COMPARISON OF PHOTOEMISSION AND RESISTIVITY IN ALKALINE-EARTH COMPOUNDS OF C-60

Photoemission spectra of CaxC60, SrxC60, and BaxC60 show successive fi lling of the t(lu)-derived LUMO and the t(lg)-derived LUMO + 1 bands, for 0 < x < 6. For x < 2 compounds of Ca and Ba exhibit a well defined Fermi cut-off; compounds of Sr do not. In this range of composition, the resistivity remains activated, suggesting that the metallic charac ter in photoemission is due to a surface phase. Stoichiometric compoun ds with formula A(3)C(60) differ in both structure and electronic prop erties. The Ca compound retains the fee stacking of C-60 and is an ins ulator. The Sr compound transforms partially to the A15 structure but remains an insulator. The Ba compound has the A15 structure and is wea kly metallic. For 3 < x < 6 metallic behavior is weak in the Ca and Sr compounds, but well defined in the Ba compound, indicating that the L UMO + 1 band is filled progressively by Ba. This is in accord with the superconductivity of these systems, which is found in compounds with x near 5, when the Fermi level is in the t(lg) band.