A. Dworkin

Fullerene C60, 2CCl4 solvate. A solid-state study

By slowly evaporating solutions of fullerene C60 in CCl4 at room temperature, a stable solvate (C60, 2CCl4) crystallizes in a simple hexagonal system (Laue class 6/mmm) with a=10.10(5) A and c = 10.75(5) A. This solvate undergoes a phase transition at 210–220 K and decomposes at 397 K. Atomic force microscopy of the (001) face shows a C60 molecular packing analogous to that which exists in the {111} planes of face-centered cubic C60. It seems likely that C60 and CCl4 molecules are orientationally disordered at room temperature.

Thermal studies of C60 transformed by temperature and pressure treatments

Raman spectroscopy, X-ray and electron diffractions have been applied to study samples of fullerene C60 after they have undergone pressure and temperature treatments up to 8 GPa and 1073 K. It is confirmed that mixtures of rhombohedral and tetragonal structures are formed in the 2–4 GPa and 673–1073 K pressure-temperature domain. Traces of a hexagonal phase are also observed in the same range. Only the rhombohedral one is obtained in the 4–8 GPa range in the same temperature interval. The material obtained at temperatures between 473 and 673 K in the whole pressure range which has previously been described in terms of a cubic structure can be understood in terms of an orthorhombic one. It may also contain mixtures of different phases including a cubic one and even a cubic superstructure. The widened poorly resolved X-ray profiles indicate the existence of disorder within the materials formed and the increasing complexity of Raman spectra as temperature increases at fixed pressures (from 2 to 8 GPa) suggests that this disorder is related to phase mixtures in almost all samples. A rhombohedral sample formed at 6 GPa-873 K reverted to a mixture of rhombohedral and tetragonal phases at 2.5 GPa-873 K. Thus thermodynamic equilibrium between these two kinds of systems could exist.

Molecular packing of fullerene C60 at room temperature

A single crystal of C60 grown from a toluene solution has been studied by X-ray diffraction at room temperature. The lattice is face-centered cubic, with space group Fm3m and Z = 4, which agrees with previous powder diffraction measurements. It is shown that, contrary to what is obtained in other plastic crystals, the Pauling-Fowler model (the so-called free rotation one), which implies an isotropic molecular disorder, gives a better description of the molecular packing than the site model does. It is concluded that the molecules undergo a rotational diffusion as previous molecular dynamics simulations have described.

Crystalline thiophene. II: a comprehensive study of stable and metastable phases by means of heat capacity, thermally stimulated currents and raman spectroscopy measurements

Abstract This calorimetric study of crystalline thiophene between 77 and 250 K reveals the existence of four phase transitions at 174.50, 170.49, 136.8 and 111.26 K, respectively. The transition II-III (170.49 K) is easily avoided during cooling, leading to metastable phases, which, in turn, exhibit their own phase transitions. These findings explain previously confused observations. The existence of stable and metastable phases was confirmed by thermally stimulated current measurements. Intermolecular Raman spectra were observed between 10 and 300 K and assigned to the observed phases. Some predictions are made concerning the low-temperature behaviour of thiophene. Tentative rules are proposed to number metastable phases relative to stable ones.

Heat capacity of a giant single crystal of C60

Abstract The heat capacity of a giant single crystal of C 60 (8×6×6 mm 3 ) was measured from 6 to 350 K by adiabatic calorimetry. A very sharp first-order phase transition arising from the orientational ordering of C 60 molecules was observed at 262.1 K, and a glass transition due to freezing of the reorientational motion of C 60 molecules was also found at 84.6 K. The transition temperature and the height of the heat capacity peak are much higher than any other calorimetric results reported previously. The transition enthalpy and entropy are also large in comparison with the values determined hitherto for a powder sample. These facts reveal that the phase transition of C 60 is strongly affected by crystallinity as well as purity. The C 60 giant single crystal used here is supposed to be free from these effects. The present results may be considered as reference thermodynamic values and behavior for crystalline C 60 .

Morphology of C60 decagonal crystals grown from n-hexane

Abstract Ten-sided prismatic crystals of C 60 grown by evaporation of n-hexane solutions have been characterized by scanning electron microscopy and X-ray diffraction. Various unusual morphologies have been observed. X-ray precession photographs confirm the decagonal symmetry and they are analyzed by means of twinning of monoclinic variants related by π/5 rotations. Possible twinning mechanisms are discussed.

The decagonal twinning in C60 crystals grown from n-hexane

The authors report results which unambiguously show that C60 crystals grown from n-hexane solutions (denoted C60-hexane) form twins and that the twinning mechanism may generate a tenfold symmetry. They also compare the characteristics of the C60-hexane crystals with those of C60-pentane

Cooling device for low‐temperature thermal analysis studies

The exhaust gas of a liquid nitrogen tank is pumped to force evaporation of the liquid in a chamber thermally linked to the measuring heads of a thermal analysis device (Perkin Elmer DSC 2C):cooling down to 80 K and long thermal treatments are currently performed.

Evidence for a low temperature phase transition in DI-μ-chloro-tetrakis (ethylene diamine) dinickel(II) chloride

Abstract The temperature dependence of the heat capacity for the title compound exhibits a broad peak between 10 and 27 K, with a maximum of 35.3 joule mole −1 K −1 at 22.8 K. This heat capacity variation cannot result only from the single-ion zero field splitting and the interaction between Ni(II) ions. It is suggested that a crystallographic phase transition occurs. Low temperature X-ray diffraction diagrams confirm that the compound undergoes a monoclinic → triclinic transition. The consequences of this transition on the interpretation of the magnetic measurements are discussed.