Electronic materials such as BaTiO₃ (BT), Pb(Zr_1−xTi_x)O₃ (PZT), Pb(Mg_1/3Nb_2/3)O₃ (PMN), and Pb(Mg_1/3Ta_2/3)O₃ (PMT) have long held significant importance in various applications. However, in modern scientific and engineering societies, the demand for ultra-precise electronic and electrical materials with eco-friendly characteristics has grown. The utilization of electrical and electronic materials containing Pb, PZT, and PMN is diminishing. Instead, there is a shift towards developing environmentally friendly materials, such as BT and (K_1/2Na_1/2)NbO₃-type ferroelectrics. The discovery of ferroelectricity in BT, distinguished by its BO₆ oxygen octahedral structure unrelated to hydrogen bonding, has emerged as an important research topic for physicists and materials scientists. Figure 1 shows the unit cell of a conventional cubic BT perovskite. BT exhibits a ferroelectric phase owing to the pronounced spontaneous polarization arising from the asymmetrical displacement of Ti⁴⁺ ions in body-centered and O²⁻ ions in face-centered positions, concerning the arrangement of Ba²⁺ ions at cube corners. This occurs at a temperature below transition temperature (T_C), which is approximately 120 °C [1–3].

Figure 1. Typical perovskite BT with Ba²⁺ ions in the center of the unit cell and Ti⁴⁺ ions inside the oxygen octahedron.

Substituting a small number of donor ions for pure BT enhances its insulating properties, resulting in a large energy gap of approximately 3.05 eV [4]. However, owing to the distinct valences of Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺ ions compared to Ba²⁺ and Ti⁴⁺ ions, a charge imbalance and compensation phenomena occur. Consequently, the ferroelectric and electronic properties of BT can exhibit variability. The process of preparing powders and the conditions of heat treatment within the BT ferroelectric fabrication process significantly affect the crystal structure and overall physical properties of BT. This influence is accompanied by the doping mechanism and defect generation [5].

This study analyzes the characteristics of electrical and electronic materials, along with the demand for environmentally friendly Pb-free ferroelectric devices required for ultra-precision AI systems. The investigation focuses on substituting BT with small amounts of trivalent (Bi³⁺, Sb³⁺) and pentavalent (Nb⁵⁺, Ta⁵⁺) ions. Moreover, their microstructures and dielectric properties are analyzed to examine their applicability as electronic and electrical materials.

The specimens were obtained by synthesizing over 99.5 % of the raw materials, such as BaCO₃, TiO₂, Nb₂O₅, Ta₂O₅, Bi₂O₃, and Sb₂O₃. Additionally, analysis specimens were synthesized in four different compositions by introducing substitutions of Nb₂O₅, Ta₂O₅, Bi₂O₃, and Sb₂O₃ at a concentration of 0.1 mol%. For each dopant-substituted specimen, a heat treatment was conducted within an alumina crucible, employing different temperatures between 1,250 and 1,350 °C. These temperature ranges represent the optimal sintering conditions for the four compositions, and the heat treatment duration ranged from one to four hours. This process resulted in the successful synthesis of a high-density impurity BT ferroelectric. To confirm the reaction state and structure of the specimens, both an x-ray diffractometer (XRD) and field emission scanning electron microscopy (FESEM) were utilized. The XRD analysis showed the stabilization of perovskite phases, with no evidence of pyrochlore phases. Analysis of the sintered body through FESEM data analysis revealed robust growth characterized by high density and fine grains.

The essential dielectric properties, dielectric constant (K) and dielectric loss (tanδ) were measured using an impedance analyzer for specimens where dipoles were aligned by applying an electric field (DC 15 kV/cm). These measurements were conducted as functions of temperature (20–220 °C) and frequency (0.1–10⁶ kHz).

Figure 2 shows the typical XRD pattern of perovskite BT doped with Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺in a 0.1 mol% ratio. Except for the Bi³⁺-doped BT sample, phases such as BaCO₃, TiO₂, Nb₂O₅, Ta₂O₅, Bi₂O₃, and Sb₂O₃ were not clearly detected by XRD in the remaining samples. This indicates that the diffraction peaks did not provide conclusive evidence of phase coexistence for each case. However, distinct contrasts emerge when comparing the XRD patterns of the four compositions: BT doped with Nb⁵⁺and Ta⁵⁺clearly differs from that doped with Bi³⁺ and Sb³⁺. In the original BT sample, both tetragonal and cubic phases were evident, while only BT-Nb5 and BT-Ta5 exhibited strong two-phase formation.

Figure 2. Typical XRD patterns of BT doped with Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺. (a) BT-Nb5, (b) BT-Ta5, (c) BT-Bi3, and (d) BT-Sb3.

The XRD intensities of the tetragonal (002), tetragonal (200), and cubic (200) planes, observed at 45–46°, confirm the coexistence of tetragonal and cubic phases. The division of the (002) and (200) peaks in Figs. 2(a) and 2(b) indicates the tetragonal phase, whereas the (200) main peak in Figs. 2(c) and 2(d) indicates the cubic phase. Both the single-crystalline and polycrystalline forms of BT exhibit a combination of these tetragonal and cubic phases. Notably, in BT that underwent sintering with partial Nb⁵⁺and Ta⁵⁺ escape, the XRD diffraction showed a strong diffraction pattern between 45.10 and 45.45°, indicating the formation of two BT phases [3].

The substitution of additives results in significant changes that transform the intrinsic properties of BT into the desired properties. Figures 3(a)–(d) shows the energy dispersive X-ray spectroscopy (EDS) and FESEM (inset) images of BT-Nb5, BT-Ta5, BT-Bi3, and BT-Sb3 samples, wherein Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺ are introduced as substitutes. The figure clearly shows the original BT-series pattern despite the differences in dopants. The sintered densities of Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺-doped BT, synthesized under different heat-treatment conditions, reached approximately 85–89 % of the theoretical density, indicating their fabrication as relatively dense solid solutions. Figures 3(a), 3(b), and 3(d) indicate that substituting Nb⁵⁺, Ta⁵⁺, and Sb³⁺ dopants correlates with a partially inhomogeneous grain distribution. Although this experiment does not depict it, the EDS data for pure BT synthesized in our laboratory reveals atomic percentages of Ba²⁺, Ti⁴⁺, and O²⁻ as 18.95, 18.09 and 62.96 %, respectively. In contrast, for dopant-substituted BT, the atomic percentages of Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺ ions range from 0.79 to 3.54 %. The atomic percentages range from 14.85–37.12 % for Ba²⁺, 11.87–26.21 % for Ti⁴⁺, and 33.16–66.17 % for O²⁻ when substituting with Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺. The non-uniform distribution of Bi³⁺ and Sb³⁺ and sample segregation form a non-ferroelectric region in the BT sample, leading to the degradation of its dielectric properties. Specifically, compositions substituted with Bi³⁺ and Sb³⁺ may exhibit non-ferroelectric regions owing to non-uniformity and segregation, resulting in the degradation of dielectric characteristics.

Figure 3. EDS and FESEM (inset) images of Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺substituted BT. (a) BT-Nb5, (b) BT-Ta5, (c) BT-Bi3, and (d) BT-Sb3.

The observed trends in atomic percentages between pure BT and the four substituted compositions underscore significant changes. This shift in atomic composition plays a crucial role owing to its impact on diffusion kinetics during heat treatment, particularly related to charge compensation defects observed within Ba²⁺ and Ti⁴⁺ sites. The substitution of Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺ has a decisive effect on the crystallization kinetics of the solid solution. Essentially, replacing highvalence ions with low-valence ions leads to charge imbalances and defect structures within the four types of BT dopants. As Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺ ions partially substitute the Ba²⁺ and Ti⁴⁺ sites within the perovskite structure, the pore concentration in the crystal appears to be charge-compensated owing to the variances in atomic radii and valences. The substitution of suitable ions at the cube corners and body center of the perovskite oxide necessitates energy input, resulting in compensation defects owing to the differences in ionic radii and valences. Therefore, the solubilities of BT-Nb5, BT-Ta5, BT-Bi3, and BT-Sb3, designed to compensate defects at two different lattice positions, diverge based on the type of additive. This variation leads to differences in heat treatment conditions, crystal grain sizes, and the resulting dielectric, pyroelectric, and piezoelectric properties. Further insights are provided in Figs. 3(a)–(d), illustrating the cross-sections of the sintered bodies obtained through FESEM for the four BT types. Notably, all solid solutions exhibit grain growth within the range of 1.5–2.5 µm. Specifically, Ba-Nb5 and Ba-Ta5 display grain sizes of approximately 1.5–2.5 µm, while BT-Bi3 and BT-Sb3 exhibit sizes of approximately 1.5–2.0 µm.

In this experiment, the introduction of off-valent impurities into BT had a significant impact on both its ferroelectric properties and electrical conductivity. The results, depicted in Figs. 3(a)–(d), reveal the potential to improve dielectric properties by stabilizing grain growth by incorporating minute concentrations of Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺ into BT. These dopants play a pivotal role in influencing grain boundary mobility, thus significantly affecting charge compensation. Since the process of charge compensation holds substantial importance, these ions act as substitute ions along grain boundaries. This signifies that upon substituting Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺ for Ba²⁺ and Ti⁴⁺, the differences in radii and valencies among Nb⁵⁺, Ta⁵⁺, Bi³⁺, Sb³⁺, Ba²⁺, and Ti⁴⁺lead to a change in the vacancy concentration of the crystal to compensate for the charge imbalance. Through this study, it was established that the high concentration of Nb⁵⁺, Ta⁵⁺, Bi³⁺, and Sb³⁺ ions within the solid solution facilitates the suppression of abnormal grain growth, promotes the formation of fine grains, and results in a densely packed BT structure during the sintering process [6].

Impurities introduced into pure BT significantly alter its mechanical and semiconductor properties, while also significantly influencing its dielectric properties. Figure 4 shows a representative plot showing the dielectric constants across the 0.1–10⁶ kHz range for the BT-Sb3 specimen. This composition displays the characteristics of a weakly diffused ferroelectric, where maximum dielectric constant (K_m) decreases while T_C increases as the frequency increases.

Figure 4. Dielectric constant vs. temperature behavior at various frequencies for composition BT-Sb3. The inset shows the dielectric constant vs. temperature behavior at various frequencies for composition BT doped PMN-PT composition.

The frequency dependence of BT-Sb3 was confirmed through measurements of K and tanδ across a frequency ranging from 0.1 to 10⁶ kHz. Figure 4 shows the partial diffusion characteristics of K, as measured for the BT-Sb3 specimen. An inset in Fig. 4 emphasizes the frequency-dependent dielectric constant of PMN-PT doped with BT, displaying a diffuse ferroelectric configuration fabricated for comparative analysis. However, when BT is doped with Sb³⁺, the composition displays characteristics of a partially relaxor ferroelectric. Figure 4 also reveals nuanced dynamics: K_mexperiences a marginal reduction with increasing frequency, while the T_C continuously shifts toward the high-temperature side.

Figure 4 displays the frequency vs. K plot of a typical relaxor ferroelectric featuring PMN-PT combined with BT, which was fabricated for comparative analysis. However, when BT is doped with Sb³⁺, the resulting composition shows characteristics similar to those of a partially relaxor ferroelectric.

Figure 5 displays the dielectric constant vs. temperature profiles for various dopants after polarization. The Nb-doped BT-Nb5 composition exhibits a K_m of approximately 8294, followed by BT-Ta5 at approximately 9940, BT-Sb3 at 6092, and BT-Bi3 at 4897. As depicted in Fig. 5, the dielectric loss exhibits a more diffuse temperature dependence than PMN, PMT, and PSN. Shifting to BT with trivalent or pentavalent ion dopants results in the highest K_m value and a narrower dielectric constant half-width. Introducing small quantities of Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺ at Ba²⁺ and Ti⁴⁺ sites induces favorable changes and improvements in physical properties. Substituting higher-valence ions in the BT system with lower-valence ions significantly influences charge balance in the crystal lattice. However, owing to the limited amounts (<0.1 mol%) of Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺ dopants used in this experiment, achieving a stable distribution within the solid solution posed challenges. Consequently, their effect on ferroelectric parameters such as dielectric, pyroelectric, and piezoelectric constants remained insignificant. Ferroelectric solid solutions led to significant changes in phase and domain boundaries, necessitating a statistical distribution of various composition types and concentrations. Therefore, the phase transition mechanism in the diffusion of ferroelectric complex perovskites requires satisfying two or more phase equilibrium conditions. These phenomena indicate the active physical and chemical effects of dopants, including atomic radius, valence, electronegativity, and dopant concentration.

Figure 5. Temperature dependence of the dielectric constant of BT with different dopants added. The inset shows the temperature dependence of the dielectric loss of BT with different dopants added. (a) BT-Nb5, (b) BT-Ta5, (c) BT-Bi3, and (d) BT-Sb3.

As shown in Fig. 6, a noticeable reduction in the dielectric constant is apparent across the 10–100 kHz frequency range for all compositions. Consequently, the temperature response, K vs. T characteristics, and rapid phase transition within the T_C region were observed in the four BT specimens substituted with Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺. The chemical behavior of solid solutions containing these substitutions in BT is believed to depend on the mixing of two different microstructures: heterogeneous components and pre-existing nonferroelectric states, both present below T_C. However, in the tanδ vs. frequency graph depicted in the inset of Fig. 6, the tanδ values of samples doped with Nb⁵⁺ and Ta⁵⁺ display a continuous decrease as the frequency increases. Conversely, in samples doped with Bi³⁺ and Sb³⁺, a slight increase in dielectric loss is observed above 10⁴ Hz. The overall frequency dependence illustrated in Fig. 6 reveals that the dielectric constant of the doped BT compositions, which require more energy for dipole orientation within the solid solution, experiences a significant decline above 10² Hz. The phenomenon observed in the BT system closely resembles findings reported by numerous researchers [6,7]. When BT is doped with Bi³⁺ and Sb³⁺, which exhibit similarity in ionic radius and electronegativity, the probability of occupying the perovskite cube-corner lattice site increases. Therefore, while substituting into the A site of perovskite BT, the dopants tend to better maintain charge balance. For example, the probability of the occurrence of electron and/or barium vacancies increases, such as Bi_Ba→Bi*_Ba+e′ and Bi_Ba→Bi*_Ba+(1/2)V″_Ba. Here, Bi*_Ba signifies the ionization of donor Ba²⁺, V″_Ba represents a vacancy owing to Ba ionization, and e′ denotes an electron. Replacing a low concentration of Ba²⁺ leads to compensation through Bi_Ba→Bi*_Ba+e′. Consequently, substituting an appropriate concentration of Bi³⁺ for pure BT reduces the degree of disorder at the Ba²⁺ atomic site, inducing changes in the structure of the local state of Ti⁴⁺ [7,8].

Figure 6. Frequency dependence of dielectric constant of (a) BT-Nb5, (b) BT-Ta5, (c) BT-Bi3 and (d) BT-Sb3 compositions. The inset shows the frequency dependence of the dielectric loss of BT with different dopants added. (a) BT-Nb5, (b) BT-Ta5, (c) BT-Bi3 and (d) BT-Sb3 compositions.

Substituting BT with a small molar ratio of acceptor or donor dopants, such as Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺, poses challenges in achieving a uniform crystal structure. Despite this difficulty, the advantage lies in its minimal effect on permittivity parameters such as T_C. The substitution of these dopants in the BT lattice sites of Ba²⁺ and Ti⁴⁺ results in microscopic relaxor characteristics. These characteristics are subjected to a phenomenological analysis utilizing the modified Curie– Weiss law, 1/K=1/K_m+[(T-T_m)^γ/(2K_mδ2)]. In this equation, K represents the dielectric constant, K_m denotes the maximum dielectric constant, T temperature, Tm the maximum temperature of the dielectric constant, γ the critical exponent, and δ the diffusion parameter. Specifically, γ of a ferroelectric is 1, while γ of a diffused ferroelectric is known to be approximately 2. As shown in Figs. 7(a) and 7(b), the partial replacement of Ba²⁺ lattice sites in BT with Nb⁵⁺ and Ta⁵⁺ results in subtle microscopic changes. While the diffusive parameters remain relatively stable, researchers can analyze these intricate microstructural changes.

Figure 7. Plot of ln[(1/K)-(1/K_m)] vs. ln(T-T_m) when different dopants are added to BT. The inset shows the temperature dependence of the reciprocal dielectric constant of BT with different dopants added. (a) BT-Nb5, (b) BT-Ta5, (c) BT-Bi3, and (d) BT-Sb3.

While the diffusivity of the substituted BT varies from that of the Pb-complex perovskite, the phenomenon of the diffusion phase transition can still be observed on a macroscopical scale. By calculating γ using this method, the diffusivity of BT when substituted with Nb⁵⁺ and Ta⁵⁺ can be estimated. The substitution of Nb⁵⁺ and Ta⁵⁺leads to a reduction in γ. According to the reported data, the diffusion phenomenon arises from localized compositional changes within microregions, resulting in different T_C values for each microregion. This disparity in T_C is attributed to the variation in grain growth prompted by the substitutions of Nb⁵⁺ and Ta⁵⁺.

For Ba-Bi3 and BT-Sb3, a continuous reduction in K was observed with decreasing temperature in the transition region, showing a singularity different from pure BT [5]. In Fig. 7(c), it is evident that the substitution of Bi³⁺ ions for Ba²⁺ ions induces a ferroelectric state across a wide temperature range, such that γ<1. The diffusivity of BT-Bi3, wherein partial substitution of Bi³⁺ for Ba²⁺ takes place, is expressed as γ in Fig. 7(c). Although not mentioned in this study, it was confirmed that elevating the heat treatment temperature decreases the value of γ in BT-Bi3. Previous studies by various researchers have attributed this diffusion phenomenon to the appearance of complex microregions within the crystal structure owing to localized compositional fluctuations. The change in the inverse dielectric constant of BT-Bi3 is illustrated in the inset of Fig. 7, depicting the variation in 1/K for the BT-Bi3 solid solution as a function of temperature.

The composition of BT-Bi3, with a Bi³⁺substitution at a low concentration of 0.1 mol%, exhibits a relatively stable effect on intrinsic dielectric phenomena, such as the Curie temperature and phase transition. This stability arises owing to the presence of only partial nonuniformity. Substituting BT-Bi3 with Bi³⁺ at a low concentration (0.1 mol%) somewhat stabilizes the T_C, owing to localized non-uniformity. These dielectric properties have gained significance in the analysis of the physical behavior of BT-Bi3, where Ba²⁺ and Ti⁴⁺ are substituted for BT. In this study, data including ln(T-T_m), 1/K vs. T for BT-Nb5, BT-Ta5, BT-Bi3, and BT-Sb3, with substitutions of Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺, respectively, demonstrate typical ferroelectric behavior, distinguishing them from transfers. This behavior closely aligns with previously studied dielectric properties of BT-Bi3. The experiment reveals that the T_C for BT-Bi3 and BT-Sb3, where Bi³⁺ and Sb³⁺ are substituted, can be shifted to lower temperatures, allowing the determination of diffusivity across a wide temperature range. In Fig. 7(d), upon replacing Ba²⁺ with Sb³⁺, the diffusion exponent varies, enabling the utilization of γ for tabular analysis. Figure 7 shows the analysis of γ for BT-Sb3, involving Ba²⁺ substitution with Sb³⁺. γ is derived by fitting the modified Curie–Weiss law. For BT-Sb3, γ exhibits a slight decrease inversely proportional to the rise in heat treatment temperature [9–12]. As for the characteristics of BT-Sb3, EDS/FESEM observations in Fig. 3 indicate that specimen grains enlarge with higher heat treatment temperatures. Material scientists interpret this diffusion as forming multiple layers of microdomains, each with different Tc values owing to localized compositional fluctuations. The inset chart in Fig. 7 elucidates the changes in 1/K for BT-Nb5, BT-Ta5, BT-Bi3, and BT-Sb3. In particular, BT-Sb3, with Sb³⁺ substituting Ba3+, exerts a relatively minor influence on T_C owing to its non-uniform distribution. Researchers have extensively investigated these changes in physical properties to synthesize BT substitutes at cube corners and bodycentered sites. The inset in Figs. 7(a)–(d) shows the variation in 1/K of Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺ substituted BT across temperatures ranging from 20 to 200 °C. While Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺ of BT are synthesized into BT by substituting Ba²⁺ and Ti⁴⁺, the crystal lattice undergoes a slight distortion without altering the structural form of BT, consequently shifting Curie maxima to lower temperatures. The inset of Fig. 7 shows 1/K of BT saturated with Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺. Given the structural non-uniformity observed in the four BT compositions with Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺substitutions, their effect on T_C can be considered modest. In the case of BT substitution, dielectric response characteristics were analyzed across a wide temperature range as a factor for lowering K in the region above T_C.

This study analyzes the grain growth and dielectric constant of BT with substitutions of Bi³⁺, Sb³⁺, Nb⁵⁺, and Ta⁵⁺. The optimal sintering conditions varied slightly among the four dopants; however, specimens suitable for these conditions were successfully synthesized through heat treatment at 1,250–1,350 °C for 1–4 h in air. It was confirmed that the four BT compositions, substituted with both trivalent and pentavalent ions, exhibited robust growth, forming relatively high-density particles. Additionally, the basic dielectric properties of these ferroelectrics generally met the required standards. For BT-Nb5 and BT-Ta5, achieved by substituting 0.1 mol% of Nb⁵⁺ and Ta⁵⁺, respectively, XRD analysis revealed a clear split between the (200) and (002) planes within 45.1–45.45°. However, the BT-Bi3 and BT-Sb3 compositions with trivalent Bi³⁺ and Sb³⁺ showed no clear evidence. In particular, the phase T_Cs for all four BT compositions with dopant substitutions exhibited gradual shifts toward lower temperature ranges, showing only marginal differences from pure BT. Moreover, the ln[(1/K)- (1/K_m)] vs. ln(T-T_m) and 1/K vs. T analyses indicated a departure from the behavior of pure BT. Instead, the phase transition characteristics on the higher-temperature side demonstrated a relatively favorable alignment with the modified Curie–Weiss law.

This study was supported by a research fund from Dankook University in 2023.

The authors declare no conflicts of interest.