**4. Conclusions**

Amorphizable ceramics such as LiNbO3, ZrSiO4, and Gd3Ga5O<sup>12</sup> were irradiated with 200 MeV Au ions at an oblique incidence angle. Line-like homogeneous ion tracks and bell-shaped hillocks are observed by TEM. The ion track and hillock diameters are similar for all the amorphizable ceramics, although the hillock diameter is found to be slightly larger than the ion track diameter. The TEM images of ion tracks in STO and Nb-STO irradiated with 200 MeV Au show similar features to those observed in the non-amorphizable ceramics. For example, the ion track diameter is markedly smaller than the hillock diameter, and they exhibit inhomogeneity. However, the hillocks in these ceramics are found to be amorphous which is in contrast to the crystalline feature of hillocks observed in the non-amorphizable ceramics. Therefore, it can be concluded that STO and Nb-STO are intermediate ceramics between amorphizable and non-amorphizable ceramics. No marked difference is observed between hillock formation in STO and that in Nb-STO. The material dependence of nanostructure formation can be ascribed to the intricate recrystallization process. The present results support that (1) simplicity of lattice structure and (2) the strength of ionic bonding can be the factors that determine the recrystallization effectiveness.

**Author Contributions:** Conceptualization, N.I.; TEM observation, N.I. and T.T.; Sample preparation, N.I.; Ion irradiation, N.I. and H.O.; Manuscript writing, N.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** Part of the present work was financially supported by JSPS KAKENHI Grant Number 20K05389.

**Acknowledgments:** The authors are grateful to the technical staff of the tandem accelerator at JAEA-Tokai for supplying high-quality ion beams. One of the authors (N.I.) thanks A. Iwase and A. Kitamura for their constant support during the research.

**Conflicts of Interest:** The authors declare no conflict of interest.
