Relationship between Crystal Structures and the Relaxor Property of SrBi2(Ta2-x V x )O9 Ceramics.

Here, V2O5 was used to substitute Ta2O5 in SrBi2Ta2O9 (SBT) and SrBi2(Ta2-x V x )O9 (SBTV) ceramics were formed. This study revealed that the substitution of Ta2O5 by V2O5 enhances the maximum dielectric constant (εm), increases the transition temperature, and reduces the sintering temperature of SBT ceramics. The diffraction intensity of c-axis-preferred orientation of SBTV ceramics increases with the increase in V2O5 concentration and sintering temperature. Disk-type grains were observed in the SBTV ceramics at higher sintering temperatures. By the Curie-Weiss law and modified Curie-Weiss law, the phase transitions of the SBTV ceramics were discussed. The dielectric properties of SBTV ceramics revealed that the relaxor-type ferroelectric characteristics became more obvious than the normal-type ferroelectric characteristics at high V2O5 concentrations and sintering temperatures. Raman spectroscopy was successfully used to study the lattice vibrational modes and structural transitions of SBTV ceramics. The spectra proved that as V2O5 was added to SBTV ceramics the octahedral TaO6 and VO6 structures exhibited a high frequency mode.

Introduction

In recent years, ferroelectric materials have gained popularity for their application in nonvolatile memory devices.[1,2] These materials particularly possess high permittivity, high piezoelectric and pyroelectric coefficients, and reliable polarization switching.[3] The leading candidate for this application is PbZr1–TiO3 (PZT) perovskite, which has a high Curie temperature (TC) and large remanent polarization. However, PZT ceramics exhibit serious polarization fatigue during electric field cycling. Moreover, the release of harmful compounds of Pb from electrical industries is an environmental hazard and some implemented regulations such as waste from electrical and electronic equipment, restriction of hazardous substances, and end-of-life vehicles forbid the emission of harmful waste materials.[4] More recently, a significant breakthrough in controlling fatigue problems was reported in which ferroelectric materials belonging to the layered perovskite family, such as SrBi2Ta2O9 (SBT) and BaBi4Ti4O15 ceramics, demonstrated no polarization fatigue during electric field cycling.[5]

Based on their structure, ferroelectric oxides may be classified into four types: (1) perovskite, (2) pyrochlore, (3) tungsten bronze, and (4) layer-type bismuth (Bi) compounds. In 1961, Smolenskii et al. observed ferroelectricity in the series of Bi-layered perovskite ABi2B2O9.[6] As Bi-layered structure ferroelectrics serve a crucial role in dielectric and ferroelectric devices, their crystal structure and material properties have been investigated widely.[7−9] Bi-layered pseudoperovskite SBT ceramics have gained considerable attention as a promising ferroelectric material and are a suitable lead-free alternative for PZT ceramics in memory devices.[10,11] The SBT crystal structure comprises [Bi2O2]2+ layers and perovskite-type [SrTa2O7]2– units with double TaO6 octahedral layers. SBT ceramics provide a high fatigue endurance, low switching voltage, and small polarization. However, the low remanent polarization and high crystallization temperature are the major drawbacks of SBT ceramics. Several studies have been conducted to increase the polarization of SBT ceramics by doping various cations at the A- and/or B-sites. SrBi2Ta2O9 belongs to the Aurivillius family of Bi-layered perovskites with the general formula (Bi2O2)2+(ABO3)2–, where m = 1–5. The substitution of A- and B-site cations has a pronounced influence on the ferroelectric properties of SBT ceramics.[12]

The substitution of Ca2+, Bi3+, Pr3+, Nd3+, and La3+ with Sr2+, whose radii are similar, at the A-site of SBT ceramics was widely investigated, and considerable modification of the polarization properties was observed. Pérez et al. reported the enhanced polarization properties of Ca2+-substituted SBT compounds.[13,14] Li et al. indicated that when Nd3+ is substituted into the bismuth-layered perovskite structure of SBT by magnetron sputtering, the remnant polarization (2Pr) improves and coercive field reduces.[15] Coondoo et al. reported the addition of tungsten (W) in SBT at the B-site to form SrBi2(Ta1–W)2O9 and observed effective improvement in the dielectric, electrical, ferroelectric, and piezoelectric properties.[16]

In this study, Ta2O5 substituted with different concentrations of V2O5 in SBT ceramics was used to synthesize SrBi2(Ta2–V)O9 (SBTV, x = 0.1–0.4) ceramics. By substituting Ta2O5 with V2O5 in SBT ceramics, SBTV ceramics could be formed and the ferroelectric properties were enhanced.[17] To study the effect of V2O5 content and sintering temperature on the microstructure, Raman characteristic and normal- or relaxor-type characteristic of SBTV ceramics were investigated. The results of Perez et al.,[18] Zhu et al.,[19] and Yu et al.[20] have indicated that Raman scattering is useful for exploring the microscopic origin of ferroelectric materials because it is a sensitive technique for investigating the lattice vibrational modes, which can indicate the changes in the lattice vibrations and positions occupied by the added ions. Dielectric characteristics of the normal- or relaxor-type characteristic of SBTV ceramics were measured. The distinction between normal-type ferroelectrics and relaxor-type ferroelectrics (RFE) can be distinguished by three qualitatively different features in the temperature dependence of the dielectric susceptibility. First, in normal ferroelectrics, the real part dielectric constant (εr) shows a sharp narrow peak as a function of temperature (T), whereas the εr–T curves show a broad and rounded peak in the relaxor ferroelectrics. Second, RFE is a strong frequency dependence in the peak position. Last, the polarization in FE goes to zero at transition temperature and in the relaxors the polarization extends well beyond the transition temperature. Many researchers have reported that the Curie–Weiss law and modified Curie–Weiss law may be used to describe diffusion during phase transition. In this study, the Curie–Weiss law and modified Curie–Weiss law were used to investigate the phase transition between the characteristics of normal- and relaxor-type ferroelectrics for the SrBi2(Ta2–V)O9 ceramics.

Results and Discussion

The observed morphologies of the SrBi2(Ta2–V)O9 (SBTV) ceramics sintered at 1020 °C with different V2O5 concentrations through scanning electronic microscopy (SEM) are illustrated in Figure ; grain growth was observed in all SBTV ceramics. Based on the surface morphology of the SrBi2Ta1.9V0.1O9 (x = 0.1) ceramics presented in Figure a, several small anisotropic platelike grains were observed and the average aspect ratio of the anisotropic platelike grains was approximately 6.67. When the V2O5 concentration was increased to 0.2 (x = 0.2), the sizes of the anisotropic platelike grains and the average aspect ratio of SrBi2Ta1.8V0.2O9 ceramics were slightly larger than those of SrBi2Ta0.9V0.1O9 ceramics. When the V2O5 concentrations were 0.3 and 0.4, the SrBi2Ta1.7V0.3O9 and SrBi2Ta0.6V0.4O9 ceramics revealed anisotropic plate-type grains. Moreover, the average grain size of the ceramics increased and the aspect ratio of the disk-type grains increased from 8.05 to 8.72. The different aspect ratios of the SBTV ceramics were caused by the addition of V2O5 because the low melting point of V2O5 acts as a liquid-phase-sintering aid and promotes the growth of the (00l) planes.[17] However, the small voids or pores on the SrBi2Ta1.6V0.4O9 ceramics were more apparent compared with those on the SrBi2(Ta2–V)O9 (x = 0.1-0.3) ceramics presented in Figure a–c. This result is caused by the upright anisotropic plate-type grains that grow on the plane anisotropic plate-type SrBi2Ta1.6V0.4O9 ceramics. Moreover, based on the results presented in Figure and ref (21), the addition of V2O5 in SrBi2Ta2O9 (SBT) ceramics can lower the sintered temperature required to densify the SBT-based ceramics.

Figure 1

SEM images of the 1020 °C-sintered SrBi2(Ta2–V)O9 ceramics as a function of V2O5 content. (a) x = 0.1, (b) x = 0.2, (c) x = 0.3 and (d) x = 0.4, respectively.

Figure illustrates the X-ray diffraction (XRD) patterns of the SBTV ceramics with different V2O5 concentrations; Figure a–c illustrates the characteristic peaks of SBT (JCPDS no. 49-0609) ceramics. Only a single-phase layered perovskite structure was observed in all SBTV ceramics, and no secondary or unknown phases were detected. These results suggest that Ta5+ ions (Ta5+ = 0.64 Å) were completely substituted by V5+ ions (V5+ = 0.54 Å) in the SBTV ceramics. The XRD patterns of the SrBi2Ta1.6V0.4O9 ceramics revealed unknown phases. The diffraction intensities of the (00l) planes increased and those of the (111), (113), (115), (200), (206), (208), (220), (224), (1113), and (315) planes slightly decreased with the increase in V2O5 concentration. The (00l) planes observed at 2θ values of 21.4, 28.6, 35.9, 43.4, and 51.1° corresponded to the (006), (008), (0010), (0012), and (0014) planes, respectively. The increase in the diffraction intensities of SBTV ceramics with (008) and (0010) planes was more obvious. Huang et al.[17] found that the (00l) plane-preferred orientation of the layered perovskite in V2O5-doped SBT ceramics increased faster than the orientation in nondoped SBT ceramics.[20] This result was observed because V2O5 that has a low melting point changes to the liquid phase during sintering to promote a faster growth of the c-axis-preferred orientation (00l) planes due to a high surface energy.[22] Simultaneously, the formation of trace amount of unknown phases in SrBi2Ta1.6V0.4O9 ceramics indicated that the crystal structure of layered perovskite became defective. The unidentified phases maybe because the V5+ ions did not occupy the Ta5+ sites in the structure with higher V2O5 concentration.[16]

Figure 2

XRD patterns of the 1020 °C-sintered SrBi2(Ta2–V)O9 ceramics as a function of V2O5 content. (a) x = 0.1, (b) x = 0.2, (c) x=0.3, and (d) x = 0.4.

The Raman spectra of SBTV ceramics with different V2O5 concentrations are shown in Figure . Raman bands at approximately 163, 210, 319, 356, 455, 600, 805, and 853 cm–1 were observed in all SBTV ceramics. These characteristic bands agreed well with those in other reports that presented characteristic bands at 163, 210, 600, and 805 cm–1, and the other weak bands at 319, 356, and 455 cm–1.[19,23,24] The Raman peaks at 805 and 853 cm–1 of SBTV ceramics corresponded to the TaO6 and B-site substitution modes.[18] The peak intensity of SBTV ceramics at 853 cm–1 increased with the increase in V2O5 concentration, and the TaO6 and VO6 octahedral structures exist in SBTV ceramics. The substitution of V2O5 at Ta2O5 sites in the SBT ceramics apparently changes the high Raman modes of SBTV ceramics. V2O5 is a pentavalent compound and substitutes Ta2O5 sites inside the octahedral cage. In the low Raman mode range, the SBTV ceramics do not demonstrate any evident variation. However, only the 163.7 cm–1 band shifts to 165.3 cm–1, as shown in Figure . This result was caused by the replacement of Ta+5 ions by the low-mass V+5 ions, and the substitution number of V+5 ions increases with the increase in V2O5 concentration. The band at 163.7 cm–1 is attributable to the vibration of the Ta+5 ion along the z direction (TO mode A1g),[25] and the band at 210 cm–1 represents the TO mode of SrO with a rock salt structure.[26] The peaks at approximately 600 and 805 cm–1 are associated with the internal vibration of the TaO6 octahedron. However, the oxygen ions contributing to the two bands are different. The vibration of the oxygen ions (O2) at the apex of the TaO6 octahedron forms the 600 cm–1 band, whereas the vibration of oxygen ions (O4, O5) in the TaO6 octahedron forms the 805 cm–1 band.

Figure 3

Raman spectra of the 1020 °C-sintered SrBi2(Ta2–V)O9 ceramics as a function of V2O5 content. (a) x = 0.1, (b) x = 0.2, (c) x = 0.3, and (d) x = 0.4.

Figure 4

Raman spectra of the 1020 °C-sintered SrBi2(Ta2–xV)O9 ceramics at 163 cm–1 band as a function of V2O5 content. (a) x = 0.1, (b) x = 0.2, (c) x = 0.3, and (d) x = 0.4.

Figure shows the dielectric constant (εr) and loss tangent (tan  δ) of the SBTV ceramics sintered at 1020 °C that are measured at 10 kHz, 100 kHz, and 1 MHz as a function of the measurement temperature. The dielectric constant and loss tangent of the ferroelectric materials in most cases depend on factors such as composition, grain size, and secondary phases.[14] As shown in Figure a–d, the dielectric constants of the all SBTV ceramics increased with measurement temperature and reached their maximum at the transition temperature. All ferroelectric materials were characterized by a transition temperature known as the Curie temperature, TC, at which the dielectric constant exhibits a maximum value. The significantly enhanced dielectric constant at high measurement temperatures may be due to the mobile charge carriers, related to the oxygen vacancies. The oxygen vacancies play a crucial role in the electrical polarization of perovskite materials.[27] The oxygen vacancy-induced dielectric relaxation is considerably higher at higher measurement temperatures than that at lower measurement temperatures. At low measurement temperatures, the energy required to overcome the diffusion energy barrier is insufficient and the oxygen vacancy-induced dielectric relaxation is negligible. At high measurement temperatures, there is sufficient energy and the contribution of the oxygen vacancy-induced dielectric relaxation is substantial. Another valuable observation is that the dielectric constants at low measurement temperatures were the same, regardless of the frequencies used for the measurements, whereas the dielectric constants at high temperatures varied considerably. The aforementioned results imply that the mechanism contributes to the dielectric polarization at high temperatures, but not at low temperatures. Moreover, the transition temperature causes transition of the crystal structure. At measurement temperatures higher than the transition temperature, the SBTV exhibits a layered tetragonal structure. However, below the transition temperature, the SBTV exhibits an orthorhombic structure. For a given system, the tetragonal structure imposes a lower activation energy compared with that of the orthorhombic or other lower symmetric structure.[28]

Figure 5

Temperature–dielectric constant curves of the SrBi2(Ta2–V)O9 ceramics as a function of V2O5 contents. (a) x = 0.1, (b) x = 0.2, (c) x = 0.3, (d) x = 0.4.

The maximum dielectric constants of the SrBi2Ta0.9V0.1O9 ceramics measured at 10 kHz, 100 kHz, and 1 MHz were 182, 161, and 153, respectively. The maximum dielectric constant of the SrBi2Ta0.9V0.1O9 ceramics decreased with the increase in measured frequency. The same phenomenon was observed in all SBTV ceramics because the oxygen vacancy-induced polarization becomes more predominant at low frequencies due to the inertia of oxygen ions. This explains the substantial enhancement in the dielectric constants at low measurement frequencies. Moreover, the maximum dielectric constants of the SBTV ceramics increased as the V2O5 concentration increased from x = 0.1 to 0.3. As the same sintering temperature was used, the crystallization and grain size of the SBTV ceramics for x = 0.3 is better than those of the SBTV ceramics for x = 0.2 and x = 0.1, as shown in Figures and 2. Because of the larger diffraction intensity of (0010) peak and larger grain size of the SrBi2Ta0.7V0.3O9 ceramic as compared with those of the SrBi2Ta0.9V0.1O9 and SrBi2Ta0.8V0.2O9 ceramics. Therefore, except for the effect of the lower melting temperature of V2O5, the effect of liquid-phase sintering caused by the low-temperature eutectic in the SrO–V2O5 system is considered another reason for the improvement of crystallization and grain growth.[29] Because V2O5 concentration was x = 0.4, the maximum dielectric constants of SrBi2Ta1.4V0.4O9 ceramics decreased slightly. Moreover, some small pores were found in the SrBi2Ta1.4V0.4O9 ceramics, as shown in Figures d and 4d. This result is caused by the growth of the upright anisotropic plate-type grains in the plane anisotropic plate-type SrBi2Ta0.6V0.4O9 ceramics. Moreover, compared with those of SBT ceramics, SBTV ceramics exhibit a higher maximum dielectric constant and a lower sintering temperature at 1020 °C.[30] This result can be demonstrated by the dielectric mixture rule. The most commonly used dielectric mixture rule is Lichtenecker’s equation[31]where V is the volume fraction and ε is the dielectric constant. This study assumed that the pores of the volume fraction, V1 (dielectric constant ε1 = 1), uniformly distributed in the matrix of the SrBi2(Ta2–V)O9 ceramics. The volume fraction of the SBTV ceramics is V2 (dielectric constant ε2).

The transition temperature of the SBTV ceramics as a function of V2O5 concentration measured at 10 kHz, 100 kHz, and 1 MHz is presented in Figure . The transition temperature of the SBT ceramic was observed at 320 °C. The transition temperature of the SBTV ceramic decreased slightly with the increase in measurement frequency. The degree of reduction in the transition temperature of the SBTV ceramics increased as the V2O5 concentration increased from x = 0.1 to 0.3 and decreased as the V2O5 concentration increased from x = 0.3 to 0.4. Moreover, the transition temperature of the SBTV ceramics shifted from 399 to 419 °C as the V2O5 concentration increased from x = 0.1 to 0.3. Then, the transition temperature shifted to low temperature from x = 0.3 to 0.4 at all measurement frequencies. The transition temperature of the SBTV ceramics is higher than that of the SBT ceramics. In general, in isotropic perovskite ferroelectrics, doping at the B-site (located inside an oxygen octahedron) with smaller ions results in the shift of the transition temperature to a higher measurement temperature, thus leading to a larger polarization due to the enlarged “rattling space” available for smaller B-site ions.[32] In the current study, Ta5+ ions (Ta5+ = 0.64 Å) were completely substituted by V5+ ions (V5+ = 0.54 Å) in the SBTV ceramics. As a larger amount of V2O5 was used to substitute Ta2O5 in the SBT ceramics, the transition temperature was higher. This result suggests that a higher V2O5 substitution at the Ta2O5 site in the SBT ceramics shifts the transition temperature to a higher temperature. The transition temperature also indicates enhanced polarizability,[33,34] and this result confirms the aforementioned reason for the increase in the maximum dielectric constants of the SBTV ceramics, as shown in Figure .

Figure 6

Transition temperature–frequency of the SrBi2(Ta2–V)O9 ceramics as a function of V2O5 contents at different frequencies.

The loss tangent of the SBTV ceramics measured at 10 kHz, 100 kHz, and 1 MHz as a function of measurement temperature is illustrated in Figure . The loss tangent of the SBTV ceramics rapidly increased at the measurement temperature because the measurement temperature was higher than the transition temperature. In particular, the change at 10 kHz was significantly faster than that at 100 kHz and 1 MHz. V2O5 doping in SBT ceramics increases loss tangent significantly. The dissipation in ferroelectric materials occurs due to various reasons such as domain wall relaxation, space charge accumulation at grain boundaries, dipolar loss, and direct current conductivity.[14] The presence of oxygen vacancies, which act as space charge and contribute to the electrical polarization, can be attributed to the loss tangent.[35−37]

To identify SBTV ceramics with the characteristics of the normal-type ferroelectric structure or relaxor-type ferroelectric structure, the Curie–Weiss law and modified Curie–Weiss law were used to analyze the εr–T curves. Figure shows the plot of the inverse dielectric constant versus the temperature for SBTV ceramics at different V2O5 concentrations. The dielectric constant of a normal-type ferroelectric higher than the Curie temperature follows the Curie–Weiss law that is described as follows[38]where T0 is the Curie–Weiss temperature, C is the Curie–Weiss constant, T is the measurement temperature, and T > TC. The degree of deviation from the Curie–Weiss law can be defined using ΔTm as followswhere TCW denotes the temperature at which the dielectric constant begins to deviate from the Curie–Weiss law and Tm is the temperature corresponding to the maximum dielectric constant. TC is determined from the T–1/ε by conducting extrapolation of the dielectric constant reciprocal of the paraelectric region, and the values obtained are listed in Table . Figure demonstrates three distinct regions: the ferroelectric state at temperatures lower than Tm, the state in which polar clusters exist at temperatures between Tm and TCW, and the paraelectric state at temperatures higher than TCW. The modified Curie–Weiss law has been proposed by many research groups to describe the diffusion during a phase transition[38]where γ and C′ are assumed to be constants. The γ value is between 1 and 2, and gives information about the phase transition characteristic. The γ values for normal-type ferroelectrics and ideal relaxor-type ferroelectrics are 1 and 2, respectively.[39,40]Figure represents the plot of log(1/ε′ – 1/εm) vs log(T – Tm) for SBTV ceramics measured at 1 MHz. A linear relationship was observed in the SBTV ceramics, and the slopes of the fitting curves determined the γ value. After assigning the experimental data to the modified Curie–Weiss law, the obtained γ values for SrBi2(Ta1.9V0.1)O9, SrBi2(Ta1.8V0.2)O9, SrBi2(Ta1.7V0.3)O9, and SrBi2(Ta1.6V0.4)O9 ceramics were 1.12, 1.35, 1.62, and 1.42, respectively. The γ values of the SrBi2(Ta2–V)O9 ceramics at different V2O5 concentrations were compared. The comparison results revealed that SrBi2(Ta1.9V0.1)O9 (γ = 1.12), SrBi2(Ta1.8V0.2)O9 (γ = 1.25), and SrBi2(Ta1.6V0.4)O9 (γ = 1.42) ceramics exhibit normal-type ferroelectric characteristics and that SrBi2(Ta1.7V0.3)O9 (γ = 1.62) ceramics relatively present the relaxor-type ferroelectric characteristics.

Figure 7

Plots for the temperature–1/ε curves of SrBi2(Ta2–V)O9 ceramics as a function of V2O5 content.

Table 1

Relational TCW, Tm, ΔTm, and γ Values of the Curie–Weiss Law for the SrBi2(Ta2–V)O9 Ceramics as a Function of V2O5 Contents

composition	x = 0.1	x = 0.2	x = 0.3	x = 0.4
Tm or Tc (°C)	403	411	415	409
TCW (°C)	415	431	443	424
ΔTm = TCW – Tm (°C)	12	20	28	15
γ	1.12	1.35	1.62	1.42

Figure 8

log(1/ε – 1/εm)–log(T – Tm) plots of SrBi2(Ta2–V)O9 ceramics with different V2O5 contents (symbols, experimental data; solid line, simulation data).

To understand the sintering temperature effect of the SBTV ceramics, the SrBi2(Ta1.7V0.3)O9 ceramics were further investigated as a function of the sintering temperature. The surface SEM images of the SrBi2(Ta1.7V0.3)O9 ceramics are investigated as a function of the sintering temperature, and the results are presented in Figure . At a sintering temperature of 940 °C, the SrBi2(Ta1.7V0.3)O9 ceramics revealed a porous structure and no grain growth. When sintered at 960 °C, the morphology of the SrBi2(Ta1.7V0.3)O9 ceramics revealed small square-shaped grains; the average grain size of the SrBi2(Ta1.7V0.3)O9 ceramics increased gradually as the sintering temperature was increased from 960 to 980 °C. Several anisotropic platelike grains developed at 1000 °C, and the aspect ratio of the anisotropic platelike grains was 4.75 at this temperature. At higher sintering temperatures (e.g., 1020 °C), the SrBi2(Ta1.7V0.3)O9 ceramics exhibited anisotropic platelike grains. The average grain size of the SrBi2(Ta1.7V0.3)O9 ceramics increased, and the aspect ratio of the disklike grains increased to 8.05. These differences in the microstructures of the SrBi2(Ta1.7V0.3)O9 ceramics were observed due to the addition of V2O5 because the low melting point of V2O5 acts as a liquid-phase-sintering aid and causes rapid growth of the (00l) planes.[17] Zhang et al. posited that SBT ceramics densify and shows the disklike grains after sintering at 1280 °C for 3 h.[21] By comparing the results presented in Figure d, it is evident that the addition of V2O5 in SBT ceramics can lower the temperatures required to densify SBT-based ceramics.

Figure 9

SEM images of the SrBi2(Ta1.7V0.3)O9 ceramics as a function of sintered tmeperature. (a) 940 °C, (b) 960 °C, (c) 980 °C, (d) 1000 °C, and (e) 1020 °C.

Figure S1 shows the XRD patterns of SBTV ceramics sintered at different temperatures. The XRD patterns of SrBi2(Ta1.7V0.3)O9 ceramics exhibited the characteristic peaks of SBT (JCPDS no. 49-0609) ceramics. As shown in Figure S1, only the single-phase layered perovskite structure was present and no secondary or unknown phases were detected, thus suggesting that V5+ ions completely substituted Ta5+ ions in the SrBi2(Ta1.7V0.3)O9 ceramics. In the (00l) planes, the 2θ values of 21.4, 28.6, 35.9, 43.4, and 51.1° corresponded to the (006), (008), (0010), (0012), and (0014) planes, respectively. The increase in the diffraction intensities of SBTV ceramics with (00l) planes was more obvious especially when the sintered temperature was at 1000 and 1020 °C. This result agrees with the SEM images presented in Figure . Huang et al. reported that with the increase in sintering temperature the diffraction intensities of the preferred orientation (i.e., (00l) planes) of the layered perovskite V2O5-doped SBT ceramics increased faster than those of the undoped SBT ceramics.[17]

Figure shows the measurement temperature dependence of the dielectric constant at 1 MHz in SrBi2(Ta1.7V0.3)O ceramics at different sintering temperatures. All of the samples exhibit their respective transition temperature. A shift in transition temperature to high measurement temperatures and a corresponding increase in the maximum dielectric constant values with the increase in sintered temperature are observed. Here, the transition temperature of the SrBi2(Ta1.7V0.3)O9 ceramics shifted to a high measurement temperature due to the substitution of a higher amount of Ta5+ ions with V5+ ions in the SrBi2(Ta1.7V0.3)O9 ceramics. Moreover, the maximum dielectric constant of the SrBi2(Ta1.7V0.3)O9 ceramics increased with an increase in the degree of crystallinity (as shown in Figure ). The dielectric constant of ferroelectric materials in most cases is based on the composition and grain size.

Figure 10

Measurement temperature–dielectric constant of the SrBi2(Ta1.7V0.3)O9 ceramics as a function of sintering temperatures.

The plot of log(1/ε′ – 1/εm) vs log(T – Tm) for SrBi2(Ta1.7V0.3)O9 ceramics at different sintering temperatures measured at 1 MHz is represented in Figure . A linear relationship is observed in the SrBi2(Ta1.7V0.3)O9 ceramics, and the slopes of the fitting curves are used to determine the γ value. After assigning the experimental data to the modified Curie–Weiss law, the obtained γ values of SrBi2(Ta1.7V0.3)O9 ceramics are 1.16, 1.25, 1.43, 1.54, and 1.62 at sintering temperatures of 940, 960, 980, 1000, and 1020 °C, respectively. The γ values of the SrBi2(Ta1.7V0.3)O9 ceramics at different sintering temperatures were compared. The comparison results revealed that the SrBi2(Ta1.7V0.3)O9 ceramics sintered at 1020 °C present relatively a higher number of relaxor-type ferroelectric characteristics than normal-type ferroelectric characteristics. The values are listed in Table S1.

Figure 11

log(1/ε – 1/εm)–log(T – Tm) plots of SrBi2(Ta1.7V0.3)O9 ceramics at different sintering temperatures (symbols, experimental data; solid line, simulation data).

The sintering temperature effects of the SrBi2(Ta1.7V0.3)O9 ceramics can be further explained as follows. In the SBTV ceramics, oxygen vacancies could be generated during the sintering process due to the presence of the reduced valence state of vanadium ions. Although the partial substitution of Ta+5 with V+5 is intended at the B-sites of the layered perovskite structure, V+4 may form and enter the crystal structure. Two tetravalent vanadium ions entering the crystal structure form one oxygen vacancy to maintain the electroneutrality, and the overall reaction is given as follows:where VÖ is the oxygen vacancy with two effective positive charges and OO is the oxygen ion at an oxygen site with zero effective charge. According to thermodynamics, sintering at low temperatures results in further oxidation of the low-valence-state ions. When V is further oxidized during sintering at temperatures from 940 to 980 °C in air, the SrBi2(Ta1.7V0.3)O9 ceramics takes oxygen from air to fill the oxygen vacancy, thus leading to an evident decrease in the oxygen vacancy-related dielectric relaxation. The contribution of oxygen vacancy-induced dielectric relaxation to the dielectric constant is considerably higher at high temperatures than that at low temperatures, particularly below the paraferroelectric transition temperature. Oxygen vacancy-induced dielectric relaxation is a diffusion-related process, which is an activated process and is exponentially dependent on temperature. At low temperatures, the energy required to overcome the diffusion energy barrier is insufficient and the oxygen vacancy-induced dielectric relaxation is negligible. When the ceramics are sintered at high temperatures (1020 °C), the energy is sufficient to overcome the diffusion energy barrier and the contribution of the oxygen vacancy enhances the dielectric relaxation phenomenon.

Conclusions

SBTV ceramics, which are a type of bismuth-layered perovskite ferroelectrics, were successfully synthesized using the solid-state reaction method. When the x value in the V2O5 concentration was in the range of 0.1–0.4 and ceramics was sintered at 1020 °C, only disc-type grains were observed in the surface SEM images. Compared with the SBT ceramics, the sintering temperature for the grain growth and densification of SBTV ceramics was decreased to a lower value. The XRD intensities of the (00l) planes increased with the V2O5 concentration from x = 0.1 to 0.3, and the preferential orientation phenomenon (c plane) was confirmed. The modified Curie–Weiss law revealed that SBTV ceramics sintered at 1020 °C exhibited normal-type ferroelectric characteristics for the V2O5 concentrations with x = 0.1, 0.2, and 0.4, and an incline to relaxor-type ferroelectric characteristics was obtained for the V2O5 concentration with x = 0.3. Compared with those of SBT ceramics, the higher dielectric constants and transition temperature of SBTV ceramics were attributable to the partial substitution of Ta5+ ions by a smaller amount of V5+ ions at the B-sites. Moreover, the substitution of V5+ at the Ta5+ site of SBT ceramics to form SBTV ceramics presented an evident influence on the BO6 mode, as confirmed by the Raman spectroscopy results.

Experimental Section

The investigated ceramic compositions were as follows: SrBi2(Ta2–V)O9 (SBTV) with x = 0.1, 0.2, 0.3, and 0.4. The SBTV ceramics were prepared using the solid-state reaction method based on the following chemical reactionFor substitution of Ta2O5 by V2O5, the equation was modified as followsReagent-grade SrCO3, Bi2O3, Ta2O5, and V2O5 powders (purity, >99.5%) were mixed for 2 h in polyethylene bottles containing zirconia balls and deionized water. The obtained slurry was then dried and ground. The powder obtained after drying and grinding was calcined at 850 °C for 3 h. Based on the X-ray diffraction (XRD) analysis results, the calcined powder revealed a single-layered perovskite structure. The SBTV powder was uniaxially pressed in a steel die with a diameter of 12 mm to form compact disks. These pellets were sintered at different temperatures for 4 h in air.

The phase identification of SBTV ceramics was conducted through XRD with 1.54 Å Cu Kα radiation, and the microstructure of SBTV ceramics was observed through scanning electronic micrography (SEM). Raman spectroscopy was used to analyze the SBTV ceramics by a 532 nm YAG laser, which was operated at a 100 mW power. A laser with a beam diameter of approximately 2 μm was focused on the surface of SBTV ceramics by a 1000× objective lens. Before the dielectric characteristic of the SBTV ceramics were analyzed, the Ag–Pd paste was applied on the surface of the ceramics and sintered at 600 °C as electrodes. An LCR meter (model HP4294A) measured the temperature-dependent dielectric constant (εr) and loss tangent (tan  δ) of the SBTV ceramics in a temperature-programmable testing chamber.