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Page 6 of 11 Qin et al. Microstructures 2023;3:2023035 https://dx.doi.org/10.20517/microstructures.2023.34

Figure 4A presents the temperature-dependent behavior of relative permittivity (ε ) and dielectric loss (tanδ)
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at 10 kHz. It reveals that as the Bi O content increases, the dielectric peak becomes narrower, and the
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maximum relative dielectric constant (ε ) gradually increases, indicating a reduction of diffuse behavior. It is
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evident from the spectra that the dielectric peaks of the ceramics exhibit asymmetry, which is related to the
presence of a core-shell structure within the ceramic grains. As plotted in Figure 4A, the tanδ of ceramics
exhibits an abrupt increase around the T (temperature exhibiting the maximum ε ), suggesting that there is
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m
a transition from the diffuse ferroelectric to the paraelectric phase [Figure 4B]. Figure 4C illustrates the
relaxation factor (γ) calculated at 10 kHz, which demonstrates that the value of γ decreases as the x content
increases, ranging from γ = 1.79 at x = -0.01 to γ = 1.27 at x = 0.04. Figure 4D displays the variation of ΔT
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between 1 kHz and 1 MHz for each component in the temperature spectrum. It is evident that ΔT tends to
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decrease with increasing x content, which reveals that the ferroelectricity of the ceramic becomes more
prominent, aligning with the decreasing γ depicted in Figure 4C.

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The complex impedance (Z ) plots of B F-BT ceramics at 400 °C are shown in Figure 5A, where Z' and Z''
1+x
represent the real part and imaginary part of Z , respectively . At 400 °C, the total impedance initially
[57]
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increases from 36.5 kΩ·cm at x = -0.01 to 44.4 kΩ·cm at x = 0.01 and then decreases with incorporating more
Bi O content, which indicates that the composition of x = 0.01 is the most electrically resistive. The Z" and
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M"/ε plots of x = 0.01 at 300 °C are plotted in Figure 5B, illustrating the electrical heterogeneity associated
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with various conductive components. Three peaks are found in the plots corresponding to the three
conductive components. Z" exhibits a single peak related to the grain boundary response (component 1),
while M"/ε shows a strong peak in the low-frequency region and a weak peak in the high-frequency region,
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which is ascribed to the electrical heterogeneity from the core-shell structure. In this study, the strong peak
represents the shell response (component 2), whereas the weaker peak is considered as the core response
(component 3). The resistance (R) and capacitance (C) of all conductive components at 325 °C were
calculated based on the peaks of Z" and M"/ε [Table 3]. The R values of components 1 and 2 reach the
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maximum in the composition of x = 0.01. However, the R value of component 3 did not change significantly
with the increase of Bi O content. Additionally, the resistance of components 1 and 2 is two orders of
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2
magnitude higher than that of component 3, which matches the frequency of the peaks of the three
components in Figure 5B. It is worth noting that component 3 exhibits a capacitance that is an order of
magnitude higher than components 1 and 2, indicating the formation of an electrically conducting core and
a nonconductive shell. Figure 5C shows the Arrhenius plots of the grain shell, core, and boundary, and the
calculated activation energy calculated by fitting is shown in Figure 5D. The activation energy of the shell
(1.06-1.14 eV) is generally lower than that of the core (1.15-1.28 eV) and grain boundary (1.09-1.16 eV).

Figure 6A illustrates the PE loops of the B F-BT ceramics at 60 kV/cm under a frequency of 1 Hz, and the
1+x
corresponding P and E are plotted in Figure 6B. The PE loops show typical ferroelectric features without
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C
observations of leakage characteristics at high field amplitudes. However, when x = -0.01, 0.00, the loops
demonstrate the phenomenon of leakage conduction, resulting in relatively high values of P and E , which
C
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is mainly attributed to the formation of Bi and O vacancies caused by the Bi O volatilization . When
[43]
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x ≥ 0.01, the leakage conductivity of BF-BT is significantly reduced, leading to stable values of P and E . The
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C
electric SE loops of ceramics measured at 60 kV/cm are shown in Figure 6C. It can be seen that the strain
value increases as x increases from -0.01 to 0.01, reaching a maximum value of 0.146% at x = 0.01, and then
the strain value decreases. The strains are calculated by averaging the positive strains obtained at ±60 kV/cm
and used for deriving the values, as plotted in Figure 6D. Notably, the highest = 243 pm/V and
d = 217 pC/N are achieved at x = 0.01, evidencing that the suitable amount of compensation of Bi O is
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effective in improving the piezoelectric properties of the BF-BT ceramics. In addition, the inset image of
Figure 6D depicts the temperature dependence of d for x = 0.01. It demonstrates that the d value has a
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