Electronic localization in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Rb</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>60</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>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\ifmmode\pm\else\textpm\fi{}100 K and a diamagnetic temperature-independent part ${\mathrm{\ensuremath{\chi}}}_{\mathit{d}}$=-3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ 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.