## 4 Ion Irradiation-Induced Amorphisation of Cobalt Nanoparticles
### Results
In contrast, Figures 17-21 demonstrate the opposite for Co NPs where an irradiation-induced transformation from FCC (stable at high-temperature/high-pressure in bulk material) to HCP (stable at ambient conditions in bulk material) is apparent. The FCC-to-HCP transformation in bulk Co is martensitic and only proceeds when, in the presence of external forces, a critical shear energy is exceeded [24]. In free-standing Co NPs [24], this barrier (\(\sim\)0.016 kJ/mol) can be overcome via the shear stress applied through mechanical grinding. In free standing SiO\({}_{2}\) films irradiated with swift heavy ions, large shear stresses result from the rapid thermal expansion of the cylindrical ion track [94, 95, 96] and depending on the energy of the incident ion, the single-ion induced local stress can be in excess  
Figure 17.2 Temperature dependence of the \(\sigma^{2}\) for HCP and FCC bulk standards, unirradiated FCC NPs and phase transformed HCP NPs. The lines represent the fits with the correlated anharmonic Einstein model (Equation (1)). \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Sample** & \begin{tabular}{c} _BL_ \\ **(Å)** \\ \end{tabular} & \begin{tabular}{c} \(\sigma^{2}\) \\ **(10\({}^{-3}\) Å\({}^{2}\))** \\ \end{tabular} & \begin{tabular}{c} _C3_ \\ **(10\({}^{-5}\) Å\({}^{3}\))** \\ \end{tabular} & \begin{tabular}{c} _CN_ \\ **(10\({}^{-5}\) Å\({}^{3}\))** \\ \end{tabular} & \
\begin{tabular}{c} \(\Theta_{E}\)** \\ **(atoms)** \\ \end{tabular} &
\begin{tabular}{c} \(\sigma^{2}_{S}\) \\ **(K)** \\ \end{tabular} \\ \hline Bulk FCC & \(2.492(0.002)\) & \(3.00(0.1)\) & \(-3.2(2.1)\) & \(12\) & \(286(1)\) & \(0.16(0.05)\) \\ FCC NPs & \(2.488(0.004)\) & \(2.79(0.2)\) & \(-4.0(2.3)\) & \(11.4(0.5)\) & \(285(2)\) & \(0.04(0.01)\) \\ Bulk HCP & \(2.482(0.002)\) & \(2.46(0.06)\) & \(-3.0(1.7)\) & \(12\) & \(296(1)\) & \(0.05(0.01)\) \\ HCP NPs & \(2.476(0.004)\) & \(3.31(0.3)\) & \(-3.3(2.7)\) & \(10.8(0.5)\) & \(292(2)\) & \(0.35(0.05)\) \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Structural parameters, Einstein temperature (\(\Theta_{E}\)) and structural disorder (\(\sigma^{2}_{S}\)) for HCP and FCC bulk standards, unirradiated FCC NPs and phase transformed HCP NPs. of 300 MPa. The effects of both LEII and SHII on the stress state of constrained SiO\({}_{2}\) films have also been intensively studied [96, 97, 98, 99]. Two opposite trends in the stress state have been measured and are dependent on the irradiation temperature, energy and fluence. Low fluence irradiation results in a transformation from compressive to tensile stress. This transformation is due to the structural changes that compact the SiO\({}_{2}\) network. After a maximum tensile stress is reached, the stress relaxes back to a compressive state via Newtonian viscous flow and also saturates at a few hundred MPa. This saturation in compressive stress is a result of a dynamic competition between the generation of compressive anisotropic stress (with energy-dependent strain rate, or deformation yield) and stress relaxation due to Newtonian viscous flow. Thus, the shear stress in SiO\({}_{2}\) resulting from the formation of an ion track or the macroscopic build-up of stresses in the constrained SiO\({}_{2}\) host matrix may well drive the FCC-to-HCP phase transformation in Co NPs. It was recently demonstrated that the radial density distribution of an ion track resulting from SHII is consistent with a frozen-in acoustic pressure wave [100]. Figure 22 compares the interaction cross section for the irradiation-induced FCC-to-HCP phase transformation for Co NPs in SiO\({}_{2}\) (derived from Equation (3)) with the irradiation-induced latent ion-track cross-section in SiO\({}_{2}\) (derived from SAXS measurements [100]) as a function of \(S_{e}\). The two are reasonably well correlated though, as noted above, the interaction cross-section appears to saturate at high \(S_{e}\), potentially the result of a similar rate of stress generation at these high energies.