NMR evidence for Mott-Hubbard localization in(NH3)K3C60

${}^{13}\mathrm{C},$ ${}^1\mathrm{H},$ and ${}^{39}\mathrm{K}-\mathrm{N}\mathrm{M}\mathrm{R}$ measurements of $\left({\mathrm{NH}}_3\right){\mathrm{K}}_3{\mathrm{C}}_{60}$ unambiguously demonstrated that its magnetic properties are described as a $S=1/2\mathrm{}$ localized spin system over the entire temperature range (300 K–4.2 K). A possible antiferromagnetic structure that appeared below 45 K is compatible with the orientational order of ${\mathrm{K}-\mathrm{N}\mathrm{H}}_3$ pairs on the octahedral (O) site. The present NMR study strongly suggests that the Mott-Hubbard localization occurs due to the removal of the ${\mathrm{C}}_{60}$ $t_{1u}$-orbital degeneracy.