Conduction electron spin resonance inRb3C60

The conduction electron spin resonance (CESR) of ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ has been observed for the first time and the spin susceptibility found to be ${\mathrm{\chi }}_{\mathit{s}}$=(12.4±4)×${10}^{\mathrm{-}4}$ emu/mol, in agreement with static measurements. The CESR linewidth is anomalously large, 1300 G at 30 K giving a spin lattice relaxation rate ${\mathit{T}}_1^{\mathrm{-}1}$=9.0×${10}^{\mathrm{-}11}$ ${\sec }^{\mathrm{-}1}$, and we suggest that it is caused by large molecular disorder. The ESR line formerly assigned to ${\mathrm{Rb}}_3$ ${\mathrm{C}}_{60}$ arises from an unidentified nonsuperconducting phase ${\mathrm{Rb}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$ which has a structural transition near 400 K and a thermal history dependent metal-insulator transition at lower temperatures.