Phase Transitions, Superconductivity, and Ferromagnetism in Fullerene Systems
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Figure 1. Logarithm of the NMR orientational correlation time for solid Cgoft in picoseconds) plotted against 11J. Two regions of activated rotation are clearly discerned. The inset shows a snapshot of the high-temperature rotator phase from Monte Carlo studies.7
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molecules rotate about some preferred axis. C6o and C7o satisfy the criteria for such orientationally disordered solids and exhibit rich phase behavior in the solid state.12 Since Cffl has high electron affinity, it forms anion salts with alkali and alkaline-earth metals as well as with strong organic donor molecules. With tetrakis dimethylaminoethylene (TDAE), which is a very powerful electron donor, C m forms a 1:1 solid that is ferromagnetic.3 C^-TDAE is the molecular organic ferromagnet with the highest Tc (of 16 K) known to date. Some of the alkali and alkaline-earth fullerides, on the other hand, show superconductivity, with transition temperatures going up to 33K.4 We shall briefly examine some of these solid-state properties. Phase Transitions At ambient temperature, Ceo, like solids of rare gas atoms such as argon or xenon, crystallizes in an fee structure. The molecules in the solid rotate at frequencies almost comparable to those in the gas phase. Cooling the solid slows down and restricts the free rotation. The low-temperature phase of C a is simple cubic.1 NMR spectroscopy permits one to probe the two regimes of rotational behavior as a function of temperature.5 Figure 1 shows the activation plot for the orientational correlation time, obtained from NMR spectroscopy. Above 260 K, the barrier to rotation is small, giving rise to a free rotator phase. Below this temperature, the rotational activation barrier increases to 3000 K, and the molecules ratchet between preferred orientations. Computer simulation studies have thrown much light on the phase behavior. The inset in Figure 1 shows a snapshot of the high-temperature phase of
Cm, obtained from Monte Carlo simulations.7 One can see that the centers of the molecules define a cubic lattice, but that the orientations of individual molecules are random. Infrared8 and Raman9 spectroscopies have been used to follow the effect of the ordering transitions on the intramolecular vibrational modes. Raman spectra across the transition show unusual cycling effects attributed to laser-induced electronic excitation.9 The effect of pressure on Cffl is to increase the transition temperature and enrich the phase diagram. There is evidence that application of pressure results in the observation of two phase transitions instead of one.10 Pressure also modulates the electronic properties of the solid. For example, with increasing pressure, the photoluminescence bandgap of solid CM decreases,11 associated with a decrease in activation energy for conduction. There is also much recent interest in pressureinduced structural changes in C«), including amorphization.12 Under nonhydrostatic pressure, Ceo can be converted into diamond and diamondlike carbon.14 The lower symmetry of C^ compared to C a gives rise to a
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