Unusual conductivity patterns in reduced mesoporous titanium, niobium, andtantalum oxides with one-dimensional potassium fulleride wires in the channels

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 w ires. The oxidation state of the encapsulated fulleride phase can be tuned by addition of potassium naphthalene to the mesostructured composite. Surpr isingly, 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-Angstrom pore size, mesoporous tantalum oxide wit h a 22-Angstrom pore size, and mesoporous titanium oxide with a 22-Angstrom pore size were treated with K3C60 and characterized by elemental analysis, nitrogen adsorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray ph otoelectron spectroscopy (XPS), electron spin resonance spectroscopy (ESR), and superconducting quantum interference device (SQUID) magnetometry. Thes e materials were then further reduced with small aliquots of potassium naph thalene in sequential steps up to a fulleride oxidation state of n = 4.5, a nd each material was fully characterized as described above. For each serie s of materials, two conductivity maxima were observed, the first at approxi mately n = 2.5 and the second at roughly n = 4.0, indicating that this doub le-maxima behavior is general to other one-dimensional alkali fulleride mes ostructures. 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, cons istent with the high levels of conductivity of the fulleride at this oxidat ion state.