Microwave study using the cavity perturbation technique revealed that the conductivity of the antiferromagnet (NH(3))K(3-x)Rb(x)C(60) at 200 K 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 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.

We studied the method of complex-electrical-conductivity measurement for powders placed in a sealed tube at microwave and millimeter-wave frequencies, using a cavity perturbation technique. We applied this technique to a molecular superconductor K3C60, whose electrical conduction is well known in single crystals, and reproduced the metallic conductivity with the magnitude of 0.5–1.2 mΩcm at 20 K which is in good agreement with the data for single crystals. From this result, it was found that the cavity perturbation technique works well for the conductivity measurement of anaerobic powders, with respect to metallic materials.

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This paper describes the Ω oIrdestructive inspect ． ion me む hod Ibr inner 血 ult ofoon α ete pole by elastic i皿 pulse waves method ． Detected signals are analysed by delay t− i皿 e of propagt ， i ・