Knudsen cell mass spectrometry and simultaneous Knudsen-torsion effusion technique were used to study K/C60binary system. Activities of potassium and/or C60were measured in all heterogeneous regions of the K-C60phase diagram and the Gibbs energies of formation of KxC60(x=l, 3, 4, 6) were calculated. Gaseous species KC60(g) was identified and the Gibbs energy of formation of KC60(g) was calculated directly from the experimental data.
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