Rare-Earth Based Fullerides: tuning the onset of valency transitions

Abstract

In this thesis, the recent advances in studies on rare-earth metal intercalated fullerene solids with emphasis on their structural, electronic, and magnetic properties. The investigations on the rare-earth based fullerides have been concentrated on their structural, electronic, and magnetic properties. Intercalation of C(_60) with rare-earth metals results in interesting compounds not only for the appearance of superconductivity but also for the magnetic properties and mixed valence phenomena related to the localised 4f electrons. Of particular interest, I discuss the results obtained from various experiments on rare-earth based mixed valence fullerides, of which displays a remarkable sensitivity of rare-earth valency to external stimuli, such as temperature and pressure. Among the family of rare-earth fullerides, Sm(_2.75)C(_60) was the first known molecular-based material to show valence fluctuation associated with the highly-correlated narrow-band behaviour of the 4f electrons in Sm ions. Improvement in the synthetic technique to produce single-phase rare-earth doped fullerides have opened the way to carry out detailed and systematic study of the structural properties of the RE(_2.75)C(_60) (RE = Sm, Eu, and Yb) as a function of temperature and pressure, which were carried out using the synchrotron X-ray powder diffraction technique. The obtained results have lead us to find a rich variety of temperature- and pressure-driven abrupt or continuous valence transitions. In addition, we have observed that by taking precise control on the nature of dopants, the tuning of the onset temperature and pressure of this valence transition were possible. Direct measurements on the valence states of the rare-earth ions in the fulleride salts as a function of temperature were carried out using X-ray absorption spectroscopy using the alkaline-earth and rare-earth mixed compound, (Sm(_2/3)Ca9_1/3))(_2.75) C(_60). The obtained spectra have provided clear evidence to confirm the electronic nature of the low-temperature first-order valence transition.