Static and dynamic Jahn-Teller effect in the alkali metal fulleride saltsA4C60(A=K,<

Abstract

We report the temperature dependent mid-and near-infrared spectra of K 4 C 60 , Rb 4 C 60 , and Cs 4 C 60 . The splitting of the vibrational and electronic transitions indicates a molecular symmetry change of C 60 4− which brings the fulleride anion from D 2h to either a D 3d or a D 5d distortion. In contrast to Cs 4 C 60 , low temperature neutron diffraction measurements did not reveal a structural phase transition in either K 4 C 60 and Rb 4 C 60 . 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 K 4 C 60 and Rb 4 C 60 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.

Interplay between Coulomb and Jahn-Teller effects in icosahedral systems with triplet electronic states coupled toh-type vibrations

“…2 High-temperature spectroscopic data on K 4 C 60 and Rb 4 C 60 suggest that these materials contain C 4− 60 ions that are decoupled from the lattice and act as isolated ions subject to dynamic JT distortions of D 5d or D 3d symmetry. 41 These results can both be explained by taking the same ranges of quadratic coupling constants that we have deduced hold for the monoanion if the Coulomb interactions result in a high-spin ground state, where the JT effect results in a T ⊗ h problem with distortional symmetries as in Fig. 2(a).…”

Electronic and ionic conductivities in superionicLi4C60

“…On the other hand, the larger size of the alkali ion hinders the polymer formation in K 4 C 60 . Although there is a Jahn-Teller distortion 30 even in this case, the distortion of the C 60 ball is smaller than in polymers and the IR active modes emerging from the lower symmetry are weak.…”

Infrared Signatures of the Dynamic Jahn-Teller Effect in Fullerene-Based Materials

“…In CS4C60, on warming a structural phase transition occurs from orthorhombic to tetragonal [11], but in K4C60 no such transition was seen [10]. The vibrational spectra, however, change in both cases (Figure 2) to a pattern corresponding to a D3 j/D 5^ symmetry.…”

“…At room temperature they saw a twofold splitting. Our previous infrared measurements [9] found this twofold splitting changing to a threefold one at low temperatures, while the crystal structure remained tetragonal [10]. In this paper, we compare the vibrational spectra of A2C60 and A4C60 salts in various crystal structures and their temperature dependences, in order to establish the factors influencing the symmetry of the molecular ions.…”