Electronic localization in ${\mathrm{Rb}}_4$${\mathrm{C}}_{60}$ 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 ${\mathrm{Rb}}_4$${\mathrm{C}}_{60}$. The magnetic susceptibility reveals thermally activated behavior with an activation energy of 1200±100 K and a diamagnetic temperature-independent part ${\mathrm{\chi }}_{\mathit{d}}$=-3×${10}^{\mathrm{-}4}$ 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 ${\mathrm{C}}_{60}$ 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 ${\mathit{t}}_{1\mathit{u}}$ LUMO level degeneracy is lifted.