Electronic and geometric structure of La@xaC82andC82: Theory and experiment

Core-level and valence-band photoemission studies of films of La@${\mathrm{xaC}}_{82}$ and ${\mathrm{C}}_{82}$ demonstrate charge transfer from La to the fullerene cage and the formation of a nonmetallic solid with the uppermost band centered 0.64 eV below the Fermi level. Comparison to results for ${\mathrm{C}}_{82}$ demonstrates that there are modifications to the electronic-state distribution throughout the valence band. Bonding of the molecular radicals in the pure solid is found to be significantly stronger than in empty fullerene solids which sublime at much lower temperatures. Detailed calculations for isolated La@${\mathrm{xaC}}_{82}$ molecules allow comparison with the experimental results. They show the tendency of the La ions to move from the center of the molecule toward the cage. Comparisons of calculations for La@${\mathrm{xaC}}_{82}$ based on ${\mathit{C}}_{3\mathit{v}}$ and ${\mathit{C}}_2$ structures show that much of the energy difference between the two structures is lost upon La incorporation. Bond-length analyses allow comparison with extended x-ray-absorption fine-structure data for the metal-carbon coordination which has been measured for Y@${\mathrm{xaC}}_{82}$. They show sizable La-induced effects on the structure of the carbon cage in the proximity of the La ion.