Antiferromagnetism and Phase Diagram in Ammoniated Alkali Fulleride Salts

Intercalation of neutral ammonia molecules into trivalent face-centered-cubic (fcc) fulleride superconductors induces a dramatic change in electronic states. Monoammoniated alkali fulleride salts $\left({\mathrm{NH}}_3\right){\mathrm{K}}_{3-x}{\mathrm{Rb}}_x{\mathrm{C}}_{60}$, forming an isostructural orthorhombic series, undergo an antiferromagnetic transition, which was found by the electron spin resonance experiment. The Néel temperature first increases with the interfullerene spacing and then decreases for $\left({\mathrm{NH}}_3\right){\mathrm{Rb}}_3{\mathrm{C}}_{60}$, forming a maximum at 76 K. This feature is explained by the generalized phase diagram of Mott-Hubbard transition with an antiferromagnetic ground state.