69,71Ga NMR studies of a superconducting Ga 84 cluster compound

We present ^69,71Ga-NMR experiments onmicrocrystalline samples of the recently discoveredsupramolecular compound Ga ₈₄ [ N ( SiMe ₃ ) ₂ ] ₂₀ Li ₆ Br ₂ ( thf ) ₂₀ ^. 2 toluene, which is composed ofligand-coordinated Ga₈₄ metal clusters,packed together in a fully ordered crystalline matrix. Thecompound is highly conducting and even shows superconductivitybelow T _c ~ 7.2 K. Our preliminaryresults between 10-300 K show a metallic-like behavior: thenuclear spin-lattice relaxation rate T ₁ ^-1 follows the Korringa law ⁶⁹ ( T ₁ T ) ^-1 = 0.36 s ^-1 K ^-1 , but with a relaxation rate approximately threetimes smaller than in bulk -Gametal. No quantum-size effects are observed, the Korringa lawbeing followed down to 10 K, whereas the quantum-gaps forindividual clusters should amount to ~10 ³ K. These results therefore suggest a transport process based onintermolecular charge transfer, similar as in alkali-dopedfullerenes and silicon-clathrates.