Synthesis and systematic studies of the electronic and magnetic properties of alkali fulleride intercalates in mesoporous tantalum oxide.

摘要

Herein we report the synthesis and systematic study of a new family of alkali fulleride A3C60 (A = Na, Li, Rb) doped mesoporous tantalum oxide nanocomposites. This study allowed us to chart the dependency of conductivity and electromagnetic properties on the oxidation state of one-dimensional fullerides and elucidate the role of absolute carbon loading level in the pores of mesostructure. The role of alkali metal in the conducting properties of these composite materials and possible mechanisms of electron transfer upon reduction of fulleride is also discussed.; Results presented in this work indicated that in case of Na and Rb fullerides the conductivity pattern as a function of Na and Rb reduction level showed a minimum at n = 3.0 and maxima at n ≈ 4.0 states. These results suggest that the superconducting behavior in A3C60 is due to accidental vacancies in the structure and the actual composition differs from the nominal. In contrast, the Li fulleride composite displayed completely insulating behavior and could not be further reduced, thus hindering a study of conductivity as a function of reduction.; Electrochemical studies of the Na and Li fulleride composite revealed a substantial difference in electrochemical behavior in these composites. The Li fulleride composites showed rather irreversible behavior upon Li intercalation/de-intercalation, while the Na fulleride composites exhibited a more reversible pattern. The irreversible electrochemical behavior of Li fullerides was attributed to their insulating nature imposing charge transfer restrictions through the channels. On the other hand the semiconducting nature of the Na fulleride composites likely enhances the electron transport properties.; In order to probe into the structural and coordination geometry of fulleride units and in efforts to make meaningful comparison of alkali fulleride composites with corresponding bulk phase solid-state NMR studies were conducted on all fulleride composite materials reported in this thesis. Solid-state 13C NMR studies revealed a great degree of non-stoichiometry and presence of multiple fulleride species in all composites, except the Li materials, which showed a single fulleride species. These results suggest that conductivity in our composite materials likely arises from the presence of mixed fulleride species at certain oxidation states and the conductivity mechanism occurs via the electron hopping between these states. The n = 3.0 Na composite was completely insulating and showed only C603- by 13C NMR. This result strongly supports the view that A 3C60 materials are hole doped Mott-Hubbard insulators.