## 3 Results and Discussion
This gave evidence of a phase transformation from the Hf-Ta-O superstructure to each separated oxide form, such as Hf-O and Ta-O. The color changes also support this phase separation, considering that the colors of HfO\({}_{2}\) and Ta\({}_{2}\)O\({}_{5}\) are bright (e.g., white-like), as revealed in the inset photographs in Figure 1.4  
In addition, this phase change would cause the different peeling behaviors of the samples shown in Figures 5 and 6. It is worth noting that the chemical form of Ta oxide is Ta\({}_{2}\)O\({}_{2,2}\) rather than Ta\({}_{2}\)O\({}_{5}\)  
Figure 1.5 SEM micrographs and EDS results of the (**a,c**) HT-1 and (**b,d**) HT-3 samples after heat treatment in a 4% H\({}_{2}\)-Ar atmosphere. Figure 1.6 XRD pattern of the residual powder collected from HT-3 after heat treatment in a 4% H\({}_{2}\)-Ar atmosphere. as presented in the XRD results. This reduction should occur by hydrogen during the thermal cycle. Provided that phase separation occurs from the Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) superstructure, the residual powders are expected to be in the form of HfO\({}_{2}\) and Ta\({}_{2}\)O\({}_{5}\). However, in the XRD analysis, the Ta oxide appears to be slightly reduced to Ta\({}_{2}\)O\({}_{2.2}\), while the Hf oxide form is maintained. Due to the lack of thermodynamic information on Ta oxides, it is difficult to precisely calculate the oxygen potential leading to the reduction process. Instead, the observed reduction form empirically shows that the Ta oxide can be reduced under the sintering conditions. More importantly, the reduction of Ta oxide would give rise to breakage of the crystal structure, resulting in the different peeling behavior. The crystal structure of Ta\({}_{2}\)O\({}_{5}\) is known to be hexagonal, while Ta\({}_{2}\)O\({}_{2.2}\) is known to be monoclinic [13]. This difference in crystal structure leading to different lattice constants would result in severe lattice distortion and may finally cause destruction of the lattice; i.e., phase-transformation-induced peeling behavior was expected to occur. Accordingly, the amount of reduced Ta oxide affects the degree of lattice distortion; in other words, a higher Ta content facilitates lattice destruction due to reduction. It is therefore expected that the degree of lattice distortion should be mild in the HT-1 sample compared to the distortion in the HT-2 and HT-3 samples. This is supported empirically by the peeling behaviors: note that the outermost layer was maintained at the surface of the HT-1 sample, whereas it peeled off in the HT-2 and HT-3 samples. Based on this, to minimize phase-transformation-induced destruction during thermal cycling in a reducing atmosphere, either the Ta content should be lowered or Ta reduction should be suppressed. As in the case of the sintering in pyroprocessing, if the oxygen potential during thermal cycling is expected to be sufficiently low, resulting in a reduction of Ta oxide into a lower oxidation state (e.g., Ta\({}_{2}\)O\({}_{2.2}\)), lowering the initial Ta content should be effective. In this case, the Ta content should be in the range allowing the Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) superstructure to be formed.