## 4 Ion Irradiation-Induced Amorphisation of Cobalt Nanoparticles
### Results  
Figure 17 shows ED patterns (a) before and (b) after SHII with 89 MeV to a fluence of 2 \(\times\) 10\({}^{13}\) /cm\({}^{2}\) with (c) displaying the radially integrated intensity for each pattern in (a) and (b). The indexed lattice spacings (\(d\)) labelled (a)-(f) are listed in Table 7 and confirm the complete FCC-to-HCP phase transformation. No evidence of an oxide component was apparent in the ED data. Significant changes in the NP shape were also not apparent with TEM. At greater fluences (as discussed in the following  
section), the NP shape was progressively transformed from spherical to rod-like with a decrease in mean volume as a result of NP dissolution into the matrix, consistent with previous SHII studies of embedded Co NPs [78]. XANES spectra for bulk standards, unirradiated NPs and 185 MeV irradiations are shown in Figure 18(a) as a function of fluence. The smooth transformation from FCC to HCP Co is readily apparent. Circles represent the linear-combination fits to the experimental data and Figure 18(b) shows an example of the XANES fitting with separate FCC and HCP contributions to the spectrum for the 185 MeV, 1 \(\times\) 10\({}^{12}\) /cm\({}^{2}\) sample. The XANES spectrum for HCP Co agrees well with previous reports [79, 80, 81, 82]. While multiple-scattering calculations for FCC Co predict the amplitude of the B feature should exceed that of the C feature [20, 83], such a difference in relative amplitudes is not apparent in our XANES spectrum for FCC Co. However, the latter is consistent with XANES spectra for FCC Ni [84] \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Peak** & \begin{tabular}{l} **FCC** \\ **(hkl)** \\ \end{tabular} & \begin{tabular}{l} **\(d\)** \\ **(Å)** \\ \end{tabular} & \begin{tabular}{l} **Unirr \(d\)** \\ **(Å)** \\ \end{tabular} & \begin{tabular}{l} **HCP** \\ **(hkl)** \\ \end{tabular} & \
\begin{tabular}{l} **\(d\)** \\ **(Å)** \\ \end{tabular} &
\begin{tabular}{l} **2 \(\times\) 10\({}^{13}\)\(d\)** \\ **(Å)** \\ \end{tabular} \\ \hline a & \((111)\) & \(2.05\) & \(2.05(0.01)\) & \((100)\) & \(2.17\) & \(2.17(0.01)\) \\ b & \((200)\) & \(1.77\) & \(1.78(0.01)\) & \((002)\) & \(2.03\) & \(2.03(0.01)\) \\ c & \((220)\) & \(1.25\) & \(1.26(0.01)\) & \((101)\) & \(1.92\) & \(1.91(0.01)\) \\ d & \((311)\) & \(1.07\) & \(1.07(0.02)\) & \((110)\) & \(1.25\) & \(1.25(0.02)\) \\ e & \((222)\) & \(1.02\) & \(1.02(0.02)\) & \((103)\) & \(1.15\) & \(1.16(0.02)\) \\ f & – & – & – & \((112)\) & \(1.07\) & \(1.07(0.02)\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Theoretical and experimental lattice spacing (\(d\)) determined from the ED patterns of Figure 17.  
Figure 17: ED patterns for **(a)** Co NPs unirradiated and **(b)** after SHII with 89 MeV Au ions to a fluence of 2 \(\times\) 10\({}^{13}\) /cm\({}^{2}\) and **(c)** corresponding radially integrated electron diffraction patterns. The lattice spacings are listed in Table 7.  
and thin FCC Co films grown on (111) Cu substrates [85]. Figure 18(c) shows that the most obvious difference in the XANES spectra of our two standards is the more pronounced depression between features B and C for the FCC phase. Nonetheless, fitting to quantify the HCP and FCC fractions was performed over the entire 7690-7760 energy range and thus included the additional phase-dependent features. The fraction of Co atoms in an oxidized environment was also quantified using XANES with approximately 5% of the Co atoms in a Co\({}_{3}\)O\({}_{4}\)-like atomic configuration after the complete FCC-to-HCP phase transformation. Furthermore, modelling of the XANES spectra demonstrates that the changes observable upon irradiation cannot be the result of Co atoms in an oxidized environment only and stem from the FCC-to-HCP phase transformation [86]. Figure 18: (**a**) XANES spectra for bulk standards, unirradiated and swift heavy ion irradiated (185 MeV) NPs as a function of fluence (/cm\({}^{2}\)).