Static and dynamic Jahn-Teller effect in the alkali metal fulleride salts A₄C₆₀ (A=K,Rb,Cs)

We report the temperature dependent mid- and near-infrared spectra of K₄C₆₀, Rb₄C₆₀, and Cs₄C₆₀. The splitting of the vibrational and electronic transitions indicates a molecular symmetry change of C₆₀⁴⁻ which brings the fulleride anion from D_2h to either a D_3d or a D_5d distortion. In contrast to Cs₄C₆₀, low temperature neutron diffraction measurements did not reveal a structural phase transition in either K₄C₆₀ and Rb₄C₆₀. This proves that the molecular transition is driven by the molecular Jahn-Teller effect, which overrides the distorting potential field of the surrounding cations at high temperature. In K₄C₆₀ and Rb₄C₆₀ we suggest a transition from a static to a dynamic Jahn-Teller state without changing the average structure. We studied the librations of these two fullerides by temperature dependent inelastic neutron scattering and conclude that both pseudorotation and jump reorientation are present in the dynamic Jahn-Teller state.