## 4 Discussion
As it is known, from phase diagrams, there is a wide range of concentrations where c + t or t phases can be formed, but different studies gave conflicting results. In our study, we provide the refined composition between the m, c, and t phases for the CeO\({}_{2}\)-ZrO\({}_{2}\) system, which has been obtained by using charge from micron-sized powders. According to our experiments, the clear boundaries of the m, t, and c phases at room temperature can be distinguished, and the results of [11; 12] can be elaborated. The analysis of the cross-sectional SEM images of all of the samples revealed that variations in CeO\({}_{2}\) content in the CeO\({}_{2}\)-ZrO\({}_{2}\) system have a great effect on the microstructure and morphology of the sintered ceramics. When the ceramics completely consisted of thecubic phase (pure ceria or doped ceria), it was found that the morphology of the fractured surfaces included large grains with pronounced grain boundaries and intergranular pores. In tetragonal FSZ ceramics, a small-grain, pore-free structure was observed. For mixed-phase samples, the morphology was represented by polygonal grains with a wide size range together with feather-like grains. The SEM micrographs at low x values (domination of the monoclinic zirconia phase) showed that the microstructure of these ceramics contains intergranular cracks and pores. The behavior of the mean grain size vs. concentration curve can be associated with a phase composition change when a high amount (more than 50%) of the tetragonal phase is formed in mixed-phase samples. For pure CeO\({}_{2}\) and doped CeO\({}_{2}\) ceramics, exaggerated grain growth was possible because of the high concentration of oxygen vacancies. These vacancies can be formed because of the multivalent nature of Ce ions (Ce\({}^{3+}\), Ce\({}^{4+}\)) [25; 27]. To maintain electronic balance in the crystal lattice, some anions should leave the structure during heating. In this case, mass transport during sintering is high because of point defect migration in the crystal lattice. When the zirconia content in the initial charge increases, the process of exaggerated grain growth is located only in cubic phases or totally disappears, resulting in a smaller grain size. It was observed that the tetragonal zirconia phase did not show a tendency to form large grains. A possible reason for this is that CeO\({}_{2}\) acts like a grain growth inhibitor for zirconia. Some papers clearly state that stabilization additives hinder grain growth by accumulating in the grain boundary area [28; 29]. Nevertheless, this effect is more pronounced in the case of high-content zirconia ceramics because a sharp drop in the average grain size value for doped CeO\({}_{2}\) with ZrO\({}_{2}\) was not observed. It is known that for small-grain ceramics, the possibility of pore formation is lower because, during sintering, pores can migrate through grain boundaries to the surface [5]. Also, in large grain-size ceramics, the granular pore concentration could be higher than in small grain-size ceramics. So, the main reason for the low porosity of FSZ ceramics (x = 0.20-0.25) is explained by the formation of a small-grain tetragonal phase. In addition, as previously mentioned, the decrease in volumetric shrinkage may be caused by the growth of large grains. High ceria content ceramics demonstrate a very pronounced frequency dispersion of permittivity. This feature is described by the Maxwell-Wagner polarization phenomenon, where polarization occurs at the conductive grain/high resistance grain boundary interface [30]. The spectra become similar to that of Debye relaxation, but with hidden maxima at very low frequencies. The tangent loss frequency dependencies also showed pronounced dispersion throughout the whole measured range. This feature can be explained by the relatively high conductivity of ceramics [3]. If a metal oxide consists of multivalence cations, its conductivity becomes higher due to the hopping electron mechanism [31].