## 3 Results and Discussion
However, the proportion of the tetragonal phase in 2-4 mol% MnO\({}_{2}\)-doped CSZ was lower, ranging from 17% to 53.6%, compared with 65.9% in 9-CSZ. Additionally, the fraction of the cubic phase increased dramatically in 2-4 mol% MnO\({}_{2}\)-doped CSZ, from 38.7% to 73.7%, compared with only 7% in 9-CSZ. These results suggest that the increase in Mn\({}^{2+}\) ions resulting from MnO\({}_{2}\) doping in 9-CSZ leads to an increase in oxygen vacancies due to the difference in valence states. The increased concentration of oxygen vacancies promotes the stabilization of the cubic phase, which may result in a rapid increase in the cubic phase fraction during thermal shock. The morphological characteristics of all ZrO\({}_{2}\) samples with varying amounts of MnO\({}_{2}\) were examined using high-resolution TEM, and the resulting images are presented in Figure 5, and the related data are summarized in Table 4. The (111) plane of tetragonal ZrO\({}_{2}\) exhibits an interplanar spacing in the range of 3.00-3.10 A. In the case of the cubic phase, the interplanar spacing of the (111) plane is specifically 2.97 A. For the tetragonal phase, the O-Zr-O bond angle is about 71.05-71.32\({}^{\circ}\), while in the cubic phase, the Zr-Zr atomic angle on the (111) plane is 60\({}^{\circ}\), and the Zr-O-Zr bond angle is approximately 109.5\({}^{\circ}\)[22; 23]. Figure 3: FE-SEM images of (a) 9-CSZ, (b) 2 MnO\({}_{2}\)-doped 9-CSZ, (c) 3 MnO\({}_{2}\)-doped 9-CSZ, (d) 4 MnO\({}_{2}\)-doped 9-CSZ. The interplanar spacings of 2.91 A and 3.81 A observed in Figure 5a are similar to those of the (\(\overline{1}11\)) and (011) planes of the monoclinic phase, respectively (ICSD 98-001-5983). The measured angle between the atomic planes is 87.58\({}^{\circ}\), which differs from the theoretical value of 91\({}^{\circ}\). However, this value is considered as consistent when possible interface angle distortions during TEM analysis are taken into account. Furthermore, such a combination of interplanar spacing and angle cannot be observed in the cubic or tetragonal phases; thus, the structure is identified as monoclinic. Figure 5b,c show interplanar spacings in the range of 2.99-3.16 A, with atomic arrangement angles of 69.96\({}^{\circ}\) and 71.32\({}^{\circ}\), respectively. These values are very similar to the interplanar spacing and O-Zr-O bond angles of the tetragonal phase; therefore, these regions are presumed to correspond to the (111) plane of the tetragonal phase. Figure 5d exhibits an interplanar spacing of 2.96-2.98 A and an atomic arrangement angle of 59.93\({}^{\circ}\). These values are nearly identical to the interplanar spacing (2.97 A) and atomic angle (60\({}^{\circ}\)) of the (111) plane of the cubic phase, indicating that this region is likely to correspond to the (111) plane of the cubic phase. The TEM image analysis provides indirect evidence that MnO\({}_{2}\)-doped CSZ retains the tetragonal and cubic phases even after thermal shock. Figure 4: XRD diffraction patterns of 9-CSZ powder after thermal cycle (\(\triangle\)400 \({}^{\circ}\)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.