The effect of encapsulation in carbon nanotubes on properties of Fe-Ni nanoalloys with cubic and helical structures

The effect of encapsulation in carbon nanotubes on Fe-Ni nanoparticles (NPs), NP assemblies and nanowires (NWs) is studied by means of atomistic simulations with empirical potentials. The BCC Fe, L1(0), FeNi, L1(2) FeNi3 and FCC Ni stable phases of the bulk alloy are retrieved for both the freestanding (FS) and the encapsulated nanoalloys. As it requires large morphological changes, the BCC/L1(0) transition may be inhibited by the confinement in a nanotube. The results indicate that encapsulation has the effect to enhance Fe segregation at compositions intermediate between stable phases. When the nanotube is too narrow, encapsulated NWs do not support a cubic structure. They consist in coaxial layers with a central straight atomic row aligning with the tube axis. Each layer displays a helical structure which can be equivalently viewed as a folded atomic plane with low Miller indices. Such ultrathin helical Fe-Ni NWs, FS as well as encapsulated, behave as almost ideal solid solutions over the whole range of compositions.