Orthorhombic fulleride (CH3NH2)K3C60 close to Mott-Hubbard instability : ab initio study

We study the electronic structure and magnetic interactions in methylamine-intercalated orthorhombic alkalidoped fullerene (CH3NH2)K3C60 within the density functional theory. As in the simpler ammonia intercalated compound (NH3)K3C60, the orthorhombic crystal-field anisotropy Delta lifts the t(1u) triple degeneracy at the Gamma point and drives the system deep into the Mott-insulating phase. However, the computed Lambda and conduction electron bandwidth W cannot alone account for the abnormally low experimental Neel temperature, T-N = 11 K, of the methylamine compound, compared to the much higher value TN = 40 K of the ammonia one. Significant interactions between CH3NH2 and C-60(3-) are responsible for the stabilization of particular fullerene- cage distortions and the ensuing low-spin S = 1/2 state. These interactions also seem to affect the magnetic properties, as interfullerene exchange interactions depend on the relative orientation of deformations of neighboring C-60(3-) molecules. For the ferro-orientational order of CH3NH2-K+ groups we find an apparent reduced dimensionality in magnetic exchange interactions, which may explain the suppressed Neel temperature. The disorder in exchange interactions caused by orientational disorder of CH3NH2-K+ groups could further contribute to this suppression.