DETERMINATION OF C-13 NMR ISOTROPIC KNIGHT-SHIFT AND DEVIATION FROM BCS RELATION IN A(3)C(60) SUPERCONDUCTORS

We determined the isotropic C-13 NMR hyperfine coupling constant as a( iso)/2 pi=0.69+/-0.06 MHz for the C-60(3-) ion near room temperature ( RT). On this basis, the spin susceptibility at RT, chi(s)(RT), was est imated in various alkali-metal doped C-60 compounds A(3)C(60), which i s approximately proportional to the lattice constant a(0); we found d chi(s)/da(0)=(0.61+/-0.06)x10(-3) (emu/mole C-60)/Angstrom. The correl ation between the superconducting transition temperature T-c and chi(s )(RT) was determined. It was found that T-c of Na2RbC60, Na2KC60, Li2C sC60, and ammoniated A(3)C(60) is significantly lower than that expect ed from the BCS relation based on chi(s)(RT).