## 3 Results and Discussion  
The results of the phase analysis for doping 9 mol% CaO-stabilized zirconia (9-CSZ) with 2-4 mol% MnO\({}_{2}\) are shown in Figure 1 and Table 2. The monoclinic fraction (V\({}_{\rm m}\)) of 9-CSZ was 32.6%. In contrast, with the addition of 2, 3, and 4 mol% MnO\({}_{2}\), V\({}_{\rm m}\) decreased to 3.8%, 6.5%, and 2.5%, respectively, which indicates the progression of phase stabilization to the level of fully stabilized ZrO\({}_{2}\). The tetragonal fraction (V\({}_{\rm T}\)) for 9-CSZ was 58.2%, whereas MnO\({}_{2}\)-doped CSZ showed increased values, ranging from 88.2% to 90.3%. The cubic phase fraction (Vc) of 9-CSZ was 9.2%, and it decreased to 8.0% and 3.4% with the addition of 2 and 3 mol% MnO\({}_{2}\), respectively. Notably, with the addition of 4 mol% MnO\({}_{2}\), Vc increased to 7.2% compared with the 3.4% observed for the 3 mol% MnO\({}_{2}\)-doped CSZ. The doping limit of CaO in ZrO\({}_{2}\) is established to be 15 mol%; however, it is evident that the doping limit for MnO\({}_{2}\) is relatively lower. Ca\({}^{2+}\) ions (1.00 A) are larger than Zr\({}^{4+}\) ions (0.84 A), whereas Mn\({}^{2+}\) (0.83 A) and Mn\({}^{3+}\) (0.65 A) ions are smaller than Zr\({}^{4+}\) ions. These differences in ionic radii lead to lattice contraction and distortion, which play a significant role in phase stabilization. MnO\({}_{2}\) doping in ZrO\({}_{2}\) not only generates additional oxygen vacancies but also enhances phase stabilization through the lattice distortion caused by the contraction of the lattice due to the smaller dopant ions [18]. Therefore, both the increased concentration of oxygen vacancies and the size effect of the dopant ions synergistically contribute to the stabilization of the tetragonal phase. The Mn 2p\({}_{3/2}\) XPS spectra of 9-CSZ doped with 2-4 mol% MnO\({}_{2}\) are shown in Figure 2. The peaks at 640.8, 641.8, and 642.7 eV correspond to Mn\({}^{2+}\), Mn\({}^{3+}\), and Mn\({}^{4+}\), respectively. It was observed that Mn predominantly exists as Mn\({}^{2+}\) in all samples [19]. This indicates that manganese oxide, initially introduced as MnO\({}_{2}\) (Mn\({}^{4+}\)), is mainly present as Mn\({}^{2+}\) within the zirconia lattice, implying a change in valence state [20]. The predominant presence of Mn\({}^{2+}\) leads to the formation of more oxygen vacancies due to charge compensation, compared with Mn\({}^{3+}\) and Mn\({}^{4+}\). This results in a greater reduction in the monoclinic phase upon doping, which correlates with the phase fractions observed in the XRD analysis. The fractions of Mn\({}^{3+}\) and Mn\({}^{4+}\) gradually decreased as the MnO\({}_{2}\) doping level increased, reaching 0% in the sample with 4 mol% MnO\({}_{2}\). This result is known to be due to the high crystal field stabilization energy (CFSE) of the tetragonal phase, which makes Mn\({}^{4+}\) unstable and leads to its reduction to the lower valence states of Mn\({}^{2+}\) or Mn\({}^{3+}\) for stabilization [21]. Therefore, Mn primarily exists as Mn\({}^{2+}\) in the samples, which is associated with the decrease in the monoclinic phase and the increase in the tetragonal phase. Figure 3 shows the microstructural images of 9-CSZ and 9-CSZ doped with 2-4 mol% MnO\({}_{2}\). The 9-CSZ sample displayed a mixture of small grains approximately 2 \(\upmu\)m in size and relatively larger grains of around 10 \(\upmu\)m. In contrast, the MnO\({}_{2}\)-doped 9-CSZ specimens contained exclusively large grains exceeding 30 \(\upmu\)m, indicating significant grain growth. This pronounced grain growth suggests that MnO\({}_{2}\)-doped 9-CSZ has enhanced sinterability compared with undoped 9-CSZ. Figure 1: XRD diffraction patterns of 9-CSZ powder calcinated at 1200 \({}^{\circ}\)C: (**a**) 9-CSZ, (**b**) 2% MnO\({}_{2}\)-doped 9-CSZ, (**c**) 3% MnO\({}_{2}\)-doped 9-CSZ, (**d**) 4% MnO\({}_{2}\)-doped 9-CSZ.