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 K n C 60 (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 K 3 C 60 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 onedimensional 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.

Recent Advances in Synthesis and Applications of Transition Metal Containing Mesoporous Molecular Sieves

“…[87] The conductivity of the materials at these two states was 10 À4 and 10 À1 W À1 cm À1 , respectively. Interestingly, two conductivity maxima at n 2.6 and 4.1 were revealed in this system.…”

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“…To more fully probe the dependence of room‐temperature conductivity on oxidation state of the fulleride in the pores, the parent composite with n =0.5 was reduced in small increments up to n =4.5, the process was monitored by Raman and XPS. Interestingly, two conductivity maxima at n =2.6 and 4.1 were revealed in this system 87. The conductivity of the materials at these two states was 10 −4 and 10 −1 Ω −1 cm −1 , respectively.…”

Synthesen und Anwendungen von übergangsmetallhaltigen mesoporösen Molekularsieben

“…Um die Abhängigkeit der Raumtemperaturleitfähigkeit des Fullerids in den Poren vom Oxidationszustand näher zu untersuchen, wurde das Stammkomposit mit n =0.5 in kleinen Schritten zu n =4.5 reduziert. Interessanterweise wurden zwei Leitfähigkeitsmaxima bei n =2.6 und 4.1 festgestellt 87. Die Leitfähigkeit der Materialien betrug an diesen Stellen 10 −4 bzw.…”