X-ray photoemission investigations of binary and ternaryC60fullerides of Na, K, Rb, and Cs

Core-level photoemission studies of alkali fulleride ${\mathit{A}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ thin films indicate alkali bonding configurations that can be associated with octahedral and tetrahedral interstitial sites of the ${\mathrm{C}}_{60}$ lattice. For ${\mathrm{K}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$, the K 2p core-level results suggest ${\mathrm{K}}_3$ ${\mathrm{C}}_{60}$ nucleation for K concentration as low as x∼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∼1, forming ${\mathit{A}}_1$ ${\mathrm{C}}_{60}$ phases. For intercalation beyond x∼1, the fcc tetrahedral sites of Rb are occupied as ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ forms. Intercalation with K or Rb beyond x∼3, or with Cs beyond x∼1, results in a spectral feature that is consistent with formation of a body-centered phase with equivalent tetrahedral interstitial sites. Results for ${\mathrm{Na}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ suggest that Na ions occupy off-center positions in the large octahedral sites. Studies of ternary fullerides ${\mathrm{K}}_{\mathit{x}}$ ${\mathit{A}}_{\mathit{y}}$ ${\mathrm{C}}_{60}$ (x+y<3) demonstrate preferential filling of the octahedral sites by the larger ion.