Single-Source Facile Synthesis of Phase-Pure Na⁺- and Sr²⁺-Modified Bismuth Titanate—Structural, Optical, and Electrical Properties for Energy Storage Application ^†

Abstract

In this present study, sodium- and strontium-modified bismuth titanate—Bi_0.5Na_0.5TiO₃ (BNT) and Bi_0.5Sr_0.5TiO₃ (BST)—were synthesized using the auto-combustion technique with citric acid (C₆H₈O₇) and glycine (C₂H₅NO₂) as fuels in an optimized ratio of 1.5:1. The resulting powders were characterized using X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, UV–Visible diffuse reflectance spectroscopy (DRS), and Fourier-transform infrared (FT-IR) spectroscopy. The electrical behavior of the samples was studied using an LCR meter. XRD analysis confirmed the formation of a single-phase perovskite structure with average crystallite sizes of 18.60 nm for BNT and 22.03 nm for BST, attributed to the difference in ionic radii between Na⁺ and Sr²⁺. An increase in crystallite size was accompanied by a corresponding increase in lattice parameters and unit-cell volume. The Williamson–Hall analysis further validated the strain-size contributions. EDX (Energy-Dispersive X-ray analysis) results confirmed successful incorporation of Na⁺ and Sr²⁺ without detectable impurity phases. Optical studies revealed distinct absorption peaks at 341 nm for BNT and 374 nm for BST, and the optical bandgap (E_g), calculated using Tauc’s relation, was found to be 2.6 eV and 2.0 eV, respectively. FT-IR spectra exhibited characteristic Ti-O vibrational bands in the range of 420–720 cm⁻¹, consistent with the perovskite structure. For electrical characterization, the powders were pelletized under 3-ton pressure and sintered at 1000 °C for 3 h. The dielectric constant (ε_r), dielectric loss (tan δ), and ac conductivity (σ) of both samples increased with frequency. The combined structural, optical, and electrical results indicate that the optimized compositions of BNT and BST possess properties suitable for use in capacitors and other energy-storage applications.

1. Introduction

Modern researchers focus on perovskite-structured (ABO₃) oxide ceramics due to their optical behavior and various electrical properties, including dielectric, ferroelectric, piezoelectric, and pyroelectric nature. Because of this, they play a vital role as actuators, capacitors, transducers, long-term FeRAM and batteries, and so on [1]. Bismuth sodium titanate (Bi_0.5Na_0.5TiO₃) and bismuth strontium titanate (Bi_0.5Sr_0.5TiO₃) stand as excellent lead-free oxide-based perovskite ceramics because of their elevated Curie temperature, intense remanent polarization, and good thermal stability [1,2]. These ceramics are prepared by various methods like traditional solid-state method, hydrothermal method, sol–gel auto-combustion technique, and microwave method [3].

In general, BNT ceramics are a good dielectric material due to their high permittivity and low dissipation energy. Earlier study with BNT showed a high dielectric permittivity of ε = 3871, with a tan loss of δ = 0.995 at 100 Hz near the Curie temperature. While the DC conductivity was in accordance with Mott’s variable range hopping (VRH) mechanism, the AC conductivity mirrored Jonscher’s power and double power law behavior, demonstrating non-Debye type relaxation in the electrical response [1]. Similar studies on Sr_1−xBi_xTiO₃ ceramics discovered that the dielectric constant expanded from 1025 to 2320 at ambient temperature, implying a substantial enhancement in dielectric characteristics with increasing Bi content. The frequency-dependent dielectric response was explained using the correlated barrier hopping (CBH) model, supported by a reduction in activation energy with higher Bi substitution [4].

In this study, pristine BNT and BNT ceramics are prepared via the auto-combustion technique. The powder samples are calcinated at 950 °C for 5 h. The structural properties are studied using X-ray diffraction (XRD), while the elemental composition of the powders is determined using energy-dispersive X-ray spectra (EDX). The functional groups and the optical behaviors are studied using Fourier-transform IR Spectra (FT-IR) and UV–Visible DRS. An LCR meter is adopted to examine the electrical properties of the samples.

2. Experimental Techniques

The perovskite ceramics were prepared via the auto-combustion technique. Bi(NO₃)₃, C₁₂H₃₆O₄Ti, NaNO₃, C₂H₅NO₂, C₆H₈O₇, and HNO₃ were the chosen precursors for the preparation of BNT. The stoichiometry amount of these starting materials was employed without additional purification. The ratio of citric acid to nitric acid and glycine was fixed to be 1.5:1:1. The incorporation of nitric acid stabilizes and inhibits the formation of precipitates in the solution. The schematic diagram illustrated in Figure 1 shows the step-by-step preparation process of BNT. A similar method is followed for the synthesis of BST.

The structural behavior of the samples calcinated at 950 °C was characterized using an X-ray diffractometer (XRD) (PANalytical X’Pert, Malvern Panalytical, Malvern, UK). Energy-dispersive X-ray spectroscopy (EDX) (D-12489, BRUKER Nano GmbH, Berlin, Germany) was employed to find the elemental composition, while the functional groups were studied by utilizing Fourier-transform infrared spectroscopy (FT-IR) (FT/IR-4700, Jasco, Tokyo, Japan). The energy bandgap of BNT and BST was calculated from the absorbance spectra obtained via UV–Visible DRS (LAMBDA-35, Perkin Elmer, Shelton, WA, USA). The electrical properties (dielectric permittivity, tangent loss, and ac conductivity) were measured using an LCR meter.

3. Results and Discussion

The XRD patterns of the fabricated BNT and BST samples are depicted in Figure 2a. The perovskite structure of the samples is confirmed without any trace of impurities. This proves that Na⁺ and Sr²⁺ ions have completely diffused into the bismuth titanate matrix crystal lattice. The significant diffraction profiles indicate the excellent crystallinity of the prepared samples. No additional peaks were detected apart from the (111) and (200) reflections, indicating that the synthesized samples possess a pseudo cubic structure [5]. The most intense peak (110) of BNT inclines towards the higher angles when compared to BST, where this shift can be due to the smaller ionic radii of Na (1.02 Å in six co-ordination), while Sr is 1.18 Å in six co-ordination. In addition, the microstructural features of BNT and BST are systematically estimated through the Williamson–Hall plot, incorporating both crystallite size and lattice strain contributions. Figure 2b shows the plot displaying a positive slope, which supports the lattice extension and a shift in lattice parameters along a particular crystallographic direction. The average crystallite size (D) was calculated by adopting Debye Scherrer’s formula, and it was found that BST has a larger crystallite size than BNT. Also, other parameters, including the lattice constant (A), volume of the unit cell (V), micro strain (ε), dislocation density (δ), and packing factor (P) values, are calculated, and the results are tabulated in

Table 1. Average crystallite size (D), lattice constant (A), volume of the unit cell (V), microstrain (ε), dislocation density (δ), and packing factor (P) of BNT and BST.

Samples	D (nm)	A (Å)	V (Å)	ε (103)	δ (10−2)	P
BNT	18.60	3.8722	58.05	1.11	2.88	9.76
BST	22.03	3.9842	63.24	0.71	2.06	10.19

.

Figure 3 displays the elemental composition of BNT and BST, characterized using Electron dispersive X-ray spectroscopy (EDX). From the results, it is clear that only desired elements are present in the prepared samples, where only Bi, Na, Ti, and O were found in BNT, while Bi, Sr, Ti, and O were discovered in BST. The atomic percentage and the weight percentage of the samples are shown in Figure 3, which emerged to be extremely close to the anticipated stoichiometric values.

From the FT-IR spectra, the functional groups present in the prepared ceramics are determined. Figure 4 displays a graph plotted between wavelength (ranging from 500 to 4000 cm⁻¹) and transmittance. The broad absorption bands observed at 555.59 and 575.65 cm⁻¹ can be ascribed to Ti–O stretching vibrations [1]. In BNT, the small absorption band located in the 851.41 cm⁻¹ region arises from the fundamental stretching vibrations of the Sr-O linkage, while the one located at 1382.71 cm⁻¹ indicates the existence of the Ba-O-Ti bond [2,3]. The minor vibrational bonds interpreted around 1700 cm⁻¹ correspond to the C=O stretching vibrations [2].

Figure 5 shows the UV–Visible absorbance spectra of BNT and BST ceramic perovskite samples in the wavelength range between 200 and 800 nm. A single absorbance peak has been obtained for both the ceramics, with BNT having its peak at 341 nm, while BST at 374 nm. Also, by adopting Tauc’s formula, the bandgap energy (E_g) of the samples is determined to be 2.0 eV and 2.6 eV for BNT and BST [6].

αhυ = A (hυ − E_g)ⁿ

(1)

These values are also in accordance with the previous reports [7]. The measured bandgap value further confirms the semiconducting nature. Although BNT shows a tail up to ~800 nm and BST up to ~700 nm, the Tauc-derived bandgaps represent the true intrinsic electronic transitions. The difference in the absorption edge arises mainly from the different defect structures in the two materials. Pristine bismuth titanate shows a bandgap of 2.2–2.7 eV. Compared to this, BNT shows a longer absorption tail toward ~800 nm, where the substitution of Na⁺ slightly widens the bandgap to 2.6 eV by altering the Bi-O-Ti electronic structure. On the other hand, in BST, Sr²⁺ substitution reduces the bandgap to 2.0 eV due to increased lattice distortion and defect-induced mid-gap states [7]. The bandgap graph plotted between photon energy and (αhυ)² is illustrated in Figure 5c,d.

Figure 6 shows the room temperature dielectric permittivity measured in the frequency range from 4 Hz to 8 MHz. The real part of the dielectric permittivity signifies the energy storage capacity of the material [6].

ε_r = C_p/C₀

(2)

C₀ = ε₀A/d

(3)

Here, ‘C_p’ represents the capacitance, and ‘A’ and ‘d’ are the cross-sectional area and thickness of the prepared pellet samples. Both samples exhibited a very high dielectric permittivity at lower frequencies initially, while a sudden decline in the value was measured around 10⁴ Hz. At low frequencies, space charge has enough time to follow the electric field at the grain boundaries. However, as frequency rises, their ability to respond diminishes, where the space charge no longer contributes and only the material’s intrinsic properties remain dominant. A Debye-like relaxing characteristic is implied by the unexpected decrease in reaction at this stage [8]. This indicates that all space charges were able to relax, and space charge-induced polarization ceased to contribute beyond 10⁴ Hz. Thus, at higher frequency, BST obtained a higher ε_r = 3946 than BNT ε_r = 2420.

Also, tangent loss is determined, and it is found that both the samples, BST and BNT, showed a significant loss of δ = 2.94 and δ = 1.04. The loss factor was measured by adopting the following equation:

ε″ = ε′ Tanδ

(4)

Figure 6b,c shows the plot drawn between tangent loss and frequency of BNT and BST. From the figure, it is clear that the loss decreases at higher frequencies and increases at lower frequencies. This change can be attributed to the sample’s structural defect [9]. Additionally, ac conductivity (σ) as a function of frequency is displayed in Figure 7. There are two different regions on the AC conductivity curve. At low frequencies, a plateau is observed, representing DC conductivity, where the field has minimal influence on the jump conduction process, likely due to electrode or surface effects. At higher frequencies, conductivity gradually increases, indicating the onset of hopping conduction. This behavior is associated with charge carrier movement between localized states and reflects the contributions from grains and grain boundaries [1].

4. Conclusions

BNT and BST ceramics were successfully prepared via the auto-combustion technique. The pseudo cubic phase was confirmed by the XRD patterns, while the average crystallite size of BNT and BST was found to be 18.60 nm and 22.03 nm. From the EDX analysis, it is proven that the prepared samples have no impurities present. From the optical properties studied via UV–Visible DRS, the energy bandgap was determined to be 2.6 eV for BNT and 2.00 eV for BST. The sample BST delivered a high dielectric permittivity of ε_r = 3946 that BNT (ε_r = 2420), while a minimal tangent loss factor δ = 2.94 and δ = 1.04 was obtained for both the samples. Also, room temperature ac conductivity was measured, displaying two different behaviors corresponding to their frequency. Given their high dielectric constant, low loss factor, and moderate bandgap, the synthesized BNT and BST ceramics show promising potential for applications in capacitors, dielectric resonators, and electro-optic devices, and in piezoelectric sensors and energy harvesting systems.