69,71Ga NMR studies of asuperconducting Ga84 clustercompound

We present 69,71Ga-NMR experiments on microcrystalline samples of the recently discovered supramolecular compound Ga84[N(SiMe3)2]20Li6Br2(thf )20 . 2toluene, which is composed of ligand-coordinated Ga84 metal clusters, packed together in a fully ordered crystalline matrix. The compound is highly conducting and even shows superconductivity below Tc ~ 7.2 K. Our preliminary results between 10-300 K show a metallic-like behavior: the nuclear spin-lattice relaxation rate T1-1 follows the Korringa law 69(T1T)-1 = 0.36 s-1K-1, but with a relaxation rate approximately three times smaller than in bulk α-Ga metal. No quantum-size effects are observed, the Korringa law being followed down to 10 K, whereas the quantum-gaps for individual clusters should amount to ~ 103 K. These results therefore suggest a transport process based on intermolecular charge transfer, similar as in alkali-doped fullerenes and silicon-clathrates.