HEATS OF SUBLIMATION FROM A POLYCRYSTALLINE MIXTURE OF C60 AND C70

Knudsen effusion mass spectrometric measurements of vapors in equilibrium with a polycrystalline mixture of C60 and C70 were carried out over the temperature range 640-800 K. From the second-law method, average values obtained for the heats of sublimation of C60 and C70 from a polycrystalline C60 matrix were found to be respectively 40.1 +/- 1.3 and 43.0 +/- 2.2 kcal mol-1, at the average temperatures 707 and 739 K. It was also noted that it was necessary to heat treat the samples at temperatures of at least 170-degrees-C for greater than 12 h to achieve stable vaporization. This was consistent with the sample becoming more crystalline.