**2. Experiments**

Thin samples for TEM observations were prepared before irradiation by the following procedure. The original materials were ZrSiO<sup>4</sup> (98%) powder (Kojundo Chemical Laboratory Co., Ltd., Saitama, Japan), LiNbO<sup>3</sup> single crystals (Onizawa Fine Product Co., Ltd., Ibaraki, Japan), Gd3Ga5O<sup>12</sup> (GGG) single crystals (Rare Metallic Co., Ltd., Tokyo, Japan), SrTiO<sup>3</sup> (STO) single crystals (Shinkosha Co., Ltd., Kanagawa, Japan), and 0.5 wt% niobium-doped STO (Nb-STO) single crystals (Shinkosha Co., Ltd., Kanagawa, Japan). The original materials were finely ground using an agate mortar and pestle. For the preparation of LiNbO<sup>3</sup> specimens, first, 3 mm diameter nickel grids with 2000 lines/inch were coated using 0.5% Neoprene W in toluene (Nisshin EM Co., Ltd., Tokyo, Japan), which works as an adhesive agent, and then the ground LiNbO<sup>3</sup> powder was randomly dispersed on the grids. For the preparation of materials other than LiNbO3, the ground samples were dispersed in ethanol by an ultrasonic bath for a few minutes, and then the ethanol was dropped on 3 mm diameter 200 mesh copper grids covered with porous carbon films. The grids were then air-dried at room temperature. The samples on the grids were subsequently irradiated with 200 MeV Au32<sup>+</sup> ions at an oblique incidence angle (45◦ relative to normal direction) at room temperature in a tandem accelerator at JAEA-Tokai (Japan Atomic Energy Agency, Tokai Research and Development Center, Tokai, Japan). The charge state (32+) was chosen to ensure the charge of the incident ions to have the average value of the equilibrium charge. The samples

were irradiated with ions at 1 <sup>×</sup> <sup>10</sup><sup>11</sup> ions/cm<sup>2</sup> . The as-irradiated samples were examined using a transmission electron microscope (TEM, Model 2100F, JEOL Ltd., Tokyo, Japan) operated at 200 kV. The ion track size was measured using the TEM images taken at a low magnification, wherein clear line-like contrasts were expected to be imaged. On the other hand, the hillock size was measured using the TEM images taken at a high magnification, wherein a clear contour of hillocks was expected to be obtained. The electronic stopping power (Se) was estimated using SRIM-2008 [40,41]. JEOL Ltd., Tokyo, Japan) operated at 200 kV. The ion track size was measured using the TEM images taken at a low magnification, wherein clear line-like contrasts were expected to be imaged. On the other hand, the hillock size was measured using the TEM images taken at a high magnification, wherein a clear contour of hillocks was expected to be obtained. The electronic stopping power (Se) was estimated using SRIM-2008 [40,41].

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#### **3. Results and Discussions 3. Results and Discussions**

#### *3.1. Dimensions of Hillocks and Ion Tracks in LiNbO3, ZrSiO4, and GGG 3.1. Dimensions of Hillocks and Ion Tracks in LiNbO3, ZrSiO4, and GGG*

We have observed nanostructures created by SHI irradiation in amorphizable ceramics using TEM. Figure 1a–c show the bright field images of ion tracks in LiNbO3, ZrSiO4, and Gd3Ga5O<sup>12</sup> irradiated with 200 MeV Au at an oblique incidence angle. In the figures, the ion tracks are imaged as line-like contrasts. The diameter of the ion tracks can be estimated by measuring the width of the line-like contrasts. However, for most of the ion track images, it is difficult to distinguish the ion track contrasts from hillock contrasts. However, if the hillocks are created at the edge of the samples, it is possible to infer the hillock side-view. It can be concluded from the images that hillocks have similar diameters to those of ion tracks. Size distribution of ion tracks is shown in Figure 2. The average sizes of the hillocks and ion tracks are summarized in Table 1. We have observed nanostructures created by SHI irradiation in amorphizable ceramics using TEM. Figure 1a–c show the bright field images of ion tracks in LiNbO3, ZrSiO4, and Gd3Ga5O12 irradiated with 200 MeV Au at an oblique incidence angle. In the figures, the ion tracks are imaged as line-like contrasts. The diameter of the ion tracks can be estimated by measuring the width of the line-like contrasts. However, for most of the ion track images, it is difficult to distinguish the ion track contrasts from hillock contrasts. However, if the hillocks are created at the edge of the samples, it is possible to infer the hillock side-view. It can be concluded from the images that hillocks have similar diameters to those of ion tracks. Size distribution of ion tracks is shown in Figure 2. The average sizes of the hillocks and ion tracks are summarized in Table 1.

**Figure 1.** Bright field images of ion tracks induced in (**a**) LiNbO3, (**b**) ZrSiO4, and (**c**) Gd3Ga5O12 irradiated with 200 MeV Au32+ at an oblique incidence angle. The images were taken at relatively low magnification. **Figure 1.** Bright field images of ion tracks induced in (**a**) LiNbO<sup>3</sup> , (**b**) ZrSiO<sup>4</sup> , and (**c**) Gd3Ga5O<sup>12</sup> irradiated with 200 MeV Au32<sup>+</sup> at an oblique incidence angle. The images were taken at relatively low magnification.

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**Figure 2.** Size distribution of the track diameter, the hillock diameter, and the hillock height in (**a**) LiNbO3, (**b**) ZrSiO4, and (**c**) Gd3Ga5O12 irradiated with 200 MeV Au32+ at an oblique incidence angle. **Figure 2.** Size distribution of the track diameter, the hillock diameter, and the hillock height in (**a**) LiNbO<sup>3</sup> , (**b**) ZrSiO<sup>4</sup> , and (**c**) Gd3Ga5O<sup>12</sup> irradiated with 200 MeV Au32<sup>+</sup> at an oblique incidence angle.

**Table 1.** Average diameter of ion tracks (Dtrack), average diameter of hillocks (Dhillock), average height of hillocks (Hhillock) are listed with standard deviations for LiNbO3, ZrSiO4, and GGG irradiated with 200 MeV Au32+. The corresponding Se values are also listed. **Table 1.** Average diameter of ion tracks (Dtrack), average diameter of hillocks (Dhillock), average height of hillocks (Hhillock) are listed with standard deviations for LiNbO<sup>3</sup> , ZrSiO<sup>4</sup> , and GGG irradiated with 200 MeV Au32+. The corresponding S<sup>e</sup> values are also listed.


Previous literatures have reported ion track sizes in LiNbO3 [42–45], ZrSiO4 [46,47], and GGG [45,48]. They have confirmed that the ion tracks in these ceramics are amorphous. A good summary of the ion track data in LiNbO3 is given in Ref. [45], in which the dependence of ion track size on the electronic stopping power (Se) is shown. The Se-dependence demonstrates that an ion track size depends primarily on the electronic stopping power, whereas low ion velocity acts as a secondary factor that contributes to a bigger ion track owing to the velocity effect [49]. The literature also demonstrates that ion track sizes are in accordance with the values predicted by the thermal spike model. The present study shows that the ion track diameter in LiNbO3 irradiated with 200 MeV Au (Se = 28.1 keV/nm) is 11.7 ± 1.3 nm, whereas the previous study showed that the ion track diameter in Previous literatures have reported ion track sizes in LiNbO<sup>3</sup> [42–45], ZrSiO<sup>4</sup> [46,47], and GGG [45,48]. They have confirmed that the ion tracks in these ceramics are amorphous. A good summary of the iontrack data in LiNbO<sup>3</sup> is given in Ref. [45], in which the dependence of ion track size on the electronic stopping power (Se) is shown. The Se-dependence demonstrates that an ion track size depends primarilyon the electronic stopping power, whereas low ion velocity acts as a secondary factor that contributes to a bigger ion track owing to the velocity effect [49]. The literature also demonstrates that ion tracksizes are in accordance with the values predicted by the thermal spike model. The present study shows that the ion track diameter in LiNbO<sup>3</sup> irradiated with 200 MeV Au (S<sup>e</sup> = 28.1 keV/nm) is 11.7 ± 1.3 nm, whereas the previous study showed that the ion track diameter in LiNbO<sup>3</sup> irradiated with 201 MeV U

LiNbO3 irradiated with 201 MeV U ions (Se = 28.1 keV/nm) was around 13 nm [44,45], indicating that

the present study is in accordance with the previous results within the experimental error.

ions (S<sup>e</sup> = 28.1 keV/nm) was around 13 nm [44,45], indicating that the present study is in accordance with the previous results within the experimental error.

Previous studies reported that the ion track diameter for ZrSiO<sup>4</sup> irradiated with 10 GeV Pb ions (S<sup>e</sup> = 20.0 keV/nm) was 5.2 ± 0.2 nm [46] and that for ZrSiO<sup>4</sup> irradiated with 2.9 GeV Pb ions (S<sup>e</sup> = 33.6 keV/nm) [47] was 8 nm. The present study shows that the ion track diameter for ZrSiO<sup>4</sup> irradiated with 200 MeV Au ions is 9.5 ± 0.5 nm, exhibiting large ion tracks. Since the large ion tracks observed in the present study could be due to the relatively high electronic stopping power (S<sup>e</sup> = 29.6 keV/nm) and the velocity effect [49], there is no contradiction between the present and previous results. *Quantum Beam Sci.* **2020**, *4*, x FOR PEER REVIEW 5 of 15 Previous studies reported that the ion track diameter for ZrSiO4 irradiated with 10 GeV Pb ions (Se = 20.0 keV/nm) was 5.2 ± 0.2 nm [46] and that for ZrSiO4 irradiated with 2.9 GeV Pb ions (Se = 33.6

A good summary of the previous ion track data in GGG is given in Ref. [45]. The present study shows that the ion track diameter for GGG irradiated with 200 MeV Au (S<sup>e</sup> = 34.0 keV/nm) is 11.3 ± 0.9 nm, whereas a previous study shows a similar ion track diameter (12.4 ± 1.6 nm for GGG irradiated with 250 MeV Pb ions (S<sup>e</sup> = 34.0 keV/nm)) [45]. Therefore, the present result is consistent with the previous results. The present results are also consistent with the prediction made by the thermal spike model. keV/nm) [47] was 8 nm. The present study shows that the ion track diameter for ZrSiO4 irradiated with 200 MeV Au ions is 9.5 ± 0.5 nm, exhibiting large ion tracks. Since the large ion tracks observed in the present study could be due to the relatively high electronic stopping power (Se = 29.6 keV/nm) and the velocity effect [49], there is no contradiction between the present and previous results. A good summary of the previous ion track data in GGG is given in Ref. [45]. The present study shows that the ion track diameter for GGG irradiated with 200 MeV Au (Se = 34.0 keV/nm) is 11.3 ± 0.9 nm, whereas a previous study shows a similar ion track diameter (12.4 ± 1.6 nm for GGG irradiated

Figure 3a–c show magnified images of hillocks in LiNbO3, ZrSiO4, and GGG, respectively, irradiated with 200 MeV Au at an oblique incidence angle. As shown in the figures, hillocks are successfully observed in those materials. It is also found that the hillocks are clearly amorphous, which confirms the amorphizable nature of these ceramics. The side-view of the hillocks allows measuring both hillock diameter and height. The distribution of the hillock sizes is shown in Figure 2 together with that of the ion track sizes. The average sizes of the hillocks and track sizes are summarized in Table 1. As demonstrated in the table, the hillock diameter is always similar to the ion track size, although the former tends to be slightly larger than the latter. A hillock height seems nearly half of the hillock diameter. with 250 MeV Pb ions (Se = 34.0 keV/nm)) [45]. Therefore, the present result is consistent with the previous results. The present results are also consistent with the prediction made by the thermal spike model. Figure 3a–c show magnified images of hillocks in LiNbO3, ZrSiO4, and GGG, respectively, irradiated with 200 MeV Au at an oblique incidence angle. As shown in the figures, hillocks are successfully observed in those materials. It is also found that the hillocks are clearly amorphous, which confirms the amorphizable nature of these ceramics. The side-view of the hillocks allows measuring both hillock diameter and height. The distribution of the hillock sizes is shown in Figure 2 together with that of the ion track sizes. The average sizes of the hillocks and track sizes are summarized in Table 1. As demonstrated in the table, the hillock diameter is always similar to the ion track size, although the former tends to be slightly larger than the latter. A hillock height seems nearly half of the hillock diameter.

**Figure 3.** Bright field images of hillocks induced in (**a**) LiNbO3, (**b**) ZrSiO4, and (**c**) Gd3Ga5O12 irradiated with 200 MeV Au32+ at an oblique incidence angle. The images were taken at relatively high magnification. **Figure 3.** Bright field images of hillocks induced in (**a**) LiNbO<sup>3</sup> , (**b**) ZrSiO<sup>4</sup> , and (**c**) Gd3Ga5O<sup>12</sup> irradiated with 200 MeV Au32<sup>+</sup> at an oblique incidence angle. The images were taken at relatively high magnification.

Although the hillock shape seems to be semi-spherical in the low magnification images, the magnified images demonstrate the formation of bell-shaped hillocks. Bell-shaped hillocks can also be found in Y3Fe5O<sup>12</sup> [36,37] and Y3Al5O<sup>12</sup> [16], that are also categorized as amorphizable ceramics. Although the hillock shape seems to be semi-spherical in the low magnification images, the magnified images demonstrate the formation of bell-shaped hillocks. Bell-shaped hillocks can also be found in Y3Fe5O12 [36,37] and Y3Al5O12 [16], that are also categorized as amorphizable ceramics.

#### *3.2. Formation Process of Hillocks and Ion Tracks in LiNbO3, ZrSiO4, and GGG 3.2. Formation Process of Hillocks and Ion Tracks in LiNbO3, ZrSiO4, and GGG*

The present study shows that both hillocks and ion tracks exhibit amorphous features, and they have similar sizes in amorphizable ceramics (LiNbO3, ZrSiO4, and GGG). The present results agree with the prediction of the thermal spike model. According to the thermal spike model, SHIs cause transient melting along the ion path, when the electronic stopping power exceeds a certain threshold value. During such transient melting, thermal pressure and volume change caused by solid-liquid transition lead to surface protrusion of the melt. In the amorphizable ceramics, rapid cooling after melting results in the formation of amorphous ion tracks and hillocks. The present study shows that both hillocks and ion tracks exhibit amorphous features, and they have similar sizes in amorphizable ceramics (LiNbO3, ZrSiO4, and GGG). The present results agree with the prediction of the thermal spike model. According to the thermal spike model, SHIs cause transient melting along the ion path, when the electronic stopping power exceeds a certain threshold value. During such transient melting, thermal pressure and volume change caused by solid-liquid transition lead to surface protrusion of the melt. In the amorphizable ceramics, rapid cooling after melting results in the formation of amorphous ion tracks and hillocks.

It is important to note that, although the hillock diameter appears to be similar to the ion track diameter, the magnified TEM images shows that the former is always slightly larger than the latter. This can be ascribed to the spreading tendency of a liquid on a solid surface. It is likely that the hillock shape is determined by the balance of the adhesive (the liquid wanting to maintain contact with the solid) and cohesive forces within the liquid (both internal cohesive force and surface tension) during melting. If the adhesive force dominates, the protruded part of the melt can spread over the surface, leading to the formation of bell-shaped hillocks. Conversely, it can turn into a spherical shape, if the cohesive force dominates. Such spherical hillocks have been reported in some ceramics irradiated with high-energy fullerene ions having a very high S<sup>e</sup> [50,51]. Large volume of surface protrusion induced by high S<sup>e</sup> may be closely related to the formation of spherical hillocks. It is important to note that, although the hillock diameter appears to be similar to the ion track diameter, the magnified TEM images shows that the former is always slightly larger than the latter. This can be ascribed to the spreading tendency of a liquid on a solid surface. It is likely that the hillock shape is determined by the balance of the adhesive (the liquid wanting to maintain contact with the solid) and cohesive forces within the liquid (both internal cohesive force and surface tension) during melting. If the adhesive force dominates, the protruded part of the melt can spread over the surface, leading to the formation of bell-shaped hillocks. Conversely, it can turn into a spherical shape, if the cohesive force dominates. Such spherical hillocks have been reported in some ceramics irradiated with high-energy fullerene ions having a very high Se [50,51]. Large volume of surface protrusion induced by high Se may be closely related to the formation of spherical hillocks.

#### *3.3. Hillocks and Ion Tracks in SrTiO<sup>3</sup> and Nb-Doped SrTiO<sup>3</sup> 3.3. Hillocks and Ion Tracks in SrTiO3 and Nb-Doped SrTiO3*

Figures 4 and 5 show the bright field image of the ion tracks in STO and Nb-STO irradiated with 200 MeV Au at an oblique incidence angle, respectively. The ion track has a bright core surrounded by a dark fringe in the underfocus condition (Figures 4a and 5a), whereas it has a dark core surrounded by a bright fringe in the overfocus condition (Figures 4b and 5b). The Fresnel fringe is an indication of the formation of ion tracks with a lower density than that of the surrounding matrix [52,53]. Such focus-dependent Fresnel contrast is not found in amorphizable ceramics (e.g., YIG, LiNbO3, ZrSiO4, and GGG). Figures 4 and 5 show the bright field image of the ion tracks in STO and Nb-STO irradiated with 200 MeV Au at an oblique incidence angle, respectively. The ion track has a bright core surrounded by a dark fringe in the underfocus condition (Figures 4a and 5a), whereas it has a dark core surrounded by a bright fringe in the overfocus condition (Figures 4b and 5b). The Fresnel fringe is an indication of the formation of ion tracks with a lower density than that of the surrounding matrix [52,53]. Such focus-dependent Fresnel contrast is not found in amorphizable ceramics (e.g., YIG, LiNbO3, ZrSiO4, and GGG).

**Figure 4.** Bright field images of ion tracks induced in SrTiO3 irradiated with 200 MeV Au32+ at an oblique incidence angle. The images were taken in (**a**) underfocus and (**b**) overfocus conditions. **Figure 4.** Bright field images of ion tracks induced in SrTiO<sup>3</sup> irradiated with 200 MeV Au32<sup>+</sup> at an oblique incidence angle. The images were taken in (**a**) underfocus and (**b**) overfocus conditions.

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**Figure 5.** Bright field images of ion tracks induced in Nb-doped SrTiO3 irradiated with 200 MeV Au32+ at an oblique incidence angle. The images were taken in (**a**) underfocus condition and (**b**) overfocus condition. **Figure 5.** Bright field images of ion tracks induced in Nb-doped SrTiO<sup>3</sup> irradiated with 200 MeV Au32<sup>+</sup> at an oblique incidence angle. The images were taken in (**a**) underfocus condition and (**b**) overfocus condition. **Figure 5.** Bright field images of ion tracks induced in Nb-doped SrTiO3 irradiated with 200 MeV Au32+ at an oblique incidence angle. The images were taken in (**a**) underfocus condition and (**b**) overfocus condition.

The ion tracks in STO and Nb-STO have inhomogeneous morphology in contrast to the homogenous morphology of the ion tracks in amorphizable ceramics. The inhomogeneity of the ion tracks is a common feature observed in non-amorphizable ceramics such as CeO2 [38], CaF2, SrF2, and BaF2 [36], in which partial recrystallization after transient melting is the likely cause of the inhomogeneous ion track formation. Black contrasts corresponding to hillocks are found at the end of the ion tracks. Figures 6 and 7 show magnified images of a hillock created at the edge of the thin TEM samples of STO and Nb-STO, respectively. Bell-shaped hillocks are observed in STO and Nb-STO. In both materials, hillocks are found to be amorphous. It is interesting to find that amorphous hillocks are created, although the ion track region is recrystallized. The ion tracks in STO and Nb-STO have inhomogeneous morphology in contrast to thehomogenous morphology of the ion tracks in amorphizable ceramics. The inhomogeneity of theion tracks is a common feature observed in non-amorphizable ceramics such as CeO<sup>2</sup> [38], CaF2, SrF2, and BaF<sup>2</sup> [36], in which partial recrystallization after transient melting is the likely cause of theinhomogeneous ion track formation. Black contrasts corresponding to hillocks are found at the endof the ion tracks. Figures <sup>6</sup> and <sup>7</sup> show magnified images of a hillock created at the edge of the thinTEM samples of STO and Nb-STO, respectively. Bell-shaped hillocks are observed in STO and Nb-STO. In both materials, hillocks are found to be amorphous. It is interesting to find that amorphous hillocksare created, although the ion track region is recrystallized. The ion tracks in STO and Nb-STO have inhomogeneous morphology in contrast to the homogenous morphology of the ion tracks in amorphizable ceramics. The inhomogeneity of the ion tracks is a common feature observed in non-amorphizable ceramics such as CeO2 [38], CaF2, SrF2, and BaF2 [36], in which partial recrystallization after transient melting is the likely cause of the inhomogeneous ion track formation. Black contrasts corresponding to hillocks are found at the end of the ion tracks. Figures 6 and 7 show magnified images of a hillock created at the edge of the thin TEM samples of STO and Nb-STO, respectively. Bell-shaped hillocks are observed in STO and Nb-STO. In both materials, hillocks are found to be amorphous. It is interesting to find that amorphous hillocks are created, although the ion track region is recrystallized.

**Figure 6.** Bright field images of hillocks induced in SrTiO3 irradiated with 200 MeV Au32+ at an oblique incidence angle. **Figure 6.** Bright field images of hillocks induced in SrTiO3 irradiated with 200 MeV Au32+ at an oblique incidence angle. **Figure 6.** Bright field images of hillocks induced in SrTiO<sup>3</sup> irradiated with 200 MeV Au32<sup>+</sup> at an oblique incidence angle.

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**Figure 7.** Bright field images of hillocks induced in Nb-doped SrTiO3 irradiated with 200 MeV Au32+ at an oblique incidence angle. **Figure 7.** Bright field images of hillocks induced in Nb-doped SrTiO<sup>3</sup> irradiated with 200 MeV Au32<sup>+</sup> at an oblique incidence angle.

Since it is difficult to find inhomogeneous ion tracks in TEM, we could not find sufficient numbers of ion tracks and hillocks to perform a statistical analysis. Therefore, in this study, we only show the size range of ion tracks and hillocks observed in STO and Nb-STO in Table 2. As demonstrated in the table, there is a marked difference in the diameters of ion tracks and hillocks in both STO and Nb-STO. The large hillock diameter compared with the ion track diameter is also found in typical non-amorphizable ceramics (CaF2, SrF2, and BaF2). The present result can be a strong evidence that supports the recrystallization of the ion track regions in STO and Nb-STO. Conversely, the amorphous hillocks are signs of the failure of recrystallization. It seems that STO and Nb-STO are intermediate ceramics between amorphizable and non-amorphizable ceramics. Since it is difficult to find inhomogeneous ion tracks in TEM, we could not find sufficient numbers of ion tracks and hillocks to perform a statistical analysis. Therefore, in this study, we only show the size range of ion tracks and hillocks observed in STO and Nb-STO in Table 2. As demonstrated in the table, there is a marked difference in the diameters of ion tracks and hillocks in both STO and Nb-STO. The large hillock diameter compared with the ion track diameter is also found in typical non-amorphizable ceramics (CaF2, SrF2, and BaF2). The present result can be a strong evidence that supports the recrystallization of the ion track regions in STO and Nb-STO. Conversely, the amorphous hillocks are signs of the failure of recrystallization. It seems that STO and Nb-STO are intermediate ceramics between amorphizable and non-amorphizable ceramics.

of the previous studies claimed that they are amorphous in STO. According to Ref. [54], analysis of X-ray diffraction peaks in ion-irradiated STO supports creation of amorphous ion tracks due to single impacts. In the same literature, although crystalline tracks containing defects are observed by TEM, **Table 2.** Approximate values of ion track diameter (Dtrack), hillock diameter (Dhillock), and hillock height (Hhillock) in SrTiO<sup>3</sup> (STO) and 0.5 wt% niobium-doped STO (Nb-STO). The corresponding S<sup>e</sup> values are also listed.

Here, it is important to discuss whether the ion tracks are actually crystalline or not, since some


partially recrystallized ion tracks in non-amorphizable ceramics. Moreover, the markedly smaller ion track diameter than the hillock diameter demonstrates that the ion track region is partially recrystallized after transient melting. **Table 2.** Approximate values of ion track diameter (Dtrack), hillock diameter (Dhillock), and hillock height (Hhillock) in SrTiO3 (STO) and 0.5 wt% niobium-doped STO (Nb-STO). The corresponding Se values are also listed.  **Dtrack (nm) Dhillock (nm) Hhillock (nm) Se (keV/nm)**  STO 3~5 12~15 4~5 28.6 Nb-STO 3~4 11~13 4~5 28.5 Here, it is important to discuss whether the ion tracks are actually crystalline or not, since some of the previous studies claimed that they are amorphous in STO. According to Ref. [54], analysis of X-ray diffraction peaks in ion-irradiated STO supports creation of amorphous ion tracks due to single impacts. In the same literature, although crystalline tracks containing defects are observed by TEM, the authors claimed that the crystalline tracks are created owing to 200 keV electron beam exposure which can cause amorphous–crystalline transition of ion track areas. Conversely, our TEM results of STO and Nb-STO support creation of crystalline ion tracks rather than amorphous ion tracks. For example, an inhomogenous feature of ion tracks in STO and Nb-STO is similar to that observed in partially recrystallized ion tracks in non-amorphizable ceramics. Moreover, the markedly smaller ion track diameter than the hillock diameter demonstrates that the ion track region is partially recrystallized after transient melting.

The clear difference in the morphology between the hillocks in STO and Nb-STO was not

The clear difference in the morphology between the hillocks in STO and Nb-STO was not observed in this study. Therefore, the influence of Nb-doping on the hillock formation is not found in this study. A previous study using TEM and small angle X-ray scattering (SAXS) reported that 1 wt% Nb-doping does not affect track formation [55]. Track formation is easy in insulators since the energy of hot electrons is transferred to the lattice in insulators before being cooled down by free electrons. In contrast, track formation is difficult in metals since high electronic conductivity allows rapid energy diffusion in an electron subsystem. Although the electrical conductivity of Nb-STO is more than nine orders of magnitude higher than that of STO, the electron density may still be too low to influence the formation of hillocks and ion tracks. It should be noted that the electrical conductivity of metals is still much higher than that of Nb-STO. The difference of bonding character (metallic bonding in metals vs. ionic/covalent bonding in STO) should be also contributing to the less sensitivity of track formation in metals [55].
