## 4 Ion Irradiation-Induced Amorphisation of Cobalt Nanoparticles
### Results
Circles represent the linear-combination fits to the experimental data (spectra vertically offset for clarity); (**b**) The separate FCC and HCP contributions to the 1 \(\times\) 10\({}^{12}\) /cm\({}^{2}\) sample; (**c**) Comparison of bulk FCC _vs._ HCP standards and unirradiated _vs._ phase transformed NPs. Arrows indicate features characteristic of the FCC-to-HCP phase transformation. Figure 19 shows the HCP fraction as a function of fluence over the given range of irradiation energies. The fitted lines were generated with the overlap model [87]:  
\[\triangle C=C_{S}\left[1-\sum_{k=0}^{n-1}\left(\frac{(\sigma\Phi)^{k}}{k!} \right)\exp(-\sigma\phi)\right] \tag{3}\]  
where \(\triangle C\) and \(C_{s}\) are the relative and total increase in concentration of the HCP phase, respectively, \(\phi\) is the irradiation fluence and \(\sigma\) is the interaction cross section for the phase transformation. The best fit was achieved with \(n\) = 1 and thus a direct-impact mechanism appears operative. For irradiation energies of 9-54 MeV, the rate of transformation clearly increases with energy, demonstrating the process is governed by \(S_{e}\). Above 54 MeV, the rate saturates and is independent of irradiation energy. Irradiation of bulk Co HCP and FCC standards (of thickness 250 nm) yielded no change in crystallographic phase. **Figure 19.** HCP fraction in irradiated NPs as a function of fluence. Fitted lines were derived from the overlap model (Equation (3)). **Figure 18.**_Cont._The FT EXAFS spectra for the bulk HCP, FCC, unirradiated and transformed NPs are shown in Figure 20. Distinguishing between the FCC and HCP phase is difficult with EXAFS, as both have similar \(BL\) and \(CN\) in the first and second NN shells. One visible difference between the bulk HCP and FCC phases is an increase in amplitude for the fourth NN in FCC Co (Figure 20). This is a typical feature of FCC metals and is primarily due to the collinear multiple scattering of the photoelectron [88, 89, 90]. The slight decrease in amplitude of the first NN for the transformed NPs is due to a decrease in \(CN\) (10.8 compared to 11.3 for the unirradiated NPs) and increase in the static disorder (\(\sigma_{S}^{2}\)). The decrease in the \(CN\) is the result of the \(\sim\)5% oxide and correcting the \(CN\) for this small component results in an increase to 11.2. As shown in Section 3, the FCC and HCP phases have different vibrational properties with Einstein temperatures (\(\Theta_{E}\)) that differ by \(\sim\)10 K. As such, a change in phase to HCP should also yield an increase in the \(\Theta_{E}\) for the phase transformed NPs. The temperature dependence of the \(\sigma^{2}\) is shown in Figure 21. The solid lines are the corresponding fits with Equation (1). The \(\Theta_{E}\) values and results form the EXAFS analysis are given in Figure 17.0 The \(\Theta_{E}\) and \(\sigma_{s}^{2}\) values for bulk HCP Co, bulk FCC Co and FCC NPs given in Table 8 agree with those previously calculated in Sections 3 and 4. After irradiation (185 MeV, 1 \(\times\) 10\({}^{13}\) /cm\({}^{2}\)) the increase in the \(\Theta_{E}\) further validates the phase transformation quantified from the XANES analysis. The slight increase in \(\sigma_{s}^{2}\) for the transformed NPs is similar to the value determined in Section 3 for 5.4 nm HCP NPs and is the result of a SHII-induced increase in disorder. Figure 17.1 Phase corrected FT EXAFS spectra for bulk HCP and FCC Co, unirradiated FCC NPs and phase transformed HCP NPs with spectra offset for comparison. Open circles represent fits to experimental data. Earlier observations of swift heavy ion irradiation-induced phase transformations in bulk metals [91] and oxides [92, 93] differ from the transformation reported herein. For example, Benyagoub _et al._ observed the transformation in zirconia and hafnia from a phase stable at ambient conditions to a high-temperature/high-pressure phase intuitively consistent with a rapid, post-thermal-spike quench [92, 93].