Distortion and orientation of fulleride ions in A 4 C 60 G. Klupp, K. Kamarás, N. M. Nemes* +, C. Brown* +, J. Leao* Research Institute for Solid State Physics and Optics, P. O. Box 49, H-1525 Budapest, Hungary, *NIST Center for Neutron Research, Gaithersburg, Md , USA +Department of Materials Science & Engineering, University of Maryland, College Park, Md D 2h D 3d / D 5d Cs 4 C 60 Rb 4 C 60 K 4 C 60 C 60 D 2h D 3d / D 5d D 3d / D 5d I h A (a.u., baseline corrected)  Intermolecular t 1u  t 1u * [6]  Weaker in Cs 4 C 60 : electron hopping is more difficult because of the larger interfullerene separation.  Intramolecular t 1u  t 1g [6]  Splittings explained by group theory: two allowed transitions in D 3d and D 5d, three in D 2h : Near infraredIntroductionNeutron diffractionInelastic neutron scattering Experimental Funding Hungary: OTKA T US: NSF-INT D 5d D 3d D 2h T rise / Preparation: 4 A + C 60  A 4 C 60 dry box, 350 °C, day, 1-3 regrind  >95 % purity Near infrared: dynamic vacuum, LN 2 cooled cryostat transmission, Bruker IFS 66v/S Neutron diffraction: 4-350K Closed cycle refrigerator (CCR), NCNR BT1 Neutron Diffractometer, Cu(311) monochromator, λ = Å, 0.2Å -1 < Q <8.1Å –1 Inelastic neutron scattering: intramolecular vibrations: 8-320K CCR, NCNR BT-4 Filter Analyzer Neutron Spectrometer (FANS) Cu(220) monochromator (25-200meV incident energy), 60’-40’ collimation cooled polycrystalline Be and graphite filters, fixed final energy < 1.2meV librations: 8-320K CCR, NCNR BT-4 Triple Axis Spectrometer Cu(220) monochromator 28meV fixed incident energy, graphite filter, 60’-40’—40’-40’ collimation graphite(004) analyzer, constant Q momentum-transfer (typically 5.5 Å -1 ) No transition on cooling: always bct (I4/mmm) Disorder over two orientations at all T  At room T anion is D 2h in Cs 4 C 60, while D 3d or D 5d in Rb 4 C 60 and K 4 C 60  Transition to D 3d /D 5d in Cs 4 C 60 at high T  Consistent with Mid-IR results [4] Nonmagnetic Mott – Jahn – Teller insulator [1] Splitting of the t 1u MO by the Jahn-Teller effect in C :   Detected molecular distortions: Cs 4 C 60 (neutron diffraction): D 2h at 5 K, 293 K [2] Cs 4 C 60 (NMR): T = K: symmetry of molecule changes [3] A 4 C 60 (Mid IR) [4]: Same change of molecular distortion irrespective of the cation, though at different T:  Cs 4 C 60 (neutron diffraction) [2]: bct-bco phase transition accompanied by orientational ordering Is there a crystal structure change in K 4 C 60 and Rb 4 C 60 ? K 4 C 60 (NMR) [5]: T = 150 K: motion becomes static on NMR time scale T = 250 K: symmetry of motion changes What is the nature of this motion?        a a Conclusion Cs 4 C 60 vs Large cations bct-bco transition accompanied by anion orientational ordering [2] Fixed orientation at low T [3] Higher transition temperature [4] for the same molecular symmetry change More difficult electron hopping K 4 C 60 and Rb 4 C 60 Smaller cations No crystal structure change, only molecular distortions change Disorder over two orientations at low T Lower transition temperature [4] for the same molecular symmetry change Easier electron hopping References E MO IhIh D 5d D 3d D 2h // IntermolecularIntramolecular  Softening and broadening with increasing T  No signature of quasielastic scattering implies no free rotation of anions [7]  Rb 4 C 60 : same behaviour  The intensity of the peak maximum has a form factor that follows that expected for a librating C 60 [7]  The character of the librational peak becomes more translational at higher energy transfer Constant Q scans Constant E Scans  H u (1) mode shows splitting on cooling  Similar to splitting seen in Mid-IR [4] [1] M. Fabrizio and E. Tosatti, Phys. Rev. B 55, (1997). [2] P. Dahlke and M. J. Rosseinsky, J. Mater. Chem. 14, 1285 (2002). [3] C. Goze, F. Rachdi and M. Mehring, Phys. Rev. B 54, 5164 (1996). [4] K. Kamarás, G. Klupp, D. B. Tanner, A. F. Hebard, N. M. Nemes and J. E. Fischer, Phys. Rev. B 65, (2002), G. Klupp, F. Borondics, G. Oszlányi and K. Kamarás, AIP Conference Proceedings 685, 62 (2003). [5] V. Brouet, H. Alloul, S. Garaj and L. Forró, Phys. Rev. B 66, (2002). [6] M. Knupfer and J. Fink, Phys. Rev. Lett. 79, 2714 (1997). [7] J. R. D. Copley, D. A. Neumann, R. L. Cappelletti and W. A Kamitakahara J. Phys. Chem. Solids 53, 1353 (1992)  Librations: Intramolecular vibrations: t 1u e 2u + a 1u / e u + a 2u / b 1u + b 2u + b 3u (splitting) Splitting of the IR active T 1u vibrational mode