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Final Activity Report Summary - REFULL (Electronic, structural and optical properties of the rare-earth-metal fullerides)

Our interest has been focused on the study of the vibrational, structural and electronic properties of the lanthanide fullerides Sm2.75C60 and Yb2.75C60 under external perturbations like pressure and temperature by means of several complementary experimental techniques (Raman and m+SR spectroscopy, X-ray diffraction and absorption, resistivity measurements). The studied systems appear to be semiconducting at ambient conditions and the rare earth ions are in an intermediate valence state between +2 and +3, while their resistivity increases at lower temperatures. These results, together with our Raman spectroscopic studies at low temperature, support the scenario that the rare earth undergoes a valence transition (from +(2+e) to nearly +2) proposed for the explanation of their giant negative thermal expansion bellow 50 K, which is not accompanied by any magnetic correlations.

On the other hand, as it can be inferred by the high pressure Raman spectra the compound Sm2.75C60 exhibits a metallic behaviour at pressures higher than 4 GPa due to a pressure-induced valence transition of Sm towards +3 and the additional electron transfer to the C60 molecular cages. In addition, our high pressure X-ray diffraction measurements reveal that Yb2.75C60 for pressures higher than 5.5 GPa undergoes a similar abrupt lattice collapse to that of Sm2.75C60. Moreover, the strong orbital mixing between the metallic and the C60 ions is clearly revealed in the Raman spectra of the multinary Eu6-xSrxC60 fullerides, which has remarkable effects on the regularity of the surface of the fulleride cages and can account for their ferromagnetic and giant negative magnetoresistive behaviour at lower temperatures.

Subsequently, we have extended our studies towards carbon nanotube materials by means of high pressure Raman spectroscopy. The detailed analysis of the positions and the lineshapes of the Raman peaks, attributed to the inner and outer tubes of bundled Double wall carbon nanotubes (DWCNTs) as a function of pressure, has provided a wealth of information concerning the pressure response of individual nanotubes as well as the inner-outer tube (intratube) interactions. The outer tubes act as a protection shield for the inner tubes whereas the latter increase the structural stability of the outer tubes upon pressure application. More importantly, we have observed peculiar regularity in the strength of the intratube interaction, which is attributed to the interplay between the tube size and the intratube spacing. Finally, an electron-phonon decoupling at elevated pressures has been also observed for the metallic single- and double-wall carbon nanotubes.

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