ELECTRONIC STATES OF THE BA-C-60 COMPOUNDS

Synchrotron-radiation photoelectron spectroscopy has been used to investigate the phases that are formed upon doping of C60 with Ba and to examine the electronic properties of those phases. Core-level analysis reveals a Ba1C60 phase and confirms the formation Of Ba3C60 and Ba6C60 at 300 K. A continuous range of solubility is indicated for 3 < x < 6. Ba1C60 has states at the Fermi level but transport measurements indicate that these states are localized. The width of the t1u-derived band for Ba1C60 points to the continued importance of electron correlation. Ba3C60 is an insulator with a pp of at least approximately 0.35 eV. The insulating character of Ba3C60 reflects transfer of two electrons per Ba atom to the fullerene and complete filling of the t1u-derived band. Ba6C60 exhibits a clear Fermi cutoff, as expected from its superconducting character, and resonant photoemission shows that the states near E(F) are approximately 25% Ba in character with hybridization in both t1u and t1g-derived bands.