Unusual Conductivity Patterns in Reduced Mesoporous Titanium, Niobium, and Tantalum Oxides with One-Dimensional Potassium Fulleride Wires in the Channels

Abstract

Recent results in our group demonstrated that KnC60 (n = 3), a much-studied superconductor and molecular metal, can be encapsulated in the channels of mesoporous niobium oxide to make pseudo-one-dimensional alkali fulleride wires. The oxidation state of the encapsulated fulleride phase can be tuned by addition of potassium naphthalene to the mesostructured composite. Surprisingly, the conductivity of this series of composites has maxima at n = 2.6 and n = 4.1, rather than n = 3 as in the bulk material. In this work, we report a study on the effect of changing the pore size and wall composition of the mesoporous host lattice on the conductivity and electronic behavior of the corresponding potassium fulleride composites. Samples of mesoporous niobium oxide with a 32-Å pore size, mesoporous tantalum oxide with a 22-Å pore size, and mesoporous titanium oxide with a 22-Å pore size were treated with K3C60 and characterized by elemental analysis, nitrogen adsorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), electron spin resonance spectroscopy (ESR), and superconducting quantum interference device (SQUID) magnetometry. These materials were then further reduced with small aliquots of potassium naphthalene in sequential steps up to a fulleride oxidation state of n = 4.5, and each material was fully characterized as described above. For each series of materials, two conductivity maxima were observed, the first at approximately n = 2.5 and the second at roughly n = 4.0, indicating that this double-maxima behavior is general to other one-dimensional alkali fulleride mesostructures. There was no clear pattern in the effect of changing pore size and wall composition on the electronic properties; however, all materials near n = 4.0 showed a greater degree of reduction of the mesostructure and a greater density of states near the Fermi level as determined by XPS, consistent with the high levels of conductivity of the fulleride at this oxidation state.