported for both cooling and heating processes in the range of −100 to 100°C for the synthesised ceramics [32]. Noh et al. had synthesised Ba_(1–x)Sr _x TiO₃ (x = 0.0–0.3) and reported that the crystal structure changed from tetragonal phase to the cubic phase on increasing the amount of strontium substitution on barium titanate. They observed that the particle size decreased with increase in the Sr doping concentration (from x, 0.0–0.3) [33]. Sandi et al. synthesised strontium-doped barium titanate by the solid-state reaction method. They synthesised three different compositions of ceramics – BaTiO₃, Ba_0.9Sr_0.1TiO₃ and Ba_0.5Sr_0.5TiO₃. Among these, Ba_0.5Sr_0.5TiO₃ was cubic and the other two were tetragonal. They reported the highest dielectric constant value 196 at 1 KHz frequency [34]. Here, we have synthesised Ba_(1–x)Sr _x TiO₃ ceramic nanoparticles by sol–gel method. The crystal structure and dielectric properties of the ceramics were greatly affected by the Sr concentration. The dielectric properties of the samples were explained with the variation of both frequency and temperature. In the present work, the enhanced dielectric constant values were obtained as compared to the previously reported values.

FTIR analysis

FTIR spectrum supported the formation of Ba_(1–x)Sr _x TiO₃ nanoparticles as shown in Figure 2. FTIR is an important technique to study the reaction procedure for the preparation of materials because it is sensitive to chemical bonds [35,36]. An absorption band of O-H stretching vibration at about 3400 cm⁻¹ was observed in the spectrum. It was expected due to the presence of adsorbed moisture during the preparation of the sample with KBr powder for FTIR analysis. Two bands near 1450 and 1700 cm⁻¹ were due to symmetric and asymmetric stretching vibrations respectively of the carboxylate group that indicated the presence of an acetate group bonded to barium atoms [37]. These bands were supposed to disappear at a higher temperature (more than 700°C). The absorption band of wavenumber below 800 cm⁻¹ represented the M-O bond [38]. In the spectrum, a band near 540 cm⁻¹ was observed which indicated Ti-O stretching normal vibration. Another absorption band below 400 cm⁻¹ was assigned to the Ti-O bending normal vibration which was not covered in the FTIR region.

XRD analysis

The formation of a single-phase structure of Ba_(1–x)Sr _x TiO₃ was confirmed by XRD study without any reflection of other secondary phases. Figure 3 shows XRD patterns of Ba_(1–x)Sr _x TiO₃ nanoparticles synthesised at 700°C. It was observed that the sol–gel synthesised ceramic powders were well crystallised after calcination at 700°C. At this temperature, a well-developed cubic phase was detected for Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃ but in BaTiO₃, Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃, the tetragonal phase was found. It was observed that the crystal structure of the synthesised ceramic samples changed from cubic to tetragonal with the variation of strontium concentration. The tetragonality (c/a ratio) was maximum in case of Ba_0.7Sr_0.3TiO₃, i.e. x = 0.3 and with the increase in Sr concentration, the structure changed from tetragonal to cubic and this transformation occurred in between the stoichiometric composition where x = 0.3–0.4. The XRD graph shows eight peaks with Miller indices (0 0 1), (1 0 1), (1 1 1), (2 0 0), (1 0 2), (2 0 1), (2 1 1) and (2 0 2) for BaTiO₃. And all the peaks matched with the JCPDS card No. 81–2203 having space group P4mm. This confirmed the tetragonal crystal structure of the synthesised barium titanate. In Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃, (1 0 0), (1 0 1), (1 1 1), (2 0 0), (2 0 1), (2 1 1) and (2 0 2) peaks were observed, and these peaks matched with the JCPDS card No. 44–0093, which supported the formation of a tetragonal structure with P4mm symmetry of the ceramic. In Ba_0.6Sr_0.4TiO₃ (JCPDS card No. 34–0411), the (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 1) and (2 2 0) peaks confirmed the formation of a cubic perovskite structure with regular symmetry of Pm3m space group. Similarly, Ba_0.5Sr_0.5TiO₃ with the Miller indices of (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0), (2 1 1) and (2 2 0) (JCPDS card No. 39–1395) confirmed the formation of a cubic perovskite structure with Pm3m space group. The lattice parameters of the samples were obtained by using the software X'Pert HighScore Plus and are listed in Table 1. For Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃, the ratio of lattice parameters (c/a) is equal to 1 which supported the cubic crystal structure of the nanoparticles, but for BaTiO₃, Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃, the c-axis lattice parameter is larger than the a-axis lattice parameter; therefore, c/a ratio is greater than 1 which suggested the tetragonal crystal structure of the nanoparticle [39]. The value of c/a ratio for Ba_0.7Sr_0.3TiO₃ indicated the higher tetragonality of the nanoparticle in comparison to BaTiO₃ and Ba_0.8Sr_0.2TiO₃. The crystal size of the obtained Ba_(1–x)Sr _x TiO₃ nanoparticles were calculated by using Scherrer's equation, i.e. D = 0.9λ/(βcosθ), where D is the crystallite size, λ is the wavelength of the X-ray radiation, θ is the diffraction angle and β is the full width at half maximum in radian. The average crystallite size of the obtained BaTiO₃, Ba_0.8Sr_0.2TiO₃, Ba_0.7Sr_0.3TiO₃, Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃ nanoparticles was 17.11, 19.82, 18.6, 16.09 and 16.0 nm, respectively. Tables 2 and 3 represent the h k l, θ and D value for barium titanate and strontium-doped barium titanate, respectively.

Table 1

Lattice parameter of Ba_(1–x)Sr _x TiO₃.

BaTiO₃	Ba_0.8Sr_0.2TiO₃	Ba_0.7Sr_0.3TiO₃	Ba_0.6Sr_0.4TiO₃	Ba_0.5Sr_0.5TiO₃
a = 3.994 Å	a = 3.963 Å	a = 3.965 Å	a = 3.964 Å	a = 3.947 Å
c = 4.002 Å	c = 3.975 Å	c = 3.985 Å	c = 3.964 Å	c = 3.947 Å
c/a = 1.002	c/a = 1.003	c/a = 1.005	c/a = 1	c/a = 1

Table 2

h k l, θ and D (grain size) value of the obtained BaTiO₃ nanoparticle.

h k l	θ	D (nm)
0 0 1	11.05	24.14
1 0 1	15.89	19.76
1 1 1	19.45	17.16
2 0 0	22.94	14.66
1 0 2	25.15	23.22
2 0 1	25.64	15.97
2 1 1	28.35	10.92
2 0 2	32.97	11.11

Note: The average crystallite size of the nanoparticles was 17.11 nm and the dielectric constant value was 1175 at 1 Hz frequency.

Table 3

h k l, θ and D (grain size) value of the obtained Sr-doped BaTiO₃ nanoparticle.

Ba_0.8Sr_0.2TiO₃			Ba_0.7Sr_0.3TiO₃			Ba_0.6Sr_0.4TiO₃			Ba_0.5Sr_0.5TiO₃
h k l	θ	D (nm)	h k l	θ	D (nm)	h k l	θ	D (nm)	h k l	θ	D (nm)
1 0 0	11.19	24.5	1 0 0	11.14	24.53	1 0 0	11.27	23.98	1 0 0	11.12	23.98
1 0 1	15.85	22.1	1 0 1	15.87	20.49	1 1 0	15.96	17.70	1 1 0	15.91	17.82
1 1 1	19.67	22.1	1 1 1	19.7	20.45	1 1 1	19.47	15.58	1 1 1	19.66	17.22
2 0 0	22.80	16.3	2 0 0	22.85	15.77	2 0 0	22.93	15.63	2 0 0	22.93	14.52
2 0 1	25.64	19.2	2 0 1	25.72	18.68	2 1 1	28.49	10.47	000	24.78	17.94
2 1 1	28.28	16.0	2 1 1	28.31	15.24	2 2 0	33.47	13.16	2 1 0	28.55	12.41
2 0 2	33.19	18.4	2 0 2	33.29	16.17				2 1 1	33.46	10.69

Note: The average crystallite size of the Ba_0.8Sr_0.2TiO₃, Ba_0.7Sr_0.3TiO₃, Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃ nanoparticles were 19.82, 18.60, 16.09 and 16 nm, respectively, and the dielectric constant values were 2653, 4915, 1631 and 1230 at 1 Hz frequency, respectively.

Raman spectroscopy

Figure 4 represents the Raman spectra of the calcined ceramic samples. Raman spectroscopy is a very important technique for the structural analysis of the materials. The material belongs to ABO₃ perovskite crystal structure has 5 atoms per unit cell and 12 long wavelength optical modes. In this case, the optical modes transform to the triply degenerate modes – 3F_1u+F_2u. Here, F_1u modes are IR active, whereas F_2u is the silent mode. However, the material that belongs to the tetragonal phase has p4mm symmetry, where the triply degenerate mode (F_1u) splits to the doubly degenerate mode (E mode) and nondegenerate mode (A₁ mode), whereas the silent mode, F_2u, splits into E and B₁ modes [40,41]. In the present work, the Raman spectra supported the obtained XRD data of the samples. Two peaks centred at around 244 and 574 cm⁻¹ were observed for the Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃, whereas the bands about 170 (A₁(TO₁), 265 (A₁(TO₂), 306 (B₁/(E(TO₂), 517 (A₁(TO₃) and 720 cm⁻¹ (A₁(LO₃)/E(LO) were observed for BaTiO₃, Ba_0.7Sr_0.3TiO₃ and Ba_0.8Sr_0.2TiO₃. The peaks around 306 and 720 cm⁻¹ were specific for the tetragonal phase [40,42,43]; therefore, the tetragonal phase was confirmed for the undoped and strontium-doped barium titanate (x = 0.2 and 0.3) and the other two were in cubic phase.

Surface morphology and density measurement

Figure 5 is one of the representative Energy-Dispersive X-ray (EDX) Spectroscopy and SEM images of the calcined powdered and sintered sample of Ba_0.7Sr_0.3TiO₃ nanoparticles. SEM provided a surface morphological study of the nanoparticles. The calcinated powdered sample indicated the homogeneous distribution of nanoparticles. After sintering at 1000°C for 2 h, well-developed closely packed grains were observed. The grain size was found to be uniform in the sintered sample. In the calcinated powdered sample, larger particles were also present, which were actually a group of smaller particles that have not yet undergone grain growth. The EDX spectrum was used for the elemental analysis of the samples; here, it confirmed the presence of Ba, Sr, Ti and O species in the synthesised nanoparticles. No other impure element has been observed in the spectrum. The density of the calcined and sintered Ba_0.7Sr_0.3TiO₃ ceramic was calculated by using the Archimedes concept. The SEM image of the sintered sample showed a significant densification as compared to the calcined material. The calculated density for the calcined ceramic sample was 5.40 g cm⁻³ which increased to 5.76 g cm⁻³ for the sintered material. It was about 98.6% of the theoretical density (5.838 g cm⁻³) of the Sr-doped barium titanate. The value showed that a highly dense ceramic could be produced by the sol–gel method. The high density of the ceramic was responsible for the higher value of dielectric constant [44].

Dielectric properties

In this work, the dielectric constant was measured with an operating frequency of 1 Hz to 1 MHz. The dielectric constant value of the Ba_(1–x)Sr _x TiO₃ ceramic samples varied by changing the barium to strontium ratio. The obtained dielectric constant for BaTiO₃, Ba_0.8Sr_0.2TiO₃, Ba_0.7Sr_0.3TiO₃, Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃ were 1175, 2653, 4915, 1631 and 1230, respectively, at 1 Hz frequency. This indicated that a small amount of strontium doping increased the dielectric constant value of ceramic nanoparticles, but higher amount of Sr reduced the value. Figure 6(a) shows that there is a decrease of dielectric constant with the increase of frequency for all the obtained nanoparticles and attained a constant value above 10 KHz frequency. The dielectric constant was observed to have the maximum value at low frequency which might be due to different types of polarisation (atomic, ionic and electronic). However, at higher frequencies, the dielectric constant value decreased owing to the decreased contribution from polarisation, i.e. only electronic polarisation was responsible for the low dielectric constant value of the ceramic [45]. The value of dielectric constant could also be explained according to the lattice shape and the presence of the dipole in the Ba_(1–x)Sr _x TiO₃ lattice. In the cubic perovskite structure of Ba_(1–x)Sr _x TiO₃, Ti atoms are octahedrally coordinated by six oxygen atoms, but in tetragonal displacement of Ti atoms are along the c-axis from its centrosymmetric position in the unit cell, which creates a permanent electric dipole, and this is responsible for the high dielectric constant value of the ceramic. From Figure 7, it is clear that with the increase in strontium concentration, the dielectric constant of strontium-doped BaTiO₃ increased up to x = 0.3 and then it decreased with the increase in strontium concentration, which was due to the fact that the Sr dopant affected the lattice parameter values of the ceramic. In addition to that, the bigger crystallite size of the barium strontium titanate (with x = 0.2 and 0.3), as compared to the undoped barium titanate, was responsible for their higher dielectric constant value, throughout the measured frequency region. From the dielectric constant values, it could be concluded that the higher the c/a ratio the higher will be the tetragonality and, accordingly, the higher will be the dielectric constant value of the ceramic.

The dielectric constant values and the dissipation factor both followed the same trend with the change in frequency, i.e. the values were maximum in the lower frequency region and minimum in the higher frequency region. The variation of dielectric loss with frequency of Ba_(1–x)Sr _x TiO₃ nanoparticles (sintered at 1000°C for 2 h) is shown in Figure 6(b). In the lower frequency region, the obtained dielectric loss values for all the synthesised Sr-doped barium titanate were strongly dependent on frequency, but in the higher frequency region, the values seem to be close to the saturation phase. The dielectric losses are reduced at higher frequencies because in this region dipole contributes to the polarisation [46]. In the dielectric system, the dielectric loss is the energy dissipation, i.e. it measures how much electric field is lost in the form of energy or heat by applying electric field during the polarisation process. Dielectric loss is proportional to the imaginary part of dielectric constant. The space charge polarisation effect also affects the dielectric loss value of the material [47]. It was observed that the dielectric loss values ranged from 0.6 to 7.27. And the values were increased with the increase in Sr content as compared to the undoped barium titanate. The highest dielectric constant and the low dielectric loss value 4915 and 1.91, respectively, were obtained for Ba_0.7Sr_0.3TiO₃ at 1 Hz frequency.

The variation of dielectric constant with temperature at a frequency of 100 KHz for Ba_(1–x)Sr _x TiO₃ is shown in Figure 8. The dielectric constant of BaTiO₃ and Sr-doped BaTiO₃ with x = 0.2 and 0.3 increased gradually with the increase in temperature up to the Tc and then it decreased. The obtained Tc for the undoped barium titanate was 120°C. The Tc point for Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃ was 50 and 40°C, respectively. The position of the Tc was found to shift towards the lower temperature side with the increase in Sr concentration. Curie temperature of Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃ were not detected in the measured temperature region as their Curie temperature was below the room temperature [48]. The decrease in Curie temperature with the increase in Sr concentration could be related to the average ionic radius of the A-site cations. A substitution of Sr ion (1.18 Å) on Ba ion (1.35 Å) site causes a reduction in the average ionic radius and thereby stabilises the paraelectric phase (cubic structure) in the lower temperature region [49,50]. The maximum dielectric constant of the nanoparticles at Curie temperature was due to the thermal activation of electrons at this temperature. The dielectric behaviour of the materials with the variation of temperature could be explained on the basis of the polarisation effects. Below Tc, the materials become spontaneously polarised, i.e. electric polarisation develops in the material without the application of an external field. In this temperature region (below Tc), the dipole interacts with each other, and as a result, internal field arises which lines up the dipole; thus, at Tc, phase transition takes place from ferroelectric to paraelectric state. But above Tc, the materials are in paraelectric phase where the elementary dipoles are randomly oriented [51]. The dielectric polarisation increased with the rise in temperature which results in the increase in dielectric constant [52]. In our previous work, the composition Ba_0.7Sr_0.3TiO₃ was used for the preparation of polyimide–ceramic nanocomposite films due to their highest dielectric constant value [53].

Conclusion

Barium strontium titanate Ba_(1–x)Sr _x TiO₃ where x = 0.0, 0.2, 0.3, 0.4 and 0.5 were successfully synthesised by low-temperature sol–gel process. The synthesised ceramic nanoparticles were characterised by XRD, FTIR, Raman spectroscopy and SEM analyser. Frequency- and temperature-dependent dielectric properties were also evaluated. The phase purity and crystalline structure of Ba_(1–x)Sr _x TiO₃ nanoparticles were determined by using an XRD. The crystallite size of the particles was in the range of 16–19.82 nm. It was found that after calcination at 700°C, Ba_0.6Sr_0.4TiO₃ and Ba_0.5Sr_0.5TiO₃ ceramic samples exhibited a cubic phase with regular symmetry of Pm3m space group and BaTiO₃, Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃ exhibited the tetragonal phase with P4mm symmetry. The formation of cubic and tetragonal phase structure of the Sr-doped barium titanate was supported by Raman spectroscopic study. In comparison to barium titanate, a much higher value of dielectric constant was achieved with strontium doping. Smaller amount of doping (0.2 < x < 0.4) was beneficial to obtain a higher dielectric constant value, but with the increase in doping concentration, the dielectric values were decreased. The highest dielectric constant value 4915 at 1 Hz frequency was obtained for Ba_0.7Sr_0.3TiO₃. The crystal structure and dipole response of Ba_(1–x)Sr _x TiO₃ nanoparticles affected the variation of dielectric constant and their dissipation factor. The obtained Tc for BaTiO₃, Ba_0.8Sr_0.2TiO₃ and Ba_0.7Sr_0.3TiO₃ was 120, 50 and 40°C, respectively. The position of the Tc was found to shift towards the lower temperature side as the Sr doping concentration increased. Barium strontium titanate is a lead-free ceramic; thus, it is an environment-friendly material. Additionally, it shows high dielectric constant and low dissipation factor; therefore, it is useful for various electronic device applications such as multi-layer ceramic capacitor, DRAM and semiconductors.

Footnotes

Acknowledgements

The authors greatly acknowledge the Central Research Facility (C.I.F), Birla Institute of Technology, Mesra, Ranchi, India, and UGC-DAE Consortium for Scientific Research, Indore, India for providing characterisation of the samples.

Disclosure statement

No potential conflict of interest was reported by the authors.

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