The high symmetry and resulting electronic degeneracy of the C603- anion is viewed as the key molecular feature in the high superconducting transition temperatures of fulleride and oxidized fullerene systems. 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 elusive with only stable A1C70, A4C70, and A6C70 phases known. In this paper, we report the synthesis of fcc A3C70 phases stabilized by size-matching the tetrahedral site with the sodium cation. The structures are strongly dependent on the cooling protocol due to the existence of metastable partially or completely orientationally disordered phases. EPR data indicate that the phases are metallic but not superconducting. The densities of states at the Fermi level appear too low to give superconductivity at above 5 K, consistent with recent observations that four electrons per C70 anion are required for superconductivity. Size-matching on both the octahedral and tetrahedral sites is required for A3C70 stability - K2CsC70 is only stable at elevated temperature and Na2C70 is unstable, the composition corresponding to C70 and a sodium-rich trigonal phase.
ASJC Scopus subject areas
- Colloid and Surface Chemistry