  Microwave study using cavity perturbation technique revealed that the
conductivity of antiferromagnet (NH$_3$)K$_{3-x}$Rb$_x$C$_{60}$ at 200K is
already 3-4 orders of magnitude smaller than those of superconductors,
K$_3$C$_{60}$ and (NH$_3$)$_x$NaRb$_2$C$_{60}$, and that the antiferromagnetic
compounds are {\it insulators} below 250K 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.
