M. Haluska

Thermal expansion measurements of C60C70 mixed crystals

Abstract The results of thermal expansion and room temperature powder X-ray diffraction measurements on (C60)1−x(C70)x fullerene alloys are presented. Samples have been prepared by controlled condensation from the vapor phase. Strong anomalies in thermal expansion occur at the fcc to sc phase transition in C60-rich, and hcp to distorted hcp one in C70-rich crystals, respectively. The phase transition temperature diminishes with rising mixture ratio both on the C60-rich ( ∼ −4K 1% C 70 ) and on the C70-rich ( ∼ −20K 1% C 60 ) side of the phase diagram. This suggests the existence of an orientational glass phase on the C70-rich side of the phase diagram (in agreement with thermal expansion measurements on sample with 70% C70, which show no sign of the phase transitions at temperatures above 80 K).

Low-Frequency Elastic Response in Single-Crystal C60

The complex elastic constant of sublimation grown C60 single crystals is measured between 100K and 300K in the frequency range 0.6 Hz- 50 Hz. The order disorder phase transition at T I = 260K shows up in a negative dip-anomaly and a peak maximum in the real and imaginary parts of the complex elastic constant, respectively. A very slow relaxation (r = 0.2s) was measured near T I. In addition the elastic behaviour around the frequency dependent freezing transition at T II is measured and described by a single activated relaxation time t. From our data and the data of previous measurements we find, that the frequency shift of T II from 180 K to 80 K can be described by a simple Arrhenius behaviour over a frequency range of 11 decades.

Orientational Glass Transition in Solid C 60

The glass transition in both single- and polycrystalline C60 at about 90 K was studied using high-resolution capacitance dilatometry. This glass transition is most pronounced in samples which also have a sharp orientational-ordering transition at T≤260 K. The kinetics of the relaxation processes near the glass transition were obtained by fitting the experimental results with a simple glass- transition model. The relaxation time obeys an Arrhenius-behavior with Ea ≤ 288 meV over more than 10 orders of magnitude when including other published data.

Preparation of Single-Crystal Buckminster Fullerenes from Transport Reactions

Crystals of buckminsterfullerene were prepared from super-saturated solution in cyclohexane and by sublimation of powder. In both cases we obtained crystals with well expressed smooth and shiny faces. X-ray characterization showed that crystals prepared by sublimation were monocrystalline with long range order and crystals from solution were disordered. The crystals were further characterized by IR and Raman spectroscopy.