Temperature driven structural transitions in the graphitic-arrangement of carbon onions filled with FePd₃ nano crystals

Carbon nano-onions (CNOs) are fullerene-like structures which have recently attracted a great attention for numerous applications, such as magnetic data recording, energy storage, biomedicine and others. In this work we report the observation of novel temperature-driven structural transitions in approximately spherical CNOs structures filled with FePd₃ nano crystals through temperature dependent X-ray diffraction analyses. The Rietveld refinement method is used to demonstrate the presence of an anomalous temperature-driven increase of the 002 interplanar distance between CNOs layers from the value of approximately 0.35 nm to the value of 0.50 nm with a consequent distortion of the graphite unit-cell parameters in the temperature range of 25 °C–400 °C. These observations are confirmed also by additional high resolution transmission electron microscopy and grain-size analyses which allow to estimate the number of CNOs layers involved in the temperature driven structural transition. Specifically we find that such unusual effect is present only for the case of FePd₃ filled CNOs, however no structural-transition is found for the case of other types of CNOs filled with Fe₃C measured in similar experimental conditions.