23 Na and 13 C NMR investigation of Na 2 C 60 and Na 6 C 60 fullerides

We report on Na and C NMR measurements on Na2C60 and Na6C60 compounds. The Na NMR spectrum of A2C60 shows one line at 73 ppm and the one of A6C60 presents two lines at 73 and 147 ppm. The intensity ratio of the latter lines is about 2:1. According topreviously reported x-ray data we attribute the line at 147 ppm to the Na tetramers in the octahedral sites and the line at 73 ppm to the Na cations in the tetrahedral ones which are singly occupied. The room-temperature C NMR spectra of Na2C60 and Na6C60 samples present a narrow isotropic line at 172 and 176 ppm, respectively. The Na6C60 resonance is shifted 20 ppm more down field than the resonances of A6C60 compounds with heavier alkalis, indicating a partial charge transfer to the threefold degenerate t1u level which is totally filled in the latter compounds.


Introduction
Alkali-metal fullerides have attracted large scientific interest due to their remarkable chemical and physical properties.For smaller ions such as Na, the alkali intercalation compounds have a unique characteristic.First, Na 3 C 60 does not superconduct 1,2 and second the small ionic radius permits intercalation of clusters of up to 8-9 Na ions into the octahedral sites, yielding compounds with x up to 11. 3 X-ray analysis indicates that Na fullerides preserve the fcc of the host lattice contrary to the A x C 60 (x≥4) compounds with heavier alkalis, where the alkalis cations expand the fcc structure of the host pristine C 60 and distort it into a bct or bcc structure with single occupation of both tetrahedral and octahedral sites. 4In this paper we present clear evidence, using 23 Na NMR of the formation of Na tetramers in the octahedral sites and single occupation of tetrahedral ones in Na 6 C 60 compounds.In Na 2 C 60 the single occupation of the tetrahedral sites and the absence of sodium in the octahedral ones is also evidenced.These results are in quite good agreement with x-ray data.

Experimental
The Na intercalation into C 60 using a vapor transport method has been described previously. 3Briefly, reactions were carried out in evacuated oxygen-free high conductivity copper tubes, which are readily vacuum sealed by swaging.We prepared several samples using predetermined amounts of C 60 and Na.We used higher annealing temperatures and longer times than usually needed for C 60 a e-mail : Ferid.The NMR signal was obtained as the Fourier transform of the free induction decay after a π/2 rf pulse.All 13 C resonances were referenced to tetramethylsilane (TMS) and the 23 Na ones to a 0.1 M NaCl solution.We have applied magic angle spinning (MAS) by using a home-built NMR probe head and a specially designed spinner which allows to spin the air sensitive samples in sealed NMR tubes with frequencies up to 3.5 kHz.Approximately 40 mg of each studied compound was filled in the NMR tube.

Results and discussion
In Fig. 1 we present room-temperature 13 C NMR spectra of Na 2 C 60 .The static spectrum shows a line with an axially symmetric shape, indicating that the C 60 molecules reorient rapidly about a given symmetry axis.The chemical shift tensor components derived from the static spectrum are σ ┴ = 176.8ppm and σ // = 157.9ppm.It was reported earlier that C 60 molecules in Na 1.3 C 60 compound rotate about the (111) axis. 5We believe this model to apply to Na 2 C 60 because the Na 1.3 C 60 corresponds to a mixing of Na 2 C 60 and pure pristine C 60 as we have checked and as has also been recently reported. 6The observed powder spectrum presents a chemical anisotropy extent (50 ppm) which is small compared to that of the saturated compounds with heavier alkalis (300 ppm). 8,9his indicates a significant motional narrowing in the Na . 10,11According to our ESR data (see the inset of Fig. 2) the thermal spin susceptibility behavior is of Pauli type in the temperature range of 100 to 300 K and also the linewidth shows a linear increase with temperature in the same range.This result seems to indicate that Na 6 C 60 is metallic at least in the investigated temperature range, contrary to the previously reported photoemission data indicating an insulating behaviour of this phase. 12NMR measurements of the thermal relaxation are in progress in order to check this assumption and will be published elsewhere.Also, we think that the observed large paramagnetic shift of the isotropic line of Na 6 C 60 is due to a strong polarization of the conduction electrons on the C 60 molecules.In contrast, as we reported earlier, the observed small shift of the isotropic lines of the A 6 C 60 (A=K, Rb, Cs) compounds which is only 13 ppm more paramagnetically shifted than the pristine C 60 resonance, is rationalized by considering the small energy gap between the filled t 1u states, highest occupied molecular orbital (HOMO) and the empty t 1g states, lowest unoccupied molecular orbital (LUMO).According to second order perturbation theory a Van Vleck paramagnetic shift should result which depends inversely on the HOMO-LUMO gap.
00047-p.3 Figure 3 shows a 23 Na static NMR spectra of Na 2 C 60 and Na 6 C 60 at room temperature.In the former spectrum only one line appears at 73 ppm which is significantly shifted from the resonance of NaCl solution used as a standard reference.The latter spectrum shows two lines one at the same position as the Na 2 C 60 resonance and a second line at 147 ppm.This result indicates the presence in the studied compounds structure of two magnetically inequivalent sodium sites.From a deconvolution of the two lines in the Na 6 C 60 spectrum we found that the intensity of the line at 147 ppm is twice that of the one at 73 ppm.According to the x-ray data1 which suggested a single occupation of the tetrahedral sites and tetramers in the octahedral ones, we attribute the line at 73 ppm to the sodium in the tetrahedral sites which are equivalent in both systems and the one at 147 ppm to the sodium in the octahedral ones.Our results are a good confirmation of the previously reported x-ray data in the studied compounds.As can be seen the observed 23 Na resonances are paramagnetically shifted with respect to the used diamagnetic reference, suggesting the presence of residual charge on the sodium in both tetrahedral and octahedral sites in the studied compounds.In the case of tetramers the observed shift which is about 12% of the Knight shift of the sodium (K=1230 ppm) indicating a significant hyperfine coupling of the sodium atoms in the octahedral sites with the residual electrons on the Na clusters or with the polarized electrons on the C 60 molecules.Our NMR and ESR results indicate that the Na tetramers manifest no quantum size or surface effects as should be expected in the case of small metal particles formation as we showed earlier in the zeolite NaY loaded with sodium. 13Therefore, we can exclude the presence of small metal aggregates in the octahedral sites and claim that the studied Na tetramers are strongly ionized in good agreement with LDA calculations. 14However, contrary to the latter calculations our results show lower charge transfer to the C 60 molecules compared to the saturated fullerides were the alkali atoms are almost totally ionized.Also, 23 Na NMR results show clearly that the sodium atoms in the octahedral sites are less ionized than the ones in the tetrahedral sites, leading to a smaller down field shift of the latter atoms.By heating the Na 6 C 60 sample under vacuum from 300 to 400 K (see Fig. 4), a collapse of the two 23 Na NMR lines was observed and only one line appears at 120 ppm at 400 K.The latter line is the average of the two lines observed at ambient indicating an averaging of the inequivalent sites due to a rapid motion of sodium atoms under thermal treatment.The observed behavior is totally reversible.

Conclusion
In this paper, we confirm in accordance with previously reported x-ray data the presence of Na tetramers in the octahedral sites in Na 6 C 60 and single occupation of the tetrahedral ones in both studied compounds.We also showed the absence of small metal particles in the former compound. 13C and 23 Na NMR results indicate clearly a weaker charge transfer in Na 6 C 60 compared to the saturated compounds with heavier alkalis.

Fig. 1 .Fig. 2 .
Fig. 1.Room-temperature 13 C NMR spectra of Na 2 C 60 : (a) the static spectrum and (b) the MAS NMR spectrum with spinning the sample at 3.2 kHz.The 13 C MAS NMR spectrum shows an isotropic line at 172 ppm and no other signals were observed indicating the absence of other phases in our sample.The isotropic line at 172 ppm is about 28 ppm more paramagnetically shifted than the pristine C 60 line (143.6 ppm), due to polarization of the transferred electrons on the carbons.This is in accordance with reported ESR results on Na 2 C 60 which show a significant electrons spin magnetization at room temperature due to a singlet-triplet equilibrium of the spin state of the dianions. 7Figure 2 shows 13 C NMR spectra of Na 6 C 60 at room temperature; (a) the static spectrum and (b) the MAS spectrum with spinning the sample at 3.2 kHz.
6 C 60 , contrary to the latter compounds where the C 60 molecules are totally blocked.We calculated chemical shift tensor components of the static Na 6 C 60 spectrum and found σ 11 = 165.1 ppm, σ 22 = 175.4ppm and σ 33 = 185.6 ppm which indicate an orientationally disordered phase.The 13 C MAS NMR spectrum shows an isotropic line at 176 ppm which is 20 ppm more down field shifted than in A6C 60 (A=K, Rb, Cs) compounds. 9This shift reflects a strong polarization of the transferred electrons and a weaker charge transfer to the C 60 molecules in Na 6 C 60 in agreement with Raman results which show that the shift of the Ag(2) mode is smaller than that of A 6 C 60 by 5 cm - 1

Fig. 3 .
Fig. 3. 23 Na static NMR spectra of Na 2 C 60 and Na 6 C 60 compounds obtained at room temperature.
13ur13C and23Na NMR measurements also indicate incomplete charge transfer in Na 6 C 60 compounds, contrary to A 6 C 60 (A=K, Rb, and Cs) fullerides.