EFFECTS OF PRESSURE AND AMBIENT SPECIES ON ORIENTATIONAL ORDERING IN SOLID C-7O

The effects of hydrostatic pressure on the orientational ordering phas e transitions in solid C-70 were investigated by differential thermal analysis (DTA) using helium and nitrogen gases as pressure-transmittin g media. The different observed responses in these media for the diffe rent transitions and for different samples, as well as some irreversib le pressure/temperature cycling effects, have provided much insight in to the ordering processes and into the influence of interstitial latti ce impurities on these processes. Compression of the lattice strongly hinders the rotational motion of the C-70 molecules, especially their high-temperature tumbling motions, and results in stabilization of the ordered low-temperature phases. This feature reflects the weak van de r Waals intermolecular bonding and the high sensitivity of the rotatio nal barriers to intermolecular separation. In microscopic terms, the r esults can be understood in terms of pressure-induced enhancement of t he nesting of electron-rich and electron-poor regions of the C-70 mole cular cages. Observations of the evolution of DTA spectra during compr ession and as a function of time after release of pressure revealed a variety of nearly degenerate (in energy) configurations which are beli eved to be due to either hindered rotations of the molecules by inters titial impurities or to different stacking sequences of the monomolecu lar C-70 layers. The results are compared with earlier findings on sol id C-60.