Structure-Property Correlation in La³⁺-Substituted SrBi₂Nb₂O₉ Ferroelectric Ceramics Synthesized via Molten Salt Route

Abstract

Aurivillius type ferroelectric ceramics, SrBi_2-xLa_xNb₂O₉ were synthesized through the molten salt method to clarify how La³⁺ substitution influences both the crystal framework and the ferroelectric response. XRD data revealed the formation of a single orthorhombic A2₁am phase for all compositions which was further verified through Le Bail refinement. FTIR further supported the stability of Nb-O linkages. Microstructural examination by SEM indicated that increasing La³⁺ content suppressed grain growth. Dielectric measurements showed a systematic lowering of the ferroelectric transition temperature, consistent with enhanced orthorhombicity and the reduced stereoactivity of the Bi³⁺ 6s² lone pair. Polarization hysteresis loops demonstrated that La³⁺ modification improved room temperature polarization behavior, attributed to reduced dielectric loss and more mobile domain walls. These results establish a clear link between structural distortion and ferroelectric response in La³⁺ modified SrBi₂Nb₂O₉, underscoring the potential of this system as a model compound for understanding structure property relationships in layered ferroelectrics.

Keywords:
Aurivillius phase; dielectric properties; ferroelectric; molten salts; orthorhombic structure

1. Introduction

The rapid development of new dielectric and ferroelectric materials has expanded their range of applications and attracted considerable attention. Among these, one promising case is their use in capacitors for energy storage. Dielectric capacitors have been investigated intensively as they combine the high power density of supercapacitors with the relatively high specific energy of Li ion batteries, making them attractive candidates for high power electronics, hybrid vehicles, and next generation energy storage systems¹-³. Nevertheless, the relatively low energy storage density of conventional dielectric capacitors compared to other technologies remains a limitation. Consequently, significant efforts have been devoted over the past few decades to improving their recoverable energy density (W_rec)⁴–⁶.

Bismuth layered perovskite compounds, commonly referred to as Aurivillius phases, have emerged as strong candidates in this regard owing to their characteristic dielectric and ferroelectric properties, excellent fatigue resistance, and tunable Curie temperatures⁷-⁹. Their general crystal structure, represented by the formula [Bi₂O₂]²⁺[A_n-1B_nO_3n+1]^2-, consists of alternating pseudo perovskite slabs and [Bi₂O₂]²⁺ layers stacked along the c-axis. The number of perovskite layers (n) determines the order of the Aurivillius phase¹⁰,¹¹. Among them, SrBi₂Nb₂O₉ (SBN), a two layer member of this family, exhibits a relatively high Curie temperature (T_c), strong remanent polarization (P_r), and excellent thermal and chemical stability¹²-¹⁴. These attributes position SBN as a promising candidate for energy related applications at intermediate temperature ranges. However, optimizing its properties for specific functional requirements still demands further structural and electrical modifications¹⁵,¹⁶.

One effective strategy to achieve such improvements is cation substitution, particularly at the Bi³⁺ site in the Aurivillius structure. Rare earth substitution has been shown to alter structural symmetry, dielectric response, and ferroelectric behavior¹⁷-¹⁹. Because rare earth cations differ from Bi³⁺ in both ionic radius and electronic configuration, they can induce local lattice distortions and modify long range ordering without disrupting the integrity of the perovskite slabs²⁰,²¹.

In the present work, La³⁺ was selected as a substituent at the Bi³⁺ site. This choice is motivated not only by the relatively limited number of reports but also by the close ionic radius of La³⁺ compared with Bi³⁺, which ensures structural compatibility while simultaneously introducing controlled distortions²²,²³. In Aurivillius phases such as SBN, structural distortion generally refers to perturbations in the Bi–O layers and NbO₆ octahedra, arising from chemical substitutions, phase transitions, or lattice defects²⁴,²⁵. Such distortions often enhance orthorhombicity, which in turn facilitates the alignment of electric dipoles and strengthens the ferroelectric response²⁶. The incorporation of La³⁺ also affects ferroelectric polarization behavior, as reflected in the shape and saturation level of P–E hysteresis loops. The observation of unsaturated loops indicates partial domain switching under external fields, a feature that provides insight into domain dynamics and the potential of these materials for energy related functionalities³,²⁷.

In this study, SrBi_2-xLa_xNb₂O₉ ceramics were synthesized via the molten salt technique, with systematic variation of La³⁺ content substituted to the Bi site in the bismuth layer. This method was chosen for its advantages, including lower reaction temperatures compared to solid state synthesis, improved phase purity, and enhanced grain uniformity²¹. The results demonstrate that La³⁺ substitution induces structural distortion, increases orthorhombicity, and reduces grain size, resulting in clear changes in the ferroelectric response. Moreover, the observed decrease in T_c with increasing La³⁺ further highlights the structural sensitivity of SBN to rare earth substitution. This investigation aims to provides new insights into how controlled structural distortion can be exploited to tune ferroelectric behavior in Aurivillius phases, positioning La³⁺ modified SrBi₂Nb₂O₉ as a model system for advancing the design of layered ferroelectrics.

2. Experimental Procedure

2.1. Synthesis

The double layer Aurivillius compound SrBi_2-xLa_xNb₂O₉, substituted with La³⁺ cations were fabricated by a molten salt reaction route. Raw material used Bi₂O₃(Aldrich, 99,9%), SrCO₃ (Aldrich, 99,9%), La₂O₃ (Aldrich, 99,9%), Nb₂O₅ (Aldrich, 99,9%) and Na₂SO₄ (Aldrich, 99%) and K₂SO₄ (Aldrich, 99%), with a 1:7 molar ratio as flux media. Stoichiometric quantities of raw materials was ground for 2 h in an agate mortar crystal class: orthorhombic. The mixtures were heated at a rate of 5 °C/min to target temperatures of 750 °C, 850 °C, and 950 °C, maintaining each temperature for 5 h with intermediate grinding processes. Heating at three different temperatures is based on previous research; each heating temperature has its function²⁷-²⁹. The temperature of 750 °C is done for the Bi₂O₃ precursor to react first and not evaporate at higher temperatures because the Bi₂O₃ precursor has a melting point of 817 °C, which is lower than other precursors³⁰. Further heating at 850 °C, above the eutectic temperature of the Na₂SO₄/K₂SO₄ salt mixture (823 °C), aims to form a molten salt phase as a reaction flux medium. However, from the heating results, the salt mixture remained in powder form and cannot form a perfect molten phase. This is because the Na₂SO₄/K₂SO₄ salt has an increased melting point due to mixing with other metal oxide precursors³¹. The final heating at 950 °C was carried out to perfect the reaction and form a good molten salt phase to produce Aurivillius compounds. The resulting products were subsequently washed with hot distilled water repeatedly to eliminate sulfate flux residues and dried at 110 °C for 5 h. For dielectric measurements, the final powders were combined with 5 wt% polyvinyl alcohol as a binder and pressed into pellet form. The pellet was initially sintered at 500 °C for 5 h to remove the binder, followed by further sintering at 950 °C for 5 h to densify. The surfaces of the sintered pellets were then coated with a silver conductive electrode paste (Aldrich, 99%) to serve as electrodes and subsequently dried at 110 °C for 2 h.

2.2. Characterization

The phase characterization and product purity were evaluated using a Shimadzu XRD 7000 X-ray diffractometer with Cu Kα radiation at room temperature (RT). Additional analyses, including purity verification and determination of crystal structure, cell volume, and space group, were conducted through the Le Bail refinement method utilizing Rietica software. Bond vibrations in the product compound were studied using a PerkinElmer 1600 FTIR spectrophotometer at room temperature. The sample morphology was investigated with a scanning electron microscope (SEM Hitachi Flexsem 1000) using coated pellet specimens. Dielectric properties were measured with an LCR meter (BK Precision) over a frequency range of 50 kHz to 300 kHz at both room and elevated temperatures, while the ferroelectric properties were examined at room temperature using a ferroelectric tester (Hantex®).

3. Results and Discussion

Lanthanum, a rare earth element from the lanthanide series, is known for its stable +3 oxidation state and an ionic radius close to that of Bi³⁺. This similarity allows La³⁺ to substitute effectively at the Bi site in Aurivillius type structures without disrupting charge neutrality, making it a suitable candidate for cation substitution in layered perovskite systems²⁵. The XRD patterns of SrBi_2-xLa_xNb₂O₉ ceramic powders synthesized via the molten salt method, with La³⁺ concentrations of x = 0.025, 0.075, 0.125, and 0.25, are shown in Figure 1 and compared with the ICSD 95919 standard. The diffraction profiles within the 2θ range of 10°–90° revealed that all Bragg reflections could be indexed unambiguously and matched well with the reference pattern. This confirms the formation of a single phase Aurivillius structure without any detectable secondary phases or traces of unreacted starting materials. The high level of phase purity achieved here is noteworthy, as it is often difficult to obtain through conventional solid state synthesis. This result indicates that La³⁺ ions were successfully incorporated into the SrBi₂Nb₂O₉ (SBN) lattice, promoting homogeneous substitution without destabilizing the host framework³²-³⁴.

Figure 1
XRD patterns of SrBi_2-xLa_xNb₂O₉ (x = 0.025, 0.075, 0.125, 0.25) samples at room temperature. The enlarged XRD profile shows the shifting of 115 peak.

All compositions crystallized in the orthorhombic phase with the A2₁am space group, characteristic of two layer Aurivillius compounds (n = 2). The most intense diffraction peak, corresponding to the (115) plane, was consistently observed in all samples. This strong reflection is typical of the layered bismuth structure and is in good agreement with earlier studies on BLSF compounds, where the most intense peaks in n = 2 members are associated with (112n+1) reflections²⁹. In addition, an examination of the XRD profiles focusing on the (001) reflection, which is commonly associated with texture along the c-axis, indicates that preferred orientation effects are not significant in any of the samples synthesized via the molten salt method.

A closer inspection of the XRD profiles shows a systematic shift of the (115) peak toward higher 2θ values with increasing La³⁺ substitution. This shift can be attributed to the partial replacement of Bi³⁺ ions (ionic radius 1.38 Å, CN = 8) by La³⁺ ions (ionic radius 1.36 Å, CN = 8), leading to a slight contraction of the lattice and the development of internal strain³⁵,³⁶. Such peak shifts are widely reported as a signature of lattice distortion resulting from ionic substitution in Aurivillius type ceramics. The observed structural changes not only reflect lattice contraction but also suggest local distortions in the Bi–O layers, indicating that La³⁺ most likely replaces Bi³⁺ in the (Bi₂O₂)²⁺ layer. Furthermore, variations in peak broadening indicate possible changes in crystallite size and microstrain, which will be elaborated in the subsequent analysis.

The crystallite size of the SrBi_2-xLa_xNb₂O₉ ceramic powders was estimated from the XRD data using the Scherrer equation:

$D=\frac{K.\lambda }{\beta .\cos \theta }$

where D is the average crystallite size (nm), K is the Scherrer constant, λ is the X-ray wavelength, β is the full width at half maximum (FWHM), and θ is the Bragg angle (rad)³⁷. The calculated crystallite sizes were 43.6 nm, 41.9 nm, 39.8 nm, and 37.9 nm for x = 0.025, 0.075, 0.125, and 0.25, respectively. These values reveal a systematic reduction in crystallite size with increasing La³⁺ concentration. The decrease in size can be explained by the substitution of larger Bi³⁺ ions (1.38 Å) with slightly smaller La³⁺ ions (1.36 Å), which induces lattice strain and constrains crystallite growth during synthesis. This observation is consistent with the shift of the (115) diffraction peak toward higher 2θ values, further confirming the presence of structural distortion associated with La incorporation³⁸. These results demonstrate that the molten salt synthesis route, combined with La³⁺ substitution, enables the preparation of single phase Aurivillius ceramics with refined crystallite size and without detectable secondary phases. This contrasts with conventional solid state routes, which often face challenges such as Bi volatility and phase inhomogeneity³⁹. Moreover, the observed lattice strain and size reduction are expected to play a role in modifying domain dynamics, thereby influencing the ferroelectric response, as will be discussed in subsequent sections.

Figure 2 presents the detailed XRD analysis obtained from Le Bail refinement of SrBi_2-xLa_xNb₂O₉ ceramic powders. This refinement is used to evaluate composition dependent variations in structural parameters arising from La³⁺ substitution at the bismuth layer. The Le Bail plots show excellent agreement between the calculated profiles (black) and the observed data (red), with only small residuals in the difference curve (green). All reflections are properly indexed, and the Bragg positions (blue ticks) confirm an orthorhombic structure in the A2₁am space group. Especially, A2₁am is a non centrosymmetric (polar) orthorhombic space group an attribute consistent with ferroelectric ordering in Aurivillius phases⁴⁰. The quality of the fits is further supported by low refinement residuals (e.g., R_wp, χ²). The refinement results also reveal a systematic decrease in unit cell volume with increasing La³⁺ content⁴¹. This Vegard like contraction is rationalized by the slightly smaller ionic radius of La³⁺ (1.36 Å, CN = 8) compared with Bi³⁺ (1.38 Å, CN = 8), which introduces subtle lattice strain and local distortions in the Bi–O layers. Together with the polar symmetry, these structural changes provide a coherent basis for the ferroelectric response observed later (lower T_c and modified P–E behavior). In short, the combination of non centrosymmetric symmetry and La induced lattice contraction/orthorhombicity underpins the link between structural distortion and ferroelectric properties in SrBi_2-xLa_xNb₂O₉⁴².

Figure 2
Le Bail plot of SrBi_2-xLa_xNb₂O₉ (x = 0.025, 0.075, 0.125, 0.25) with A2₁am space group.

The refined lattice parameters and unit cell volumes are summarized in Table 1. The results show that the lattice constants a, b, and c decrease systematically with increasing La³⁺ concentration, leading to a clear reduction in the overall unit cell volume. The substitution of La³⁺ for Bi³⁺ in the [Bi₂O₂]²⁺ layer induces structural distortion within the Aurivillius framework and influences the NbO₆ octahedra. This occurs because of the strong chemical interactions between the [Bi₂O₂]²⁺ slabs and the corner sharing NbO₆ units. Hence, modification of the [Bi₂O₂]²⁺ environment by La³⁺ incorporation inevitably perturbs the NbO₆ octahedra, resulting in subtle distortions of the perovskite block²⁵,²⁹.

Table 1
The unit cell parameters resulting from the refinement of the XRD pattern using the Le Bail method of SrBi_2-xLa_xNb₂O₉ (x = 0.025, 0.075, 0.125, 0.25) with the A2₁am space group.

Parameter	x = 0.025	x = 0.075	x = 0.125	x = 0.25
Space Group	A21am	A21am	A21am	A21am
Crystal class	Orthorhombic	Orthorhombic	Orthorhombic	Orthorhombic
Z	4	4	4	4
a (Å)	5.5124(6)	5.5130(2)	5.5100(5)	5.5091(4)
b (Å)	5.5065(4)	5.5064(7)	5.5031(1)	5.5008(3)
c (Å)	25.0799(7)	25.0733(4)	25.0687(1)	25.0325(8)
V (Å3)	761.28(6)	761.15(8)	760.14(5)	758.60(9)
(a-b)/(a+b)	5.35 × 10-4	5.99 × 10-4	6.26 × 10-4	7.54 × 10-4
Rp (%)	3.24	3.18	3.15	3.13
Rwp (%)	4.60	4.22	4.11	4.08
ꭓ 2	1.414	1.173	1.062	1.011

This distortion is reflected in the enhanced orthorhombic character of the structure, as evidenced by the increasing orthorhombic ratio, (a–b)/(a+b), with higher La³⁺ substitution. In addition, the tilting of NbO₆ octahedra along the a-axis becomes more pronounced, further contributing to the overall structural modifications⁴³. The progressive increase in orthorhombicity with La³⁺ content therefore indicates growing lattice distortion, which plays a decisive role in strengthening dipole alignment and improving ferroelectric behavior¹⁰.

Furthermore, the refinement quality factors (R_p and R_wp), also listed in Table 1, confirm the accuracy and reliability of the Le Bail analysis. These values demonstrate that La³⁺ incorporation into the Bi site does not destabilize the parent SrBi₂Nb₂O₉ structure, thereby underlining the structural stability of the Aurivillius phase even under significant compositional modification. This structural robustness provides an essential foundation for correlating lattice distortion with the observed changes in ferroelectric response.

Figure 3 shows the FTIR spectra of SrBi_2-xLa_xNb₂O₉ samples in the wavenumber range of 500–1500 cm^-1, focusing on the characteristic vibrations of metal–oxygen bonds within the BO₆ octahedra of the perovskite block. A distinct absorption band around 553 cm^-1 corresponds to the asymmetric stretching of B–O bonds, while the strong band near 823 cm^-1 is assigned to the symmetric stretching of NbO₆ octahedra⁴⁴. Only slight shifts in the positions of these peaks are observed with increasing La³⁺ substitution. The near constancy of these vibrational modes suggests that La³⁺ incorporation at the Bi³⁺ site does not significantly disturb the local bonding environment of the BO₆ framework³¹.

Figure 3
FTIR spectra of SrBi_2-xLa_xNb₂O₉ (x = 0.025, 0.075, 0.125, 0.25).

The preservation of BO₆ vibrational features demonstrates that the perovskite slabs retain their structural integrity, which is essential for maintaining stable dielectric and ferroelectric properties in the Aurivillius phase. These findings are consistent with the XRD analysis, confirming that while La³⁺ substitution induces subtle structural distortions and lattice contraction, the fundamental phase stability and local bonding configuration remain intact. This balance between distortion and stability is crucial, as it allows structural modification of the Bi–O layers without compromising the robustness of the NbO₆ octahedra providing the structural basis for the enhanced ferroelectric response discussed in later sections.

Figure 4 illustrates the morphology and particle size distribution of SrBi_2-xLa_xNb₂O₉ ceramics observed on sintered pellets by scanning electron microscopy (SEM). The micrographs display a plate like grain morphology, a hallmark of Aurivillius phases consistent with their layered crystal structure⁴⁵. The average grain size decreases markedly with increasing La³⁺ content: 3.25 μm (x = 0.025), 1.31 μm (x = 0.075), 0.73 μm (x = 0.125), and 0.52 μm (x = 0.25). This grain refinement is attributed to the reduced Bi³⁺ fraction upon La³⁺ substitution. In Aurivillius systems, Bi rich compositions (and transient Bi₂O₃ at sintering temperatures) tend to promote grain coarsening, whereas partial replacement of Bi³⁺ by the slightly smaller La³⁺ (and the associated lattice strain/defect fields) suppresses grain boundary mobility and limits growth, yielding finer grains⁴¹. The trend aligns with the decrease in crystallite size inferred from XRD line broadening, indicating that La³⁺ substitution affects both the structural and microstructural length scales of the material. From a functional standpoint, finer grains increase grain boundary density, which can help stabilize the dielectric response, mitigate leakage, and influence domain wall dynamics factors that collectively shape the ferroelectric behavior of SrBi_2-xLa_xNb₂O₉ ceramics⁴⁰. This microstructural evolution is therefore consistent with the structure property link established earlier (La induced distortion → modified polarization response).

Figure 4
SEM micrograph and particle size distribution of SrBi_2-xLa_xNb₂O₉ (x = 0.025, 0.075, 0.125, 0.25).

Figure 5 shows the temperature dependence of the dielectric constant (ε_r) and loss tangent (tan δ) for SrBi_2-xLa_xNb₂O₉ ceramics in the frequency range of 50–300 kHz. In accordance with Maxwell-Wagner theory, all samples exhibit a decrease in ε_r with increasing frequency, reflecting the diminished contribution of interfacial polarization at higher frequencies. At low frequencies, grain boundaries enhance interfacial polarization, resulting in higher ε_r values, whereas at high frequencies the dielectric response is mainly governed by electronic, ionic, and dipolar mechanisms⁴⁶. Similarly, dielectric loss decreases with frequency, as reported in comparable Aurivillius systems⁴⁷. The dielectric data summarized in Table 2 indicate that ε_r, the Curie temperature (T_c, corresponding to the ferroelectric paraelectric transition), and tan δ all decrease systematically with increasing La³⁺ concentration. The reduction in T_c is strongly correlated with the enhanced orthorhombicity, expressed by the orthorhombic ratio (a−b)/(a+b). As x increases from 0.025 to 0.25, the orthorhombic distortion becomes more pronounced, consistent with the refinement results discussed earlier. This structural modification contributes to the observed lowering of T_c, in agreement with the findings of Sun et al.⁴⁸, who reported that increased orthorhombicity weakens the influence of the Bi³⁺ 6s² lone pair, thereby suppressing the transition temperature⁴⁹.

Figure 5
Temperature dependence of the dielectric constant (ε_r) and loss (tan δ) of SrBi_2-xLa_xNb₂O₉ (x = 0.025, 0.075, 0.125, 0.25).

Table 2
Electrical properties of SrBi_2-xLa_xNb₂O₉ (x = 0.025, 0.075, 0.125, 0.25).

SrBi2-xLaxNb2O9	Tc (°C)	Dielectric constant	Dielectric loss
x = 0.025	470	583.10	0.129
x = 0.075	455	474.39	0.104
x = 0.125	435	322.23	0.088
x = 0.25	325	167.71	0.066

The dielectric loss values remain relatively low (<0.13) up to 350 °C across all compositions, confirming good dielectric stability in the 25–350 °C range. This stability is attributed to suppressed charge carrier activation and minimal energy dissipation at moderate temperatures. Above 350 °C, however, tan δ rises sharply, which is attributed to thermally activated conductivity arising from oxygen vacancy defects. These oxygen vacancies originate mainly from Bi³⁺ volatility at elevated temperatures, a phenomenon commonly observed in Aurivillius ceramics⁵⁰,⁵¹.

Overall, the reduction in ε_r and T_c, combined with the increase in orthorhombicity, highlights the central role of La³⁺ substitution in tuning the dielectric response of SrBi_2-xLa_xNb₂O₉. The microstructural refinement induced by La³⁺ enhances dielectric stability across a wide temperature range, while the high temperature increase in tan δ suggests that strategies to control oxygen vacancy concentration will be essential for further improving performance⁵²-⁵⁴. This clear structure property relationship provides important insights into the coupling between lattice distortion and dielectric/ferroelectric behavior in Aurivillius systems.

The ferroelectric hysteresis (P–E) behavior of SrBi_2-xLa_xNb₂O₉ ceramics, measured at room temperature (RT, 100 Hz) under an applied electric field of 50 kV/cm, is shown in Figure 6. The inset summarizes the remanent polarization (P_r) and maximum polarization (P_m) values. The polarization increases steadily with field strength, accompanied by a corresponding rise in Pm. Nevertheless, the P–E loops remain unsaturated for all compositions, indicating that the ferroelectric domains are not fully switched at the maximum applied field.

Figure 6
Polarization versus electric field (P‒E) loops of SrBi₂Nb₂O₉ ceramic as the variation of La³⁺ substitution.

This incomplete saturation can be explained by two factors. First, the applied field of 50 kV/cm may be insufficient to overcome the coercive field required for complete domain reversal. Second, the measurements were conducted at RT, well below the Curie temperature (T_c = 325–470 °C; see Table 2). At these sub T_c conditions, domain wall motion is relatively restricted, resulting in only partial alignment of dipoles⁵⁵. The unsaturated nature of the loops is consistent with the structural and microstructural findings: La³⁺ substitution induces lattice distortion and finer grains, which influence domain dynamics and reduce dielectric loss. These combined effects enhance polarization response at moderate fields, but full domain alignment would likely require either higher applied fields or measurements closer to T_c.

Despite the unsaturated loops, the non linear increase in polarization with field amplitude reflects active domain wall motion associated with ferroelectric behavior. The open nature of the hysteresis loops further confirms the intrinsic ferroelectricity of SrBi_2-xLa_xNb₂O₉ ceramics⁵⁶,⁵⁷ The open loop profile and polarization reversal provide clear evidence of remanent polarization (P_r) and coercive field (E_c), both of which are hallmarks of ferroelectric order. As shown in the inset of Figure 6, both saturation polarization (P_m) and P_r vary with La³⁺ concentration.

La³⁺ substitution subtly modulates lattice distortion and symmetry, enhancing orthorhombicity and overall polarizability within the Aurivillius framework. Because La³⁺ has a slightly smaller ionic radius than Bi³⁺, its incorporation introduces local lattice contraction and NbO₆ octahedral distortion, which facilitates dipole alignment and improves polarization switching. These structural changes strengthen the internal electric field and promote domain alignment, in agreement with previous reports that rare earth substitution enhances ferroelectric performance in layered perovskites³⁷. Although the P_r and P_m values obtained here are moderate due to the limited applied field, the observed trends clearly demonstrate that La³⁺ doping is an effective strategy for tailoring ferroelectric functionality, particularly where low leakage and moderate remanent polarization are advantageous⁵⁸.

Figure 7 presents the energy storage parameters derived from the first quadrant of the P–E hysteresis loops, including recoverable energy density (W_rec), loss energy density (W_loss), and efficiency (η). While the relatively low polarization values (Figure 6) are constrained by the use of bulk ceramics and a modest applied field (50 kV/cm), meaningful trends can still be extracted. With increasing La³⁺ concentration, both Wrec and η improve, with the best performance observed at x = 0.25: W_rec = 25.38 mJ/cm³ and η = 73.85%. This improvement highlights the role of La³⁺ substitution in enhancing polarization reversibility and dielectric stability.

Figure 7
Energy storage calculation for SrBi₂Nb₂O₉ ceramic as the variation of La³⁺ substitution.

The observed W_rec values are lower than those reported for thin films, which can achieve 0.1–2 J/cm³ under fields exceeding 1 MV/cm due to superior breakdown strength and domain mobility²⁷,³⁷,⁵⁷,⁵⁹,⁶⁰. Nevertheless, the relatively high η combined with moderate W_rec under conservative testing conditions suggests that La³⁺ modified SrBi₂Nb₂O₉ ceramics are promising for moderate temperature capacitive energy storage applications, where thermal stability and long term cycling reliability are crucial. These results also establish a structural property link: La induced distortion not only refines the crystal and microstructural features but also translates into improved ferroelectric and dielectric responses. These correlations provide meaningful insights for energy storage optimization and offer a rational basis for future chemical substitution and compositional design strategies in Aurivillius-type ceramics.

4. Conclusion

SrBi₂Nb₂O₉ Aurivillius ceramics were successfully synthesized using the molten salt method, with partial substitution of Bi³⁺ by La³⁺ ions in the bismuth layer. X-ray diffraction confirmed that all compositions crystallized as a single phase with non centrosymmetric orthorhombic A2₁am symmetry. Le Bail refinement revealed a systematic reduction in unit cell volume with increasing La³⁺ content, consistent with lattice contraction arising from the smaller ionic radius of La³⁺ relative to Bi³⁺. Microstructural analysis showed that La³⁺ incorporation effectively suppressed grain growth, yielding finer plate like grains typical of Aurivillius ceramics. Structurally, La³⁺ substitution introduced progressive distortion, manifested by increased orthorhombicity. This modification was linked to a reduction in the ferroelectric transition temperature (T_c), attributed to the weakened activity of Bi³⁺ 6s² lone pairs. The tendency toward more diffuse ferroelectric behavior, arising from structural disorder, was also evident, though confirmation will require further temperature and frequency dependent dielectric studies.

Ferroelectric hysteresis measurements demonstrated that La³⁺ doping enhances the polarization response, with higher remanent polarization (P_r) and improved domain alignment, while maintaining relatively low dielectric loss. These results confirm that structural distortion induced by La³⁺ substitution strengthens ferroelectric functionality without destabilizing the Aurivillius framework. In summary, La³⁺ substitution in SrBi₂Nb₂O₉ establishes a clear structure property correlation: lattice contraction and enhanced orthorhombicity directly translate into modified ferroelectric response. This makes La modified SrBi₂Nb₂O₉ a promising system for dielectric and ferroelectric applications, with potential for medium temperature energy storage. Further optimization such as applying higher electric fields, tailoring defect chemistry, or exploring thin film architectures could unlock even greater performance and efficiency, paving the way for next generation layered ferroelectric materials.s

5. Acknowledgments

This work was financially supported by Universitas Andalas with project grant number: T/7/UN.16.17/PP/IS-PDU-KRP2GB-Unand/LPPM/2021.

• Data Availability
  The full dataset supporting the findings of this study is available upon request to the corresponding author.

6. References

• ¹ Cross LE. Ferroelectric materials for electromechanical transducer applications. Mater Chem Phys. 1996;43(2):108-15.
• ² Sun J, Han Y, Gao G, Yang J, Zhang Y, Dai Y, et al. Breakdown field enhancement and energy storage performance in four-layered Aurivillius films. Ceram Int. 2022;48(11):15780-4.
• ³ Wu Q, Chen X, Zhao L, Zhao Y, Zhou Y, Zhao S. The relaxor properties and energy storage performance of Aurivillius compounds with different number of perovskite-like layers. J Alloys Compd. 2022;911:165081.
• ⁴ Qiao N, Ding J, Wang P, Liu Z, Li P, Chen J, et al. Band-gap engineering in Aurivillius BaBi4Ti4O15 thin film capacitors for enhanced energy storage performance. J Power Sources. 2024;605:234482.
• ⁵ Ahn Y, Son JY. Imprint effect on energy storage performance of Aurivillius Bi3TaTiO9 thin films. J Mater Res Technol. 2022;20:4213-9.
• ⁶ Gou X, Liu Y, Jiang N, Li Y, Jiang Y, Chen J, et al. Non–ferroelectric intercalation structure based on Aurivillius phase Bi4Ti3O12: A research arena to achieve high energy storage performance. Ceram Int. 2022;48(7):9534-43.
• ⁷ Surta TW, Manjón-Sanz A, Qian EK, Mansergh RH, Tran TT, Fullmer LB, et al. Dielectric and ferroelectric properties in highly substituted Bi2Sr(A)TiNb2O12 (A = Ca2+, Sr2+, Ba2+) aurivillius phases. Chem Mater. 2017;29(18):7774-84.
• ⁸ Fang P, Zhi C, Xi Z, Long W, Li X. Giant dielectric response at low frequency of B-site modified Aurivillius oxide potassium bismuth titanate. Mater Res Bull. 2018;108:214-8.
• ⁹ Yang J, Song DP, Yin Y, Chen LZ, Chen LY, Wang Y, et al. Ferroelectric polarization and fatigue characterization in bismuth-based Aurivillius thin films at lower voltage. Mater Sci Eng B. 2019;248:114408.
• ¹⁰ Vijatovic Petrovic MM, Bobic JD. Perovskite and aurivillius: Types of ferroelectric metal oxides. In Stojanovic BD, editor. Magnetic, ferroelectric, and multiferroic metal oxides. Amsterdam: Elsevier Inc.; 2018. p. 35-49.
• ¹¹ Moure A. Review and perspectives of Aurivillius structures as a lead-free Piezoelectric system. Appl Sci. 2018;8(1):62.
• ¹² Indurkar A, Mev SK, Asthana S, Subohi O. Influence of SrBi2Nb2O9 (SBN) incorporation in K0. 5Na0. 5NbO3 (KNN) ceramics on its structural and electrical properties. Physica B. 2025;706:417144.
• ¹³ Yan S, Sun S, Chen Z, Wang X. Improved photocatalytic activity of SrBi2Nb2O9 for the degradation of ciprofloxacin hydrochloride via piezoelectric-enhanced charge transfer. Chinese Journal of Structural Chemistry. 2025;44:100569.
• ¹⁴ Basith A, Singh S, Banwal A, Narwan M, Verma M, Bokolia R. Regulating novel tunable green to red upconversion luminescence in Er3+/Yb3+ co-doped SrBi2Nb2O9 ferroelectric ceramic. Ceram Int. 2024;50:52344-55.
• ¹⁵ Axelsson A, Le F, Valant M, Mcn N. Optimisation of SrBi2(Nb,Ta)2O9 Aurivillius phase for lead-free electrocaloric cooling. J Eur Ceram Soc. 2018;38:5354-8.
• ¹⁶ Singh P, Jha RK, Singh RK, Singh BR. Memory improvement with high-k buffer layer in metal/ SrBi2Nb2O9/Al2O3/silicon gate stack for non-volatile memory applications. Superlattices Microstruct. 2018;121:55-63.
• ¹⁷ Zhang Y, Xu T, Yu H, Chu R, Li G, Zhang C, et al. Sm-modified bismuth layer-structured SrBi2Nb2O9 multifunctional ceramics with enhanced electrical properties and good photoluminescence properties. Ceram Int. 2022;48(13):18989-98.
• ¹⁸ Wei T, Jia B, Shen L, Zhao C, Wu L, Zhang B, et al. Reversible upconversion modulation in new photochromic SrBi2Nb2O9 based ceramics for optical storage and anti-counterfeiting applications. J Eur Ceram Soc. 2020;40(12):4153-63.
• ¹⁹ Afqir M, Tachafine A, Fasquelle D, Elaatmani M, Carru JC, Zegzouti A, et al. Co-precipitation-hydrothermal preparation of SrBi2Nb2O9. Mater Lett. 2017;205:178-81.
• ²⁰ Santos VB, M’Peko JC, Mir M, Mastelaro VR, Hernandes AC. Microstructural, structural and electrical properties of La3+-modified Bi4Ti3O12 ferroelectric ceramics. J Eur Ceram Soc. 2009;29(4):751-6.
• ²¹ Zulhadjri Z, Wendari TP, Ramadhani R, Putri YE, Imelda I. La3+ substitution induced structural transformation in CaBi4Ti4O15 Aurivillius phases: Synthesis, morphology, dielectric and optical properties. Ceram Int. 2021;47(16):23549-57.
• ²² de Lira SO, Burda C, Franco A. Effects of rare Earth doping on the structural and dielectric properties of sodium bismuth titanate perovskites. J Phys Chem Solids. 2024;193:112133.
• ²³ Roy R, Dutta A. Structural, optical, electrical, and dielectric relaxation properties of rare earth containing sodium bismuth titanate Na0.5Bi0.5TiO3 perovskite: effect of ionic radius. J Rare Earths. 2024;42(2):383-91.
• ²⁴ Shao L, Jiang HH, Xu CR, Ding N, Tang BY. The lattice distortion, mechanical and thermodynamic properties of A (Zr0. 2Sn0. 2Ti0. 2Hf0. 2Nb0. 2) O3 (A= Sr, Ba) high-entropy perovskite with B-site disorder: first principles prediction. Mater Des. 2022;224:111308.
• ²⁵ Min Y, Yu H, Zhang Y, Hao Y, Chen S, Liang R, et al. Observation of La3+ entering (Bi2O2)2+ layer to tune tilting of NbO6 octahedra in CaBi2Nb2O9 ceramics. Ceram Int. 2024;50(11):19392-401.
• ²⁶ Yu H, Hou Q, Chen G, Ma C, Jiang X, Hong J, et al. Ternary doping of Na+, Bi3+ and La3+ to improve piezoelectric constant and electrical resistivity simultaneously in calcium bismuth niobate ceramics. Ceram Int. 2023;49(7):10738-47.
• ²⁷ Singh A, Kharangarh P, Gupta V. Enhanced energy storage efficiency with superior thermal stability under low electric field and large electric field driven strain in environment-friendly Bi0.5Na0.5TiO3 based ferroelectric modified with LiNbO3. J Alloys Compd. 2023;945:169181.
• ²⁸ Wendari TP, Rizki A, Zulhadjri Z, Putri YE, Emriadi E, Insani A. Salt-flux synthesis of bismuth layer-structured Ca-doped Sr2Bi2Nb2TiO12 : the effect of cation substitution on structure, ferroelectric and optical properties. J Mater Sci Mater Electron. 2023;34(16):1282.
• ²⁹ Zulhadjri RA, Wendari TP, Putri YE. Structural and ferroelectric modifications in SrBi2Nb2O9 Aurivillius phase compounds by Nd3+ ion doping. Mater Chem Phys. 2024;2025:331.
• ³⁰ Zulhadjri Z, Prijamboedi B, Nugroho AA, Mufti N, Fajar A, Palstra TTM, et al. Aurivillius phases of PbBi4Ti4O15 doped with Mn3+ synthesized by molten salt technique: Structure, dielectric, and magnetic properties. J Solid State Chem. 2011;184(5):1318-23.
• ³¹ Porob DG, Maggard PA. Synthesis of textured Bi5Ti3FeO15 and LaBi4Ti3FeO15 ferroelectric layered Aurivillius phases by molten-salt flux methods. Mater Res Bull. 2006;41(8):1513-9.
• ³² Afqir M, Tachafine A, Fasquelle D, Elaatmani M, Carru JC, Zegzouti A, et al. Synthesis and characterizations of Ho2O3 modified SrBi2Nb2O9 ceramics. Chin J Phys. 2018;56(3):1158-65.
• ³³ Lavado C, Rébola AF, Machado R, Stachiotti MG. Multiferroic properties of three-layer Aurivillius compound Bi4TiFeNbO12: A first-principles and experimental study. Solid State Commun. 2020;320:114028.
• ³⁴ Mojumdar P, Shaily R, Bokolia R. Structural properties of Strontium Bismuth Niobate (SrBi2Nb2O9) ferroelectric ceramics. Mater Today Proc. 2021;47:4661-5.
• ³⁵ Prajapat D, Sagdeo A, Reddy VR. Structural, magnetic and dielectric properties of vanadium substituted four layered Aurivillius Bi5FeTi3O15 ceramics. Ceram Int. 2019;45(15):19093-7.
• ³⁶ Long C, Fan H. Effect of lanthanum substitution at A site on structure and enhanced properties of new Aurivillius oxide K0.25Na0.25La0.5Bi2Nb2O9. Dalton Trans. 2012;41(36):11046-54.
• ³⁷ Adak MK, Mukherjee A, Chowdhury A, Khatun J, Ghorai UK, Dhak D. Electrical and energy storage properties of nickel substituted barium bismuth niobate nano-ceramics prepared by chemical route. J Mater Sci Mater Electron. 2018;29(18):15847-58.
• ³⁸ Khokhar A, Goyal PK, Thakur OP, Shukla AK, Sreenivas K. Influence of lanthanum distribution on dielectric and ferroelectric properties of BaBi4-xLaxTi4O15 ceramics. Mater Chem Phys. 2015;152:13-25.
• ³⁹ Ismunandar I, Kennedy BJ. Effect of temperature on cation disorder in ABi2Nb2O9 (A = Sr, Ba). J Mater Chem. 1999;9(2):541-4.
• ⁴⁰ Zhang M, Xu X, Yue Y, Palma M, Reece MJ, Yan H. Multi elements substituted Aurivillius phase relaxor ferroelectrics using high entropy design concept. Mater Des. 2021;200:109447.
• ⁴¹ Alarcón-Suesca CE, Cardona-Vásquez JA, Salcedo-Fontecha JP, Vargas-Jiménez A, Landínez-Téllez DA, Roa-Rojas J. Ferroelectric, magnetic and structural studies of the Bi4LaSmFe2Ti3O18 multiferroic material. Physica B. 2014;455:49-52.
• ⁴² Axelsson AK, Le Goupil F, Valant M, Alford NMN. Electrocaloric effect in lead-free Aurivillius relaxor ferroelectric ceramics. Acta Mater. 2017;124:120-6.
• ⁴³ Shin HW, Son JY. Preferential growth characteristics and ferroelectric properties of epitaxial SrBi2Nb2O9 thin films along the a-axis direction due to the misfit strains. Mater Sci Semicond Process. 2021;134:105991.
• ⁴⁴ Wendari TP, Arief S, Mufti N, Blake GR, Baas J, Suendo V, et al. Lead-free aurivillius phase Bi2LaNb1.5Mn0.5O9: structure, ferroelectric, magnetic, and magnetodielectric effects. Inorg Chem. 2022;61(23):8644-52.
• ⁴⁵ Sridarane R, Subramanian S, Janani N, Murugan R. Investigation on microstructure, dielectric and impedance properties of Sr1-xBi2+(2/3)x(VxTa1-x)2O9 [x=0, 0.1 and 0.2] ceramics. J Alloys Compd. 2010;492(1–2):642-8.
• ⁴⁶ Alharbi SR, Alhassan M, Jalled O, Wageh S, Saeed A. Structural characterizations and electrical conduction mechanism of CaBi2Nb2O9 single-phase nanocrystallites synthesized via sucrose-assisted sol–gel combustion method. J Mater Sci. 2018;53(16):11584-94.
• ⁴⁷ Liu J, Duan C-G, Yin W-G, Mei WN, Smith RW, Hardy JR. Large dielectric constant and maxwell-wagner relaxation in Bi2/3Cu3Ti4O12. Physical Review B. 2004;70:144106.
• ⁴⁸ Sun L, Feng C, Chen L, Huang S. Dielectric relaxation in layer-structured SrBi2-xNd xNb2O9 ceramics (x = 0, 0.05, 0.2, 0.35). J Am Ceram Soc. 2007;90(1):322-6.
• ⁴⁹ Zulhadjri Z, Billah AA, Wendari TP, Emriadi E, Septiani U, Arief S. Synthesis of aurivillius phase CaBi4Ti4O15 doped with both La3+ and Mn3+ cations: crystal structure and dielectric properties. Mater Res. 2020;23(2):2-6.
• ⁵⁰ Wang W, Hu S, Liu Z, Jian Z, Chen W. Origin of improved Na+ ionic conductivity in the NASICON-type solid state electrolyte with Sm modification. J Phys Chem Solids. 2023;178:111365.
• ⁵¹ Dhak D, Biswas SK, Pramanik P. Synthesis and characterization of nanocrystalline SrBi2Nb2O9 ferroelectric ceramics using TEA as the polymeric matrix. J Eur Ceram Soc. 2006;26(16):3717-23.
• ⁵² Hajra S, Deepti PL, Sahu M, Patri SK, Panigrahi BK, Parida K. Dielectric, impedance and excitation performance of Aurivillius oxides: Bi4Pb2Zr2TiFeNbO18. Mater Today Chem. 2020;17:100325.
• ⁵³ Fang P, Fan H, Li J, Liang F. Lanthanum induced larger polarization and dielectric relaxation in Aurivillius phase SrBi2−xLaxNb2O9 ferroelectric ceramics. J Appl Phys. 2010;107:064104.
• ⁵⁴ Kocoń N, Dzik J, Szalbot D, Pikula T, Adamczyk-Habrajska M, Wodecka-Duś B. Synthesis and dielectric properties of Nd doped Bi5Ti3FeO15 ceramics. Arch Metall Mater. 2021;66(2):359-65.
• ⁵⁵ Kamba S, Veljko S, Kempa M, Savinov M, Bovtun V, Vaněk P, et al. Dielectric spectra of a new relaxor ferroelectric system Ba2LnTi2Nb3O15 (Ln= La, Nd). J Eur Ceram Soc. 2005;25(12):3069-73.
• ⁵⁶ Subohi O, Shastri L, Kumar GS, Malik MM, Kurchania R. Study of Maxwell-Wagner (M-W) relaxation behavior and hysteresis observed in bismuth titanate layered structure obtained by solution combustion synthesis using dextrose as fuel. Mater Res Bull. 2014;49(1):651-6.
• ⁵⁷ Adak MK, Dhak D. Relaxor lead free Bismuth layered structured ferroelectric Cobalt & Nickel substituted SrBi2NbTaO9 for energy storage application. Physica B. 2021;621:413318.
• ⁵⁸ Scott JF. Applications of modern ferroelectrics. Science. 2007;315(5814):954-9.
• ⁵⁹ Sharma S, Nandan R, Malhotra P, Kumar S, Negi NS. Low field assisted electrocaloric effect and energy storage responses through optimization of morphotropic phase boundary by sintering temperature for lead-free Ba0.95Ca0.05Sn0.09Ti0.91O3 ceramics. J Alloys Compd. 2024;1008:176519.
• ⁶⁰ Wendari TP, Rizki A, Zulhadjri Z, Putri YE, Emriadi E, Labanni A, et al. Structure, ferroelectric, magnetic, and energy storage performances of lead-free Bi4Ti2.75(FeNb)0.125O12 Aurivillius ceramic by doping Fe3+ ions extracted from Padang beach sand. Case Stud Chem Environ Eng. 2024;9:100679.

Edited by

• Associate Editor:
  Jose Eiras.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

The full dataset supporting the findings of this study is available upon request to the corresponding author.

Publication Dates

• Publication in this collection
  02 Mar 2026
• Date of issue
  2026

History

• Received
  19 Oct 2025
• Reviewed
  18 Dec 2025
• Accepted
  11 Jan 2026