Evidence for Insulating Behavior in the Electric Conduction of(NH3)K3C60Systems

Microwave study using the cavity perturbation technique revealed that the conductivity of the antiferromagnet $\left({\mathrm{NH}}_3\right){\mathrm{K}}_{3-x}{\mathrm{Rb}}_x{\mathrm{C}}_{60}$ at 200 K is already 3–4 orders of magnitude smaller than those of superconductors, ${\mathrm{K}}_3{\mathrm{C}}_{60}$ and $({\mathrm{NH}}_3{)}_x{\mathrm{NaRb}}_2{\mathrm{C}}_{60}$, and that the antiferromagnetic compounds are insulators below 250 K without metal-insulator transitions. The striking difference in the magnitude of the conductivity between these materials strongly suggests that the Mott-Hubbard transition in the ammoniated alkali fullerides is driven by a reduction of lattice symmetry from face-centered-cubic to face-centered-orthorhombic, rather than by the magnetic ordering.