In this work we demonstrate that an amorphous metallic phase can be readily achieved by low fluence ion irradiation of crystalline Co nanoparticles (NPs) embedded in an amorphous matrix. The NP size and structure were characterized by small angle x-ray scattering, transmission electron microscopy and extended x-ray absorption fine structure (EXAFS) spectroscopy. Compared to bulk material, unirradiated crystalline NPs exhibited increased structural disorder and a decreased average coordination number as a result of finite-size effects. Upon irradiation, there was no variation in NP size yet significant structural change. The coordination number decreased further while the mean value (bondlength), variance (Debye-Waller factor), and asymmetry (third cumulant) of the interatomic distance distribution all increased, as consistent with theoretical predictions for an amorphous elemental metal. Furthermore, the interatomic distance distribution for irradiated Co NPs was in excellent agreement with our molecular dynamics simulations for bulk amorphous Co, and we have thus attributed the observed structural changes to the formation of an amorphous phase. Though such a crystalline-to-amorphous phase transformation is not readily achievable in bulk material by ion irradiation, we suggest that the perturbed structural state prior to irradiation and the amorphous surrounding matrix both contribute to nucleating and stabilizing the amorphous phase in irradiated Co NPs.

David John Sprouster*, The Australian National University, Australia
Raquel Giulian, The Australian National University, Australia
Leandro Araujo, The Australian National University, Australia
Patrick Kluth, The Australian National University, Australia
Bernt Johannessen, Australian Synchrotron Company Ltd, Australia
Kai Nordlund, University of Helsinki, Finland
Mark Ridgway, The Australian National University, Australia

*presenting author