Effects of pressure and ambient species on the orientational ordering in solidC60

Abstract

The effects of hydrostatic pressure on the orientational ordering transition in solid ${\mathrm{C}}_{60}$ were investigated using helium, nitrogen, and a 50/50 (by volume) mixture of normal and isopentanes as pressure-transmitting media. The different observed responses in these media and for different samples, as well as some irreversible pressure cycling effects, have provided much insight into the ordering process. Highlights of the work include the following: (1) Lattice compression hinders the librational motion of the ${\mathrm{C}}_{60}$ molecules in the low-temperature simple cubic (sc) phase, thereby increasing ${\mathit{T}}_{\mathit{c}}$ and stabilizing the sc phase; the effect is large; (2) the pressure dependence of ${\mathit{T}}_{\mathit{c}}$, along with the accurately known volume change at the transition, yield 1.7 kcal/mole and 6.5 cal/K mole for the latent heat and discontinuous change in entropy, respectively, associated with the transition; (3) ${\mathit{T}}_{\mathit{c}}$ and its pressure dependence are influenced by the presence of ambient species in the interstitial lattice sites. Helium appears to diffuse readily into the lattice, whereas ${\mathrm{N}}_2$ (and ${\mathrm{O}}_2$) diffuse less rapidly and occupy octahedral sites; (4) a two-peak structure in differential thermal analysis spectra (also reported in specific-heat measurements) was observed and is believed to be due to the partial occupation of the octahedral sites by species such as ${\mathrm{N}}_2$ or ${\mathrm{O}}_2$; (5) solid ${\mathrm{C}}_{60}$ ‘‘reacts’’ with pentane at pressures ≥ 14 kbar and temperatures >475 K to yield an irreversible ${\mathrm{C}}_{60}$/pentane complex similar to that obtained by the cocrystallization of ${\mathrm{C}}_{60}$ and normal pentane at 1 bar. These results are discussed in terms of proposed models.