ELECTRONIC LOCALIZATION IN RB4C60 FROM BULK MAGNETIC MEASUREMENTS

The dc-magnetization measurements were performed between 50 G and 40 kG using a superconducting quantum interference device magnetometer on powder samples of Rb4C60. The magnetic susceptibility reveals thermally activated behavior with an activation energy of 1200±100 K and a diamagnetic temperature-independent part χd=-3×10-4 emu/mol. By calculation of the related magnetic moments and by comparison with the NMR and EPR data we conclude that the additional electrons are localized on the C60 molecule and form a spin-singlet ground state. The temperature-dependent paramagnetism is due to thermally excited electrons. Strong electron correlations and lattice distortions (crystal field or/and Jahn-Teller effect) are discussed as a possible origin for the lowest unoccupied molecular orbital (LUMO) level splitting and for gap formation. The experimental results are in good agreement with the existence of Jahn-Teller or crystal-field distortions, providing the t1u LUMO level degeneracy is lifted. © 1995 The American Physical Society.