Ga. Samara et al., EFFECTS OF PRESSURE AND AMBIENT SPECIES ON ORIENTATIONAL ORDERING IN SOLID C-7O, Physical review. B, Condensed matter, 53(9), 1996, pp. 5211-5216
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.