Order-disorder transition in solidC60charged withO2and withN2:A study with dielectric and13CNMR spectroscopies

Abstract

${\mathrm{C}}_{60}$ pellets, produced by uniaxial compaction and then charged with ${\mathrm{O}}_{2}$ or ${\mathrm{N}}_{2}$ under high pressure, have been studied with dielectric and ${}^{13}\mathrm{C}$ nuclear-magnetic-resonance spectroscopies. Oxygen was found to depress their fcc-sc transition temperature ${T}_{c}$ from the normal value of 260 K, as indicated by the minimum in the temperature-dependent permittivity ${\ensuremath{\varepsilon}}^{\ensuremath{'}},$ and also enlarge the drop in ${\ensuremath{\varepsilon}}^{\ensuremath{'}}$ at the transition. The magnitudes of the downward shift in ${T}_{c}$ and the drop in ${\ensuremath{\varepsilon}}^{\ensuremath{'}}$ were roughly proportional to the ${\mathrm{O}}_{2}$ occupancy at octahedral lattice sites, as estimated from the resonance heights distribution. We propose that interstitial ${\mathrm{O}}_{2}$ molecules not only exert a negative pressure effect on the ${\mathrm{C}}_{60}$ lattice, but also enhance order-parameter fluctuations below ${T}_{c}.$ In the case of nitrogen, in addition to a mobile (local) minimum in ${\ensuremath{\varepsilon}}^{\ensuremath{'}}(T),$ another that is fixed at 256 K, was also observed. The second immobile minimum reveals that parts of these pellets were free of ${\mathrm{N}}_{2},$ due, no doubt, to its lower diffusivity.