Applied Science and Convergence Technology 2023; 32(5): 101-105
Published online September 30, 2023
Copyright © The Korean Vacuum Society.
College of Engineering, Dankook University, Yongin 16890, Republic of Korea
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The structural and electrical properties of ferroelectrics can be altered through different mechanisms to meet necessary conditions, depending on the additive type in appropriate proportions. This study investigates the growth process of four types of BaTiO3 (BT), each substituted with 0.1 mol% of trivalent and pentavalent elements: Nb5+, Ta5+, Bi3+, and Sb3+. The relationship between the dielectric properties of perovskite formation and grain growth is explored across the four dopants. The results confirm the proficient growth of these four BT compositions when substituted with trivalent and pentavalent ions, yielding relatively high-density particles. Moreover, the fundamental dielectric properties of the ferroelectrics satisfied the required levels. X-ray diffraction studies revealed that both Nb- and Ta-doped BT, prepared by substituting 0.1 mol% of Nb5+ and Ta5+, demonstrated distinctive splitting of the (200) and (002) peaks within the range of 45.10–45.45∘. However, no clear evidence was observed for Bi- and Sb-doped BT compositions containing trivalent Bi3+ and Sb3+ ions. In particular, the transition temperature of the four substituted BT compositions exhibited negligible divergence from that of pure BT, with a marginal shift toward lower temperatures. In addition, contrary to pure BT, the characteristics of ln(T-Tm) vs. ln[(1/K)-(1/Km)] and 1/K vs. T, which elucidate high-temperature phase transitions, exhibited relatively well-satisfied behavior with the modified Curie–Weiss law.
Keywords: BaTiO3, Pb(Mg1/3Nb2/3)O3, Dopants substitution, Ferroelectrics, Modified Curie-Weiss law
Electronic materials such as BaTiO3 (BT), Pb(Zr1−xTix)O3 (PZT), Pb(Mg1/3Nb2/3)O3 (PMN), and Pb(Mg1/3Ta2/3)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 (K1/2Na1/2)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 (TC), which is approximately 120 °C [1–3].
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 . 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 .
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.
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 .
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.
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 .
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 (Km) decreases while TC increases as the frequency increases.
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: Kmexperiences a marginal reduction with increasing frequency, while the TC 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 Km 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 Km 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.
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 TC 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 TC. 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 BiBa→Bi*Ba+e′ and BiBa→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 BiBa→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].
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 TC. 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/Km+[(T-Tm)γ/(2Kmδ2)]. In this equation, K represents the dielectric constant, Km 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.
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 TC values for each microregion. This disparity in TC 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 . 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 TC, 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-Tm), 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 TC 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 TC 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 TC 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 TC.
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 TCs 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/Km)] vs. ln(T-Tm) 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.