Close-Packed C703- Phases - Synthesis, Structure, and Electronic Properties

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• 作者：Mark S. Denning ; Ian D. Watts ; Sandra M. Moussa ; Pierrick Durand ; Matthew J. Rosseinsky ; and Katsumi Tanigaki
• 刊名：Journal of the American Chemical Society
• 出版年：2002
• 出版时间：May 15, 2002
• 年：2002
• 卷：124
• 期：19
• 页码：5570 - 5580
• 全文大小：192K
• 年卷期：v.124,no.19(May 15, 2002)
• ISSN：1520-5126

文摘

The high symmetry and resulting electronic degeneracy of the C₆₀^3- anion is viewed as the keymolecular feature in the high superconducting transition temperatures of fulleride and oxidized fullerenesystems. The experimental evaluation of this hypothesis requires the synthesis of face-centered cubic (fcc)trivalent fulleride anion salts derived from higher fullerenes such as C₇₀, which have thus far proved elusivewith only stable A₁C₇₀, A₄C₇₀, and A₆C₇₀ phases known. In this paper, we report the synthesis of fcc A₃C₇₀phases stabilized by size-matching the tetrahedral site with the sodium cation. The structures are stronglydependent on the cooling protocol due to the existence of metastable partially or completely orientationallydisordered phases. EPR data indicate that the phases are metallic but not superconducting. The densitiesof states at the Fermi level appear too low to give superconductivity at above 5 K, consistent with recentobservations that four electrons per C₇₀ anion are required for superconductivity. Size-matching on boththe octahedral and tetrahedral sites is required for A₃C₇₀ stability - K₂CsC₇₀ is only stable at elevatedtemperature and Na₂C₇₀ is unstable, the composition corresponding to C₇₀ and a sodium-rich trigonal phase.