MAGNETIC AND ELECTRONIC-PROPERTIES OF PURE AND ALKALI-DOPED C-60 CRYSTALS

The magnetic and electronic properties of oxygen-free, single-crystal and cesium compounds of C60 have been investigated. In non-doped crystals, a remarkable effect of oxygen absorption has been observed. The resistivity increases by a factor of 10(4) on absorption of oxygen, while the susceptibility shows a paramagnetic increase which obeys the Curie law. More than 8% O2 per C60 molecule is absorbed reversibly at ambient pressure and temperature, and is considered to be in the form of an O2 molecule but in a little ionized state. For the oxygen-free single crystal, the impurity level energy is obtained from the temperature dependence of the resistivity as 0.26 and 0.15 eV for T > T(c) and T < T(c), respectively, T(c) being the rotational disorder transition temperature. In CsxC60 (x = 0-4.5), the systematic study of susceptibility, electron spin resonance and X-ray diffraction reveals the existence of the Cs1C60 phase, in addition to x = 4 and 6 phases. CS1C60, which has the NaCl, is a metallic phase. Cs4C60 is an insulator with a paramagnetic moment of 0.7mu(B) per C60 molecule and a small antiferromagnetic interaction (theta = -5 K). The appearance of the magnetic moment in the multivalent C60 compound suggests that electrons in the conduction band are localized owing to the correlation effect.