## 3 Results and Discussion  
### Evolution of Phase Composition and Microstructure  
The phase composition of the prepared Pr-substituted 5 YSZ ceramics is defined by the equilibrium phase relationships at the given firing temperature and by the kinetics of the solid-state reaction and cation diffusion. XRD analysis of the as-prepared samples (Figure 1) shows a prevailing fluorite (F) phase and the onset of a Pr\({}_{2}\)Zr\({}_{2}\)O\({}_{7}\)-based pyrochlore (P) phase. Furthermore, one also observes a clear splitting of the reflections assigned to the fluorite phase (Figure 2A-C). Figure 1: XRD patterns of as-prepared Pr-substituted 5 YSZ ceramic samples. The reflections of the fluorite phase are indexed in \(Fm\overline{3}m\) space group. The circles mark the main reflections of the pyrochlore phase (ICDD PDF no. 01-078-7481). The ternary diagrams of the ZrO\({}_{2}\)-Y\({}_{2}\)O\({}_{3}\)-Pr\({}_{2}\)O\({}_{3}\) or ZrO\({}_{2}\)-Y\({}_{2}\)O\({}_{3}\)-PrO\({}_{y}\) systems have not been reported so far. Still, it is possible to make plausible predictions on equilibrium phase compositions based on the available diagrams of the related ZrO\({}_{2}\)-Y\({}_{2}\)O\({}_{3}\)-Ln\({}_{2}\)O\({}_{3}\) systems. As the ionic radius of Nd\({}^{3+}\) is the closest to that of Pr\({}^{3+}\)[35], the ternary diagram of the ZrO\({}_{2}\)-Y\({}_{2}\)O\({}_{3}\)-Nd\({}_{2}\)O\({}_{3}\) system can be expected to resemble that of ZrO\({}_{2}\)-Y\({}_{2}\)O\({}_{3}\)-Pr\({}_{2}\)O\({}_{3}\) counterpart in the high-temperature range, when praseodymium is in a 3+ oxidation state in air (T \(\geq\) 1300 \({}^{\circ}\)C [17; 18]). Based on the most recent diagrams of the ZrO\({}_{2}\)-Y\({}_{2}\)O\({}_{3}\)-Nd\({}_{2}\)O\({}_{3}\) system by [PERSON] [36; 37], the samples with the nominal cation compositions selected in the present work (Table 1) are expected to be ternary fluorite-type solid solutions at 1600 \({}^{\circ}\)C under equilibrium conditions. The field of fluorite phase narrows with decreasing temperature, and the composition with a lower Pr content (\(x\) = 0.05) at 1400 \({}^{\circ}\)C belongs to the two-phase T + F field comprising tetragonal and cubic fluorite ZrO\({}_{2}\)-based phases [36; 37]. According to the earlier diagram of the ZrO\({}_{2}\)-Y\({}_{2}\)O\({}_{3}\)-Nd\({}_{2}\)O\({}_{3}\) system at 1600 \({}^{\circ}\)C proposed by [PERSON] and [PERSON] [38], the \(x\) = 0.05 and \(x\) = 0.15 compositions correspond to the T + F and F regions, respectively, while \(x\) = 0.10 is located at the boundary between the two fields. Furthermore, comparative analysis of available ternary diagrams of the ZrO\({}_{2}\)-Y\({}_{2}\)O\({}_{3}\)-Ln\({}_{2}\)O\({}_{3}\) systems, where Ln = La [39; 40; 41], Nd [36; 37; 38] and Sm [42], suggests also that the boundary between the T + F and F fields for a smaller Ln = Pr cation should be shifted towards a  
Figure 2: **(A–C)** Fragments of XRD patterns of Pr-substituted 5 YSZ ceramics illustrating the evolution of the phase composition with firing temperature; **(D)** example of integration of pyrochlore (222) peak and fluorite (111) peak to compute the corresponding peak ratio; **(E)** example of de-convolution of the (220) peak of the fluorite phase. Notation: F—main fluorite phase; F2—Pr-rich secondary fluorite phase; T—original tetragonal 5 YSZ; P—pyrochlore phase. slightly higher dopant content compared to the Nd-based system. At the same time, all the inspected diagrams suggest that the praseodymium contents in the sintered ceramic samples are too low for the co-existence of the pyrochlore-type Pr\({}_{2}\)Zr\({}_{2}\)O\({}_{7}\) phase under equilibrium conditions at 1400-1600 \({}^{\circ}\)C.  
Further insight into the phase evolution in the course of the direct firing of 5 YSZ samples with PrO\({}_{y}\) additions is provided by the microstructural studies. All the ceramic samples demonstrate a noticeable porosity (Figure 3).