Effect of Superstoichiometric Bismuth Addition on the Structure and Dielectric Characteristics of the Solid Solutions (1−x)BiFeO₃-xBaTiO₃

Abstract

Ceramic samples of solid solutions of the binary system (1−x)BiFeO₃-xBaTiO₃ + 2 wt.% Bi₂O₃ (0.29 ≤ x ≤ 0.33, Δx = 0.01) were prepared using the conventional solid-phase reaction method with and without mechanical activation. Using X-ray studies, it was found that the objects have a pseudocubic crystal structure, and limited solubility occurs in solid solutions of the studied composition, as evidenced by the presence of regions with an increased Bi or Ba content and similar cell parameters. A diffuse phase transition occurred from the FE to PE state in the temperature ranges of (650–850) K. Relaxor-like behavior and the smearing of the phase transition in the studied ceramics can be associated with the presence of non-interacting regions with an increased content of Bi or Ba, different modulation, and crystal lattice symmetry. The grain morphology and dielectric characteristics of the selected solid solutions were investigated. The highest piezoelectric coefficient, d₃₃ = 120 pC/N, was obtained in the mechanically activated ceramics 0.71BiFeO₃-0.29BaTiO₃ + 2 wt.% Bi₂O₃.

1. Introduction

Multiferroic materials, which have coexisting electric, magnetic, and elastic orderings, are currently among the most intensively studied objects in materials science [1] due to their wide range of potential applications, including the production of magnetic field sensors, memory elements, and spintronics devices [2,3,4]. Bismuth ferrite (BiFeO₃, BF), a representative of this class of materials, has Curie and Néel temperatures of 1123 K and 643 K, respectively, and is being considered for use in magnetoelectric structures. However, its use is limited due to several factors, such as the difficulty in obtaining it in a single phase, the presence of Fe²⁺/Fe³⁺ ions and oxygen vacancies that cause leakage currents, and the high electric coercive field required for domain switching [5,6]. However, modification by rare-earth elements or the creation of solid solutions based on BF allows for stabilizing the structure and improving the multiferroic properties of the obtained materials [7,8,9]. Among BiFeO₃-based solid solutions, the lead-free BiFeO₃-BaTiO₃ (BF-BT) system has been proposed as a potential replacement for lead zirconate titanate (PZT) ceramics, which have been widely used in piezoelectric devices due to their excellent dielectric and piezoelectric properties near the morphotropic phase boundary (MPB) between the rhombohedral (Rh) and tetragonal (T) phases [10,11]. The Curie temperature (T_C) of commercial PZT ceramics ranges from 450 K to 770 K, depending on the composition. However, with the development of modern industry, the need for devices with a wider operating range is growing. Solid solutions of BF-BT with low BT content exhibit T_C values exceeding 650 K, indicating their potential for high-temperature applications [12]. The properties of this solid solution can be further enhanced through mechanical activation during the production process and by incorporating additional elements (

Table 1. The values of the piezoelectric modulus d₃₃ and the Curie temperature for modified binary ceramics BF-BT from the works [13,14,15,16,17,18] and this work.

Modificator	Composition	d33, pC/N	TC, K
Zn2+	0.7Bi0.4Zn0.6FeO3-0.3BaTiO3 [13]	160	723
Mn4+	0.71BiFeO3-0.29BaTiO3 + 1.2 mol%MnO2 [14]	169	779
Bi3+	0.7Bi1.02FeO3-0.3BaTiO3 [15]	214	801
Ga3+	0.7Bi(Fe0.98Ga0.02)O3-0.29BaTiO3 [16]	157	740
Zr4+	0.75BiFeO3-0.25Ba(Zr0.1Ti0.9)O3 [17]	138	729
Sc3+	0.7Bi(Fe0.98Sc0.02)O3-0.3BaTiO3 [18]	165	778
Bi3+	0.71BiFeO3-0.29BaTiO3 [this work]	120	650–750

).

As is well-known, Bi is a volatile element at elevated temperatures due to its high vapor pressure, which can result in deviations from the optimal composition [19,20]. This can obviously impact the electrical properties of piezoelectric materials based on bismuth. It has been demonstrated that mechanical activation during the manufacturing process or the addition of excess bismuth to the system can effectively address this issue [21,22]. On the other hand, Bi₂O₃ is frequently used as a sintering aid for piezoelectric materials to lower their sintering temperature and modify their dielectric and piezoelectric properties. In this context, it is essential to comprehend how mechanical activation and the addition of excess bismuth affect the crystal structure, as this determines the macroscopic characteristics of the ceramic material being studied.

In this paper, (1−x)BiFeO₃-xBaTiO₃ (BF-xBT) ceramic samples with additional Bi content were prepared with a conventional solid-state reaction method using mechanical activation and without it, and the structure, dielectric, ferroelectric, and piezoelectric properties of the obtained ceramics were studied.

2. Materials and Methods

Ceramic samples of the solid solutions of the binary system (1−x)BiFeO₃-xBaTiO₃ (0.29 ≤ x ≤ 0.33, ∆x = 0.01) were fabricated using conventional ceramic technology with double solid-phase synthesis at temperatures T₁ = 1123 K and T₂ = 1143 K and holding times τ₁ = τ₂ = 10 h with following sintering at T_sin1 = 1343 K for samples prepared with mechanical activation (BF-xBTm) and T_sin2 = 1323 K for other samples (BF-xBT) within 2 h. The experimental sample density ρ_exp was determined with hydrostatic weighing; octane was used as a liquid medium. The density was calculated with the formula ρ_exp = (ρ_oct × m₁)/(m₂ − m₃ + m₄), where ρ_oct is the density of the octane, m₁ is the mass of the dry workpiece, m₂ is the mass of the workpiece saturated with octane, m₃ is the mass of the saturated workpiece suspended in octane with suspension, and m₄ is the suspension mass for the workpiece. Experimental and relative densities are shown in

Table 2. Experimental and relative densities of the studied ceramics.

Sample	ρexp, g/cm3	ρrel, %
BF-0.29BT	7.03	91.06
BF-0.3BT	7.00	90.67
BF-0.31BT	7.03	91.66
BF-0.32BT	7.05	92.40
BF-0.33BT	7.04	92.63
BF-0.29BTm	7.28	94.33
BF-0.3BTm	7.28	94.32
BF-0.31BTm	7.24	94.45
BF-0.32BTm	7.17	94.17
BF-0.33BTm	7.21	94.90

. The initial reagents were Bi₂O₃, Fe₂O₃, TiO₂, BaCO₃ with a base substance content of at least 99.95%. The mechanical activation of synthesized powders was carried out at the stage of manufacturing press powders prepared for sintering. For mechanical activation, a high-energy grinding ball mill AGO-2 manufactured by NOVITS (Novosibirsk, Russia) was used. The prepared powder was loaded into drums with an internal diameter of 63 mm together with ZrO₂ balls with a diameter of 8 mm with a total weight of 200 g. The drum with the mixture was placed in the AGO-2 planetary mill. Grinding was carried out in an alcoholic environment for 15 min; the rotation speed of the drum was 1800 rpm.

Ceramics were sintered in the form of cylinders, which were cut into disks Ø = 10 mm and h = 1 mm. Electrodes were applied to the flat surfaces of the disks by the stepwise burning of a silver-containing paste: 573 K for 20 min, 773 K for 30 min, and 1073 K for 20 min.

The X-ray study was carried out using the method of powder diffraction on the DRON-3 diffractometer (IC “Bourevestnik”, Saint Petersburg, Russia) (Bragg-Brentano focusing) using CoKα radiation.

The microstructure of the samples was studied using the scanning electron microscope JSM-6390L.

Temperature dependences of the relative complex permittivity ε*/ε₀ = ε′/ε₀ − iε″/ε₀ (ε′/ε₀ and ε″/ε₀ are the real and imaginary parts of ε*/ε₀, respectively; ε₀ – dielectric permittivity of vacuum) in the temperature range (300–873) K and the frequency range f = (50–2 × 10⁶) Hz were obtained using a measuring bench based on the Agilent 4980A LCR meter (Keysight Technologies, Inc., Santa Rosa, CA, USA).

Samples were polarized at T = 400 K in the polyethylene siloxane fluid under applied fields of 3–6 kV. Piezoelectric coefficients of samples were measured using a quasistatic YE2730A d33 METER, (APC International Ltd., West Kingston, RI, USA) at f = 110 Hz.

3. Results and Discussion

3.1. XRD Analysis

A powder X-ray diffraction study of the binary system (1−x)BiFeO₃-xBaTiO₃ (0.00 ≤ x ≤ 0.5, Δx = 0.1) [23] showed that the solid solutions demonstrate rhombohedral symmetry in the range of 0 ≤ x < 0.4, pseudocubic (Psc) in the range of 0.3 ≤ x ≤ 0.5, and at x = 0.3, both phases coexist in the same solid solution. In this study, we investigated in more detail the concentration region 0.29 ≤ x ≤ 0.33 containing MPB for compositions of the binary system (1−x)BiFeO₃-xBaTiO₃ (0.29 ≤ x ≤ 0.33, Δx = 0.01) prepared with and without mechanical activation and with the superstoichiometric addition of Bi₂O₃.

Before analyzing the results of X-ray studies of solid solutions, it is important to compare the crystallochemical characteristics of the ions involved. These include the X-ray atomic scattering factor (f), at θ = 0, for positive ions; ionic radii (R); cation electronegativity (EN); and their correspondence to the isomorphism criteria [24]. According to the isomorphism rules, the difference in ionic radii should not exceed 15% when compared to the smaller value. Similarly, the difference in electronegativities should be less than 0.4. This ensures that the ions can form stable compounds within the solid solution

From the data presented in

Table 3. Atomic scattering factors, f, ionic radii, R, electronegativity, ΔR/R_min, EN, ΔEN.

Cation	f [25]	R, Å [26]	ΔR/Rmin × 100, %	EN [27]	ΔEN
Ba2+	54	1.54 for CN 12	15	0.9	1.0
Bi3+	80	1.34 for CN 12		1.9
Ti4+	18	0.64 for CN 6	4	1.5	0.3
Fe3+	23	0.67 for CN6		1.8

, it is evident that both conditions are fulfilled for B-cations. For A-cations, the ionic radii also meet the above condition, but the ΔEN exceeds the permissible value by a factor of 2.5. This indicates that BF and BT may not form a continuous series of solid solutions. Instead, only limited solubility is possible, and the solid solution may disintegrate into several solid solutions with increased concentrations of Bi or Ba and similar cell parameters.

A similar effect has been observed in the Pb_1−xBaTiO₃ system, which has been described in detail in [28]. It would be interesting to investigate whether it is possible to improve the homogeneity of the solid solution by incorporating the mechanical activation of the synthesized product into the sintering process. In this study, we compare X-ray data with electrophysical properties of solid solutions that were prepared without mechanical activation, and those that were mechanically activated before sintering.

The heterogeneity of the solid solution should be manifested in the X-ray diffraction pattern as a slight splitting of the diffraction peaks and the emergence of diffuse maxima near them. These diffuse maxima indicate the presence of atomic segregation, as described by [29]. If an ordered arrangement of regions with a higher concentration of Bi and Ba occurs, the diffuse maxima transform into satellites of the main reflections. The intensity of these satellites is proportional to the square of the difference in atomic scattering factors (∆f)² of the atoms that form the planar defect. In our case, this planar defect represents the boundary between regions with different chemical compositions.

Figure 1 shows X-ray diffraction patterns for studied ceramics in the angle range 20 ≤ 2θ ≤ 85 (°). In Figure 1a weak peaks of the impurity phase Ba₅Fe₂O₈ can be seen. After mechanical activation, the pikes of the impurity phase disappeared. No distortion of diffraction reflections corresponding to any one symmetry was detected, so all ceramics have Psc symmetry.

Figure 2, Figure 3 and Figure 4 show the diffraction peaks of solid solutions with different compositions with x = 0.29, 0.31, and 0.33, which were prepared both with and without mechanical activation of the synthesized powder.

Figure 2a,b show the diffraction pattern for the sample prepared without mechanical activation. There is a diffuse scattering at the base of the 111 and 200 diffraction peaks, indicating the presence of atomic segregation. This means that there are regions in the crystal structure that have a different chemical composition from the main matrix. The diffuse maxima are blurred, suggesting some order in the arrangement of these segregated regions. In contrast, Figure 2c,d show the diffraction pattern of the mechanically activated sample. Here, the signs of atomic segregation have disappeared, but a splitting has appeared at the top of the diffraction peaks. This suggests that after the mechanical activation, there was a more uniform distribution of BT within the BF matrix, leading to the formation of two solid solutions with similar cell parameters.

Figure 3 shows fragments of the X-ray diffraction patterns for samples with x = 0.31. Satellite maxima, denoted by the letter “$k_{1-3}^{+}$”, are visible near the main diffraction peaks and on them (200 in Figure 3b and 111 in Figure 3c). The subscript indicates the position of the satellite relative to the main peak (“+” or “−” from the side of a large angle θ), and the superscript indicates its serial number.

The following modulation parameters were calculated: the wave number k, the modulation wavelength Λ, and the number of perovskite cells N in the modulation wavelength. The relationships between the wave numbers were also determined, and the character of the modulation was identified. The modulation wavelength, Λ, was calculated using the Formula (1):

$$\mathsf{\Lambda }={\left|{\displaystyle \frac{1}{d_{hkl}}}-{\displaystyle \frac{1}{d_s}}\right|}^{-1},$$

(1)

where d_hkl and d_s are the interplanar spacings of the main peak and satellite, respectively [29]. The calculation results are presented in

Table 4. Wave number, k, modulation wavelength Λ, number of cells in a wavelength, N, and the relationship between the wave numbers of the solid solutions with x = 0.31.

Diffraction Reflection and Composition	Wave Number	k	Λ, Å	N	Relation Between the Wave Numbers k
200 BF-0.31BT	${\mathrm{k}}_1^{+}$	0.00104	963	241	${\mathrm{k}}_3^{+}$$={\mathrm{k}}_1^{+}$$+{\mathrm{k}}_2^{+}$
	${\mathrm{k}}_2^{+}$	0.00314	318	79.6
	${\mathrm{k}}_3^{+}$	0.00424	235	59
111 BF-0.31BTm					${\mathrm{k}}_1^{+}$$=1/2{\mathrm{k}}_2^{+}$$=1/3{\mathrm{k}}_3^{+}$
	${\mathrm{k}}_1^{+}$	0.0018	556	80.5
	${\mathrm{k}}_2^{+}$	0.00359	278	40
	${\mathrm{k}}_3^{+}$	0.00539	185.6	27
	${\mathrm{k}}^{\mathrm{*}}$	0.00036	2781	403	${\mathrm{k}}^{\mathrm{*}}$$=1/5{\mathrm{k}}_1^{+}$$=1/10{\mathrm{k}}_2^{+}$$=1/15{\mathrm{k}}_3^{+}$

.

In BF-0.31BT the modulation wave propagated along < 100 > direction. The satellites of the diffraction peak at 200 (Table 4) were the first-, third-, and fourth-order satellites, as the wave numbers associated with them were related by the ratio $k_1^{+}={\displaystyle \frac{1}{3}}{k}_2^{+}={\displaystyle \frac{1}{4}}{k}_3^{+}$. However, the lack of a second-order satellite and the strong intensity of the fourth-order satellite suggest that the wave number, $k_3^{+}$, is the total wave number. These satellites occur when two modulation waves are superimposed in a polydomain crystal in one crystallographic direction, as described by [30]. In our experiment, crushed ceramics can be viewed as polydomain crystals, with regions of increased Bi and Ba concentration acting as domains.

For the BF-0.31BTm (Figure 3c,d), only traces of modulation were observed on the (200) diffraction peak, but modulation was also present along the < 111 > direction. The (111) diffraction peak satellites were related by a ratio $k_1^{+}={\displaystyle \frac{1}{2}}{k}_2^{+}={\displaystyle \frac{1}{3}}{k}_3^{+}$, which corresponds to the harmonic modulation characteristic of the concentration wave. Mechanical activation not only changed the modulation direction but also reduced the modulation wavelength. Since the atomic displacements that determine the symmetry of the crystal are modulated, we can conclude that at x = 0.31, there are regions with very small tetragonal distortion (modulation in the < 001 > direction) and regions with very small rhombohedral distortion, (modulation in the < 111 > direction). In BF-0.31BT, a phase Rh→T transition has already occurred in the bulk of the rhombohedral distorted regions, and mechanoactivation shifts BF-0.31BTm towards the Re phase, thus shifting this transition to a region with a higher BaTiO₃ content. Figure 3 shows that all the diffraction peaks have split tops, and the widths of the split peaks are approximately equal, except for the (111) peak in Figure 3c, which has a narrower and more intense peak marked by an arrow. Assuming that this wide peak is a satellite of the peak indicated by the arrow, a modulation wavelength can be calculated. An interesting result was found, as shown at the bottom of Table 4. This ratio of wave numbers is not accidental. It shows that mechanical activation of BF-0.31BTm allows us to obtain a long-range ordered structure consisting of regions with increased concentrations of Bi and Ba. Each of these regions contains approximately 400 perovskite cells. In fact, the solid solution decomposes with the spinodal mechanism, resulting in the formation of two solid solutions with similar cell parameters.

The diffraction peaks (111) and (200) of ceramics with x = 0.33 are shown in Figure 4. The X-ray diffraction pattern of BF-0.33BT clearly shows the asymmetry of the diffraction peaks. The tails on the side with larger θ angles indicate the presence of clusters in the structure with a cell parameter smaller than that of the matrix. BF-0.33BTm is more homogeneous. However, the diffuse maxima at the bases of the peaks suggest segregation.

Figure 5 shows the dependence of the cell parameter a(x) for solid solutions prepared using two methods and the linear dependence according to Vegard’s law. For the initial compounds, the parameters were obtained from the JCPDS database (BF-Set 14, card 181; BT-Set 3, card 725). For x = 0.1, 0.2, 0.4, and 0.5, the cell parameters were used from a previous study of the BF-BT system [23]. Since the cell parameter for x = 0.3, which was prepared without adding 2% Bi₂O₃, differs by 0.0001 Å from the values in this study, this difference is acceptable.

Inset on Figure 5 shows that a break in the a(x) dependence occurs in the range 0.30 < x < 0.32. The width of this break for samples manufactured without mechanical activation is Δx = 0.03 (0.29–0.32) with the jump Δa = 0.003 Å. For mechanically activated ceramics, Δx = 0.04 (0.29–0.33) and Δa = 0.005 Å. In both cases, the parameter a has a minimum value at x = 0.31.

In Figure 5, it is also clear that the curve a(x) deviates positively from Vegard’s rule. This type of dependence of a(x) is observed in non-metallic solid solutions with immiscibility regions within the homogeneous region of a given system [31]. Thus, the appearance of the X-ray diffraction patterns and the dependence of the cell parameter on x suggest that a continuous series of solid solutions is not formed in the BF-BT system with Bi₂O₃ addition.

3.2. Microstructural Characterization

Figure 6 shows micrographs of chips of the studied ceramics. The microstructure of the BF-(1−x)BT samples (Figure 6a,c,e,g,i) is primarily composed of two types of grains: small grains with a diameter of less than 1.5 µm, which form compact aggregates, and larger grains with a size of more than 3 µm in the shape of irregular polyhedra. Despite a slight reduction in the average grain size, an increase in the BT concentration has no significant impact on the microstructure of ceramics produced without mechanical activation. The grain structure of BF-0.29BTm ceramics (Figure 6b) consists of crystallites with an average size of ~5 µm. The chip passes along the grain boundaries. An increase in the BT concentration leads to the appearance of segregations of small grains (Figure 6d,f) as in BF-(1−x)BT ceramics and a decrease in the average grain size to ~1.5 µm in BF-0.32BTm and BF-0.33BTm (Figure 6h,j). The presence of a homogeneous mass of very small grains ~1 µm arranged in an ordered manner on separate fragments of the chip is characteristic of solid solutions after spinodal decomposition. This indirectly confirms the conclusions of the X-ray analysis.

3.3. Dielectric and Piezoelectric Characteristics

Figure 7 shows the temperature dependences of the real part of the complex permittivity and the dielectric loss tangent of the studied ceramics. In all samples at T < 600 K, an increase in the permittivity is observed with increasing temperature, ending in a diffuse frequency-dependent maximum corresponding to the phase transition from the paraelectric phase to the ferroelectric one. With an increase in the frequency of the measuring field, the maximum shifts to the high-temperature region. For all samples except BF-0.29BTm, the presence of BT also affects the position of the maximum, shifting it to the low-temperature region with an increase in the BT concentration from ~740 K (BF-0.29BT) and ~820 K (BF-0.30BTm) to 680 K (BF-0.33BT) and 685 K (BF-0.33BTm) (Figure 7a,c). Increasing the BT concentration shifts the T_C in the BF-BT solid solutions to the low-temperature region since the T_C of BT is much lower than the T_C of BF, which can be seen in Figure 7a. In Figure 7c, the ε′/ε₀ curves are divided into the two groups. Curves of the samples with x = 0.30, 0.31, 0.32 correspond to the solid solutions from the morphotropic phase boundary. Other curves correspond to the solid solution located outside the morphotropic phase transition region. In each of the groups, T_C decreases with increasing BT concentration. Almost all samples demonstrate extremely high conductivity, which is expressed in a sharp increase in the dielectric loss tangent after 400 K (Figure 7b,d). BF-0.29BTm was the only sample that demonstrated acceptable tangent values. The reason for the observed effects may be the already mentioned oxidation of Fe-ions and volatilization of Bi₂O₃, but we consider that such a large increase in conductivity in the samples of one research section is due precisely to Maxwell–Wagner relaxation arising from the accumulation of free charges at the interfaces of Bi- or Ba-rich regions. In the BF-0.29BTm sample, a more uniform dissolution of BaTiO₃ into BiFeO₃ occurred after mechanical activation, thus avoiding these problems.

Figure 8 shows the temperature dependences of the real part of the complex permittivity and the dielectric loss tangent in a wide range of temperatures and research frequencies. As can be seen from the figure, in the range of (650–750) K, a diffuse phase transition occurs in the sample, the temperature of which, T_m, depends on the frequency. Similar behavior is observed in the tgδ(T), but the growth of the electrical conductivity of the material at T~500 K leads to a sharp increase of the tgδ values. As a result, in the vicinity of the FE→PE phase transition, we observe insignificant anomalies at high f values. When approximating the T_m(f) dependence in the whole frequency range, we used the Vogel–Fulcher relation (2):

$$f=f_{0}exp{\displaystyle \frac{E_a}{k(T_m-T_f)}}$$

(2)

where f₀ is the frequency of attempts to overcome a potential barrier E_a, k is the Boltzmann constant, and T_f is the Vogel–Fulcher temperature, interpreted as the temperature of “static freezing” of electric dipoles or transition to the state of a dipole glass. The best result was achieved with the parameters f₀~4·10¹² Hz, E_a ≈ 0.29 eV, and the Vogel–Fulcher temperature T_f ≈ 595 K. The obtained results indicate that the solid solution BF-0.29BTm demonstrates relaxor behavior that may be associated with the presence of regions rich in Bi⁺² or Ba⁺² in the ceramic crystal structure.

Despite the fact that solid solutions from MPB in many cases exhibit extreme electromechanical properties, we did not find stable piezoresponse after poling most of the samples studied. However, it was possible to observe the piezoelectric characteristics that were stable in time in the BF-0.29BTm ceramics. The highest piezoelectric constant of 120 pC/N is much larger than d₃₃ = 65 pC/N achieved in BF-BT ceramics fabricated by conventional solid-phase reaction methods [23]. However, this value is lower than presented in Table 1, indicating that complex modification and good solubility of the solid solution components are required to improve the piezoelectric properties of the investigated system.

4. Conclusions

Ceramic samples of solid solutions of the binary system (1−x)BiFeO₃-xBaTiO₃ + 2 wt.% Bi₂O₃ (0.29 ≤ x ≤ 0.33, Δx = 0.01) were prepared using the conventional solid-phase reaction method with and without mechanical activation. Using X-ray studies, it was found that in the investigated concentration range the structure of solid solutions is modulated, in the interval 0.30 ≤ x ≤ 0.32 there is a morphotropic phase transition at the local level. Clusters with very small tetragonal cell distortion predominate in BF-0.31BT, and clusters with rhombohedral distortion predominate in BF-0.31BTm. It is concluded that the solid solutions are heterogeneous and contain regions rich in Bi or Ba. The reason for this is the violation of the rules of isomorphism during the substitution of Bi↔Ba. Mechanical activation led to a limited improvement in the homogeneity of the obtained ceramics only for the BF-0.29BTm composition. Data from the study of the microstructure and dielectric properties of the ceramics confirmed the incomplete solubility and heterogeneity of the studied compositions. The average grain size in the BF-0.29BTm sample was ~5 µm. In the remaining compositions, the average grain size varied from 1 µm to 3 µm, which is characteristic of solid solutions in the region of morphotropic phase transition or solid solution decomposition. At the same time, these samples had high electrical conductivity, which prevented the production of a stable piezoactive state. A diffuse phase transition occurred from the FE to PE state in the temperature ranges of (650–750 K) for BF-(0.29–0.33)BT samples and (650–850 K) for BF-(0.29–0.33)BTm samples. For the BF-0.29BTm sample, the relaxation process was approximated using the Vogel–Fulcher relation, with parameters E_act = 0.29 eV and T_f = 595K. We assume that relaxor-like behavior and the smearing of the phase transition in the studied ceramics can be associated with the presence of non-interacting regions with increased content Bi or Ba, different modulation, crystal lattice symmetry, and chemical composition identified by X-ray analysis. Thus, it is shown that the mismatch of the solid solution composition with the rules of isomorphism does not allow obtaining ceramics with high piezocharacteristics, and even additional mechanical treatment can only improve the technological characteristics of ceramics to a limited extent. The maximum values of the piezoelectric modulus were observed in the BF-0.29BTm ceramics (~120 pC/N). The results show that mechanically activated BF-BT ceramics can be a promising material for future applications, but a comprehensive modification considering the isomorphism rule is required to seriously improve its performance.