Orientational and magnetic transitions in ammoniated alkali metal fullerides

Monoammoniated alkali fulleride salts (NH3)K3-xRbxC60 (0 less than or equal to x less than or equal to 3), forming an isostructual orthorhombic series, show a Mott-Hubbard type antiferromagnetic transition. The Neel temperature T-N was found to first increase with the inter-fullerene spacing and then decrease for (NH3)Rb3C60, forming a maximum at 76K. Moreover, a structural phase transition, which is associated with the ordering of metal-NH3 pair, was found at 150K in all samples and a correlation between the magnetic and structural transition is expected. The identical structural transition temperature T-S is a striking contrast with T-N.