Mott-Hubbard transition in alkali ammonia fullerides

Monoammoniated alkali fullerides salt (NH 3 )K 3-x Rb x C 60 (0 ≤ x ≤ 3), forming an isostructual orthorhombic series followed by a Mott-Hubbard type antiferromagnetic transition. The Neel temperature T N was found to first increase with the interfullerene spacing and then decreases for (NH 3 )Rb 3 C 60 , forming a maximum at 76K. This behavior can not be understood by a simple localized electron model and provides direct evidence for the important role of electron correlation effects.