Fullerenes Under Pressure Studied by 13 C-NMR
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PASCALE AUBAN-SENZIER -, R. KERKOUJD -, D. JEROME -, F. RACHDI * AND P. BERNIER ** * Laboratoire de Physique des Solides (associ6 au CNRS), Universit6 Paris-Sud, 91405 Orsay, France "* Groupe de Dynamique des Phases Condens~es, UJniversit6 des Sciences et Techniques du Languedoc, 34060 Montpellier, France
ABSTRACT High pressure is an important parameter for the study of C'60 and doped fullerenes as these molecular crystals are very compressible. 13 C-NMR experiments under pressure in IK3 C6 o have given access to the determination of the 13C Knight shift and the chemical shift of this superconducting compound. These NMR data do not reveal significant effects of Coulomb correlations in Ka3 Co and support a pairing mechanism for superconductivity mediated by intramolecular vibrations. We report also a 13 C-NMR investigation of R64C 60 under pressure and temperature. The temperature dependence of the spin-lattice relaxation rate clearly shows, under pressure, the increase of a linear contribution which gradually substitutes to the exponential behaviour present at ambient pressure. The activated relaxation is attributed to intrinsic spin excitations through the direct Jahn-Teller gap whereas the closing of a small indirect gap under pressure gives rise to a semimetal and a Korringa like relaxation.
INTRODUCTION Molecular solids such as fullerenes compounds, C60 and AC 60 (where A is an alkali atom), are characterized by strong intra C60 covalent bonding together with weak intermolecular Van der Waals bonding. This explains the high compressibility measured on C60 (KQ ; 5.5. 10-(kbar)-') [1] and the slightly lower value measured on I 3 C60 (K, 3.57. 10-'3 (kbarf)-) [21 where the C60 lattice is stiffened by the intercalation of alkali atoms. By reducing intermolecular distances, the application of pressure is expected to play a significant role either on molecular dynamics or on electronic properties (namely the band structure, the density of states at the Fermi level, N(EF), and the superconducting transition. Tj)of these materials. It has been proved, for instance, by the experimental relation existing between T, and the lattice constant obtained through the study of a series of isostructural superconducting compounds with composition A3 _•BC 6o (where A and B are Na, K, Rb, Cs) [3] [4] and also through the variation of the lattice constant under
pressure [21 [5]. Thlie NMR technique is well adapted to study these two aspects and allows to work on air sensitive powders. In pure C60, 13 C-NMR properties, spin-lattice relaxation time, T1 , and spectral shapes, are dominated by molecular reorientations and structural properties. 261 Mat. Res. Soc. Symp. Proc. Vol. 359 0 1995 Materials Research Society
This has allowed us to determine thermodynamic data such as the enthalpies and volumes of activation in the two structural phases of C6o [6]. On the other hand, NMR data are also correlated to the electronic properties of doped fullerenes AXC 60 [7]. When alkali atoms are intercalated in the interstitial sites of the fa
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