X-RAY PHOTOEMISSION INVESTIGATIONS OF BINARY AND TERNARY C-60 FULLERIDES OF NA, K, RB, AND CS

Core-level photoemission studies of alkali fulleride A(x)C60 thin films indicate alkali bonding configurations that can be associated with octahedral and tetrahedral interstitial sites of the C60 lattice. For KxC60, the K 2p core-level results suggest K3C60 nucleation for K concentration as low as x approximately 0.1 with occupation of both octahedral and tetrahedral sites. In contrast, the Rb 3d and Cs 4d core-level spectra show filling of only octahedral sites until x approximately 1, forming A1C60 phases. For intercalation beyond x approximately 1, the fcc tetrahedral sites of Rb are occupied as Rb3C60 forms. Intercalation with K or Rb beyond x approximately 3, or with Cs beyond x approximately 1, results in a spectral feature that is consistent with formation of a body-centered phase with equivalent tetrahedral interstitial sites. Results for Na(x)C60 suggest that Na ions occupy off-center positions in the large octahedral sites. Studies of ternary fullerides K(x) A(y)C60 (x + y < 3) demonstrate preferential filling of the octahedral sites by the larger ion.