1 NMR EVIDENCE FOR C 60 CONFIGURATIONAL FLUCTUATIONS AROUND Na SITES IN Na 2 CsC 60 P. Matus, 1,2 H. Alloul, 1 G. Kriza, 2 V. Brouet, 1 P. M. Singer, 1 S. Garaj, 3,4 and L. Forró 3 1 Laboratoire de Physique des Solides, UMR 8052, Université Paris-Sud, Orsay cedex, France, 91405 2 Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences, Budapest, Hungary, 1525 3 Laboratoire de Nanostructures et Nouveaux Matériaux Electroniques, Institute of Physics of Complex Matter, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland, 1015 4 Department of Physics, Harvard University, Cambridge, United States of America, MA 02138 The 23 Na nuclear magnetic resonance (NMR) spectrum, spin-lattice and spin-spin relaxation, as well as spin echo double resonance (SEDOR) are investigated in the ternary alkali fulleride compound Na 2 CsC 60 in the temperature range of 10 K to 300 K. The NMR line associated with the tetrahedral sodium site is split below 170 K (T and T' lines) similarly to Rb 3 C 60 although the crystal structures of these two materials are different. SEDOR measurements prove that the T and T' sites are microscopically close. The merger of the two lines at about 170 K is attributed to motional narrowing resulting from a site exchange due to angular reorientations of the C 60 molecules. The exchange dynamics inferred from the spectra, spin-spin relaxation, and spin-lattice relaxation are all consistent and agree with inelastic neutron scattering, supporting our proposal that the observed T-T' splitting originates from different local fullerene configurations around the tetrahedral alkaline sites. Keywords: alkali fullerides, nuclear magnetic resonance, chemical exchange 1. INTRODUCTION The alkali intercalated fullerene compounds A n C 60 (A = alkali metal) have very rich physics and have been on the focus of scientific interest for more than a decade [1,2]. One of the oldest puzzles in the NMR spectroscopy of fullerenes is the so-called “T' problem” [3] described below. In alkali fullerides, alkali ions are inserted in the interstitial sites of the face-centered-cubic (fcc) host lattice of the pristine fullerene by standard chemical doping [4]. There are two different interstitial sites with octahedral (O) and tetrahedral (T) C 60 coordination, with stoichiometric ratio of O : T = 1 : 2. The site occupied depends on the alkali dopant, the octahedral sites being preferred by the larger ions [5]. Surprisingly, in A 3 C 60 (A = K, Rb) salts, NMR measurements show a splitting of the spectrum line associated with the tetrahedral site at low temperatures [3,6,7,8]. This T-T' splitting coincides with the broadening of the 13 C line [7] suggesting a relation between C 60 motion and the appearance of the T' peak [9]. However, despite numerous efforts, the origin of the T' line is still unclear [ 10 , 11 ]. Recently, we demonstrated the existence of a T' peak in Na 2 CsC 60 and attributed it to different local fullerene environments of the tetrahedral sites [12]. This assignment is based on the finding that Na 2 CsC 60 is isostructural with pure C 60 [5], in contrast to the merohedrally disordered [4] A 3 C 60 family, so that a first order phase transition (face centered cubic- simple cubic, fcc-sc) occurs near room temperature. In the sc phase the local structure is better defined than in the merohedrally disordered cases which helps us in assigning the splitting of the tetrahedral line to a difference in the local fullerene environment. In this paper we provide some additional support to this suggestion by investigating the temperature dependence of the 23 Na NMR spectra, spin-lattice and spin-spin relaxation as well as spin echo double resonance. 2. EXPERIMENTAL The Na 2 CsC 60 powder sample was prepared by the conventional solid state technique [5] and sealed in a quartz tube under He pressure. Sample purity was checked by x- ray and magnetization measurements. The superconducting transition temperature is T c = 12 K. The free induction decay (FID) NMR spectra were observed with a home-built spectrometer in B 0 = 7.5 T applied magnetic field (the corresponding 23 Na Larmor frequency: 84.4 MHz). The T 1 and T 2 measurements are performed by saturation-recovery and π/2-τ-π spin-echo technique, respectively. 3. RESULTS AND DISCUSSION 3.1 23 Na spectra and spin-echo double resonance (SEDOR) The temperature dependence of the sodium NMR spectra in Na 2 CsC 60 shown in Fig. 1 has two main characteristics: (1) a discontinuous change of the spectra at the 300-K first order fcc-sc phase transition [5] and (2) the splitting (T and T') of the tetrahedral NMR line below 170 K similarly to other A 3 C 60 salts [8]. The spectral weight of the T' line at low temperatures is 29 ± 5%. The possibility that the T' line emerges from a separate phase with different structure in the material can be excluded by SEDOR experiments demonstrated in Fig 2. This technique enables us to detect if a dipole-dipole coupling between the nuclei of the two sites occurs, and therefore if these nuclei are spatially close to each other. In the SEDOR experiment the B 1 field is set sufficiently low to restrict the frequency window of irradiation to one of these lines [13]. During the measurement, a π/2-τ-π spin-echo sequence is used at an “α” frequency, and simultaneously with the second pulse of the echo sequence, another pulse is applied at a “β” frequency. The α frequency is swept with small increments in the frequency range of the original