Cs-133 and C-13-NMR in CsC60 polymers under pressure.

Akali metal doped fullerides reveal a polymerized crystal structure with pa rallel chains of covalently bounded C-60 molecules. The electronic structur e shows a narrow pi-electron band. The existence of strong electron-electro n correlations in this fulleride is supported by the establishment of an an tiferromagnetic ground state at T-N=30K. We report the measurements of Cs-1 33 and C-13-NMR relaxation rates on CsC60 polymers which have been performe d versus pressure and temperature. Our results show that the hydrostatic pr essure suppresses the antiferromagnetic order and stabilizes a spin-singlet long range order at 5kbar. A study of the quadrupolar spin-echo of Cs-133 performed at ambient pressure reveals a second order structural transition at T-S=13.8K towards a spin-singlet ground state which coexists with the pr eexisting antiferromagnetic order.