Funding OTKA T Fulleride ions in various crystal fields studied by infrared spectroscopy G. Klupp, K. Kamarás Research Institute for Solid State Physics and Optics, P. O. Box 49, H-1525 Budapest, Hungary, Introduction to Jahn—Teller effect in fullerides D 5d D 3d D 2h Na 2 C 60 and A 4 C 60 (A=K, Rb, Cs) are nonmagnetic Mott – Jahn – Teller insulators [1]. In C 60 n- coupling of the t 1u electrons with H g vibrations leads to p n  8H g Jahn—Teller systems. [2] The I h symmetry of C 60 can be distorted by this coupling to [2]: E MO t 1u e 1u + a 2u e u + a 2u b 1u + b 2u + b 3u The shape of the C is prolate, the shape of C is oblate. [3] On the warped APES [4] either the D 3d distortions are minima and the D 5d maxima, or vice versa. The D 2h distortions are always saddle points. [5, 6] In isolated C 60 n- the Jahn—Teller effect is dynamic, pseudorotation takes place [4]: In crystals the crystal field of the cations can disturb the pseudorotation and static Jahn—Teller effect can appear [4]. The crystal field can even dominate the distortion. The C in orthorhombic Cs 4 C 60 was found to be D 2h by neutron diffraction. [7] The reduced symmetry leads to the following splittings of the HOMO: [1] M. Fabrizio and E. Tosatti, Phys. Rev. B 55:13465, [2] C. C. Chancey and M. C. M. O’Brien, The Jahn-Teller effect in C 60 and Other Icosahedral Complexes, Princeton University Press, Princeton, [3] A. Auerbach, N. Manini and E. Tosatti, Phys. Rev. B 49:12998, [4] S. Tomita, J. U. Andersen, E. Bonderup, P. Hvelplund, B. Liu, S. Brondsted Nielsen, U. V. Pedersen, J.Rangama, K. Hansen and O. Echt, Phys. Rev. Lett. 67:1886, [5] A. Ceulemans, J. Chem. Phys. 87:5374, [6] M. C. M. O’Brien, Phys. Rev. B 53:3775, [7] P. Dahlke and M. J. Rosseinsky, Chem. Mater. 14:1285, Splitting of the IR active T 1u vibrational modes: T 1u E 1u + A 2u E u + A 2u B 1u + B 2u + B 3u Infrared spectra of A 4 C 60 and Na 2 C 60 at various temperatures Cubic crystal field averaged out by rotation [11]: Weak tetragonal (D 4h ) crystal field [9, 10] with reorientation [8]: Strong tetragonal (D 4h ) crystal field [8] with reorientation [8]: Strong orthorhombic (D 2h ) crystal field [7] without reorientation: [8] G. Klupp, K. Kamarás, N. M. Nemes, C. M. Brown and J. Leao, Phys. Rev. B 73:085415, [9] R. M. Fleming, M. J.Rosseinsky, A. P. Ramirez, D. W. Murphy, J. C. Tully, R. C. Haddon, T. Siegrist, R. Tycko, S. H. Glarum, P. Marsh, G. Dabbagh, S. M. Zahurak, A. V. Makhija and C. Hampton, Nature 352:701, [10] C. A. Kuntscher, G. M. Bendele and P. W. Stephens, Phys. Rev. B 55:R3366, [11] T. Yildirim, J. E. Fischer, P. W. Stephens and A. R. McGhie, Progress in Fullerene Research, p. 235, Threefold splitting of T 1u  C : D 2h Twofold splitting of T 1u  C : D 3d or D 5d Twofold splitting of T 1u  C : D 3d or D 5d Discussion orthorhombic Cs 4 C 60 : The distortion found in [11]: Low temperature tetragonal K 4 C 60, Rb 4 C 60 : D 2h distortion  dominated by crystal field the crystal field is the largest where the K-C distances are the smallest, ie. in the c direction  a possible distortion can be “distortion A“: c a c a The effect of strong crystal field: Static D 2h distortion dominated by the crystal field. The effect of heating: The distortions on the graphs correspond to distortions along the axes shown on the molecule. B B C C D D D D A Heating BBBBBBBB CC CC CC CC DD DD DD DD A A A Cs 4 C 60 : The phase transition between 293 and 623 K [11] can lead to both static and dynamic distortion. K 4 C 60, Rb 4 C 60 : gradual decrease of crystal field due to thermal expansion and the gradual occupation of higher energy levels. If “distortion A“ is present at low temperature, then: If ”distortion A” is not the one present at low temperature, then on heating the first step is to move from the low temperature D 2h distortion to “distortion A“. The dark blue atoms are pushed closer to the center of the molecule. In the shown case the isolated molecule has D 5d minima. The scheme is analogous for D 3d minima. At low temperature the lowest minimum is that of "distortion A". The farther the axis of the distortion from the crystallographic c axis, the higher the energy of the distortion. At low temperatures only the lowest energy levels are occupied, which is the reason why the D 2h distortion appears. On heating the D 2h minimum arising from the crystal field weakens. At the same time the differences between the D 5d distortions gradually disappear. This, together with the occupation of higher lying energy levels, leads to the appearance of D 3d /D 5d distortions besides the D 2h distortion. This way on heating the molecule first starts to pseudorotate between "distortion A" and the two "distortion B"-s, meaning a static-to-dynamic transition. The confinement of the pseudorotation then gradually decreases as the temperature rises, to the state where, in the case of an averaged out crystal field, it is free. This is the case of the high temperature Na 2 C 60. As the distortion of the molecules can be detected with IR spectroscopy:  pseudorotation >  vibration