## 3 Results and Discussion
### Dielectric Properties  
In order to perform dielectric measurements, the samples were first subjected to a preparatory process. After the grinding of the samples was completed, they were subjected to a stress-relief annealing process to remove the structural stresses resulting from grinding. The process took place in a muffle furnace at the temperature of T = 723 K for t = 2 h. The samples were then cooled to room temperature. Before applying the electrodes, the disks were degreased in ethyl acetate, and then the electrodes were applied using hot platinum paste. Samples with electrodes applied were dried at a temperature of T = 473 K for t = 0.5 h in an SML 30-250 dryer and then fired in a muffle furnace at a temperature of T = 1123 K for t = 0.5 h.  
In order to investigate the dielectric properties of the Bi\({}_{7}\)Fe\({}_{3}\)Ti\({}_{3}\)O\({}_{21}\) base ceramics doped with gadolinium ions, the characteristics of the electrical permittivity versus the temperature were determined and measured in a measurement field with frequencies in the f = (1-1000) Hz range. Figure 6 shows the temperature characteristics of the real part of the electrical permittivity of Bi\({}_{7-x}\)Gd\({}_{x}\)Fe\({}_{3}\)Ti\({}_{3}\)O\({}_{21}\) ceramics for x = (0, 0.2, 0.4, 0.6), which were measured in a measurement field of f = 1 kHz, in the temperature range T = (300-800) K.  
Figure 6: Temperature characteristics of the real part of electrical permittivity measured in Bi\({}_{7-x}\)Gd\({}_{x}\)Fe\({}_{3}\)Ti\({}_{3}\)O\({}_{21}\) ceramic materials for x = (0, 0.2, 0.4, 0.6) in a measurement field with a frequency of f = 1 kHz. \begin{table}
\begin{tabular}{c c c c c} \hline  & & & \multicolumn{2}{c}{**BGFT-x**} & \\ \cline{3-5} \cline  
The gadolinium dopant induces indiscriminate changes in both the values of electrical permittivity, particularly its maximum value and the temperature at which its maximum occurs. Adding a dopant at a level of x = 0.2 results in a decrease in \(\varepsilon^{{}^{\prime}}_{\max}\) and a shift in the temperature of its occurrence T\({}_{\max}\) towards higher values. Conversely, for a dopant level of x = 0.4, \(\varepsilon^{{}^{\prime}}_{\max}\) increases again while its temperature shifts towards lower values. Furthermore, increasing the concentration of the modifier to x = 0.6 leads to a sharp increase in the value of the real part of permittivity to values exceeding 6000. The corresponding temperature T\({}_{\max}\) has practically the same value as in the case of the pure Bi\({}_{7}\)Fe\({}_{3}\)Ti\({}_{3}\)O\({}_{21}\) compound (Table 4). The addition of gadolinium ions significantly influenced the microstructure, causing grain growth and increasing their size non-uniformity. On the other hand, we also observe its effect on the electrical permittivity, raising the question: what truly accounts for the changes in dielectric properties? Is it the dopant or the change in grain size? This is a serious issue. On the one hand, in the literature, we find many examples of grain size affecting dielectric properties, especially extensively described for BaTiO\({}_{3}\)[25; 26]. On the other hand, introducing dopant ions into the lattice also affects the electrical permittivity, especially when the ionic radius of the dopant significantly differs from the ionic radii of the base compound ions. However, in the case described in the article, this is not the case. The ionic radii of bismuth and gadolinium ions are comparable (see Table 1). Therefore, it can be postulated with a high probability that the changes in dielectric properties are due to changes in the microstructure caused by the introduction of the dopant. This statement is further supported by the lack of a clearly defined trend in these changes, which is probably a consequence not only of changes in the so-called average grain size but also of changes in the volume fraction of large grains compared to small grains. Confirmation of this thesis is provided by the ceramics containing the highest concentration of gadolinium ions.