Evidence for insulating behavior in the electric conduction of (NH3)K3C60 systems -: art. no. 096401

Microwave study using the cavity perturbation technique revealed that the conductivity of the antiferromagnet (NH3)K3-xRbxC60 at 200 K is already 3-4 orders of magnitude smaller than those of superconductors, K3C60 and (NH3)(x)NaRb2C60, 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.