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College Park, Maryland      June 6 - 10 , 2004

WP64: Distortions and orientations of fulleride ions in K4C60, Rb4C60 and Cs4C60

N. M. Nemes (NIST Center for Neutron Research; Department of Materials Science and Engineering, University of Maryland), Gy. Klupp, G. Oszlanyi, K. Kamaras (Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, P. O. Box 49, H-1525 Budapest, Hungary), C. Brown (NIST Center for Neutron Research; Department of Materials Science and Engineering, University of Maryland), J. Leao (NIST Center for Neutron Research)

A4C60 compounds (A = K, Rb, Cs) are good candidates for the Mott-Jahn-Teller insulating state. We have previously shown that in these materials a splitting of vibrational lines occurs on cooling, the transition temperature depending on the counterion. We will now present new low temperature neutron diffraction, inelastic neutron scattering and near-IR data on K4C60, Rb4C60 and Cs4C60 to prove that the splitting of the vibrational and electronic excitations is not coupled to a structural phase transition; rather, the driving force behind this change is an interplay between the molecular Jahn-Teller effect and the crystal potential. Between room temperature and 5 K, Cs4C60 is orthorhombic while K4C60 and Rb4C60 are tetragonal; their low-temperature vibrational spectra are, however, identical. Since the molecular point group compatible with the crystal structure is the same in both space groups, the picture we suggest is orientational order in Cs4C60 and static disorder in K4C60 and Rb4C60 between the two standard orientations. With increasing thermal energy, the molecules start to rotate and induce a dynamic disorder. When the time scale of the rotation reaches that of the measurement, the apparent symmetry changes from biaxial to uniaxial. We will discuss this point by comparing our results to methods with different time constants (NMR and ESR).

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