X-ray diffraction has been employed to study the low temperature structural behavior of C60 crystals. The intensity (I) of selected reflections which appear at the facecentered cubic-simple cubic transition was measured as a function of temperature down to 23 K. This orientational ordering transition is found at To " 259 +1 K. It displays a hysteresis of about 1 K but no discontinuity of the intensity at ToAnomalies in d(I)/d(T) around 155 K and 85 K are clearly revealed. They are associated with changes in the C60 molecular reorientations and their origin is discussed. (*) URA 2. (**) URA D1104. (***) URA 1110. (****) URA 233. 512 JOURNAL DE PHYSIQUE I N°5 The understanding of the orientational ordering mechanism of the C60 molecules at low temperature in pure fullerene C60 remains an important issue for this new class of molecular solids. Combining results from various types of measurements it appears that the details of the molecular reorientations ant their associated dynamics are rich and still partly unravelled. At room temperature, X-ray diffraction studies established that crystalline C60 adopts a facecentered cubic (fcc) structure where the molecules are orientationally disordered [l~ 2]. This results from rapid and continuous rotations of the C60 truncated icosahedra, as shown by ~~C NMR [3, 4] and recent quasielastic neutron scattering measurements [5]. Differential scanning calorimetry [2, 6] and X-ray powder diffraction [2] revealed a first-order transition near 250 K associated with a symmetry change from fcc to simple cubic (sc) at 250-260 K. The structure of the low temperature phase has been studied by both high resolution X-ray [2] and neutron [7, 8] powder diffraction methods. From these results the following picture for the orientational ordering of the C60 molecules has been proposed [7, 8]: below the firstorder transition at To * 260 K the C60 molecules perform rapid rotational jumps about one or several directions (their frequency h 10-100 kHz at 160 K [3, 6, 9]). Two inequivalent orientations of the molecules, related by 60° rotations about , and corresponding to anticlockwise rotations # of 38° and 98° with respect to the ideal Fmlm configuration, appear to be energetically preferred. A frequency dependent elastic anomaly in the sound velocity and attenuation at m 160 K supports the existence of 2 such energy minima [9]. As the temperature is reduced the orientation that optimizes the electrostatic interactions (# = 98° is favored at the expense of that 11 = 38°) which minimizes the van der Waals contribution to the free energy. Furthermore, evidence for a possible anomaly in the cell parameter near 155 K has been reported. It was tentatively associated with a ihange from multi rotation axes (T > 155 K) to a single rotation axis (T < 155 K) for each molecule. Finally, at about 90 K~ the cell parameter variation exhibits a cusp which was attributed to the freezing of the rotational degrees of freedom of the molecules. In this paper we present the results of a X-ray single-crystal study of the orientational ordering transition. This first-order transition is found at To = 259 +1 K and it exhibits a hysteresis of about I K. Of particular interest is the first report of anomalies in the slope of the superstructure reflection intensity I(T) near 155 K and 85 K. The starting C60 material was extracted from arc-generated carbon soot with hexane-toluene mixtures and purified by column chromatography on neutral alumina. Characterization by chromatography, infrared and mass spectroscopy indicated no trace of impurity. Single crystals, shaped as distorted octahedra, were grown by slow evaporation from toluene solutions of pure C60. Typical dimensions were of the order of100 pm. Characterization by X-ray precession photography and 4rcirde diffractometry confirmed the fcc cubic symmetry with a = 14,15 I. The precession photographs also reveal a pattern of diffuse lines running along directions and which can be attributed to close-packed stacking faults in the fcc lattice. Such faulting has been previously observed, in particular in C60 films [11]. X-ray intensity measurements were performed using a home-built three-axis diffractometer (lifting-detector geometry) consisting of a rotating anode generator (CuKa, fine focus~ 55 kV, 20 mA), a doubly-bent graphite (002) monochromator and a linear position sensitive detector (PSD). The sample was attached to the cold end of a modified Displex cryogenerator. Good thermal stability (0.01 K) was assured by a flow of helium gas and the temperatures were accurate to within 0.I K. In a preliminary stage, oscillation photographs were made between 295 K and 25 K in order to probe extended regions of the reciprocal lattice. The fcc-forbidden superstructure reflections clearly appear on photographs taken at 250 K and below. Apart from the intensity buildup of these reflections as T was lowered, no further modification (such as lattice distortion or new N°5 PHASE TRANSITIONS IN C60 513 satellite reflections) could be found. No attempt was made to follow the lattice parameter

Well-developed surfaces of crystals of pure C60 grown in various experimental conditions have been examined by atomic force microscopy. To out knowledge, macroscopic “pure” faces of bulk crystals have not been studied by near-field microscopies. The crystals obtained by sublimation or by rapid evaporation of C60 solutions in toluene have a fcc structure. By slow evaporation of fullerene C60 solution CCl4 at room temperature, a stable solvate C60, 2CCl4 crystallizes in a simple hexagonal system, with a flat large (0001) surface. The morphological habits of the fcc crystal show macroscopically well-defined (100) and (111) faces, with some examples of faces which cannot be attributed unambiguously. For the first time, AFM images have been obtained for all these faces. In every case, molecular resolution has been achieved. The (100) face of the fcc crystal, which has been shown by SEM to accumulate impurities, appears at a molecular scale to be constituted by a juxtaposition of micro domains, each of which having all the angle and distance characteristics of the (100) face. The AFM images demonstrate directly that the molecular arrangement is the same on the (111) face of the fcc crystal of pure C60 as on the (0001) face of the hexagonal crystal of the C60, 2CCl4 solvate. Measurements on AFM images of a face that could not be indexed give results which induce us to assign tentatively this surface to the (311) face of the crystal.

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) angstrom and c = 10.75(5) angstrom. 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.

The plastic c.f.c. phase of quinuclidine is simulated, at different temperatures, through a Monte Carlo method which uses the compatibility matrices technique. The mean symmetry of the lattice is constrained not to change and the mass centres of the molecules are assumed to be fixed. Molecular orientations are selected at random among the 48 discernible orientations of the c.f.c. space group. So, a previous memorizing of the interaction energy of neighbouring molecules is possible and the computing time is greatly shortened. A freezing of the reorientations is observed at low temperatures and the crystal symmetry becomes monoclinic. It is interpreted in terms of a phase transition whose temperature (215 K) and energy increment (5 kJ mol-1) are in good agreement with the experimental results.

La phase plastique (c.f.c.) de la quinuclidine est étudiée à différentes températures par la méthode de Monte Carlo utilisant la technique des matrices de compatibilité. Afin de ne pas modifier la symétrie moyenne du réseau, les centres de masse de molécules sont supposés fixes. Les orientations moléculaires sont choisies de façon aléatoire parmi les 48 orientations équivalentes et discernables du groupe c.f.c. Cela permet une mémorisation préalable des énergies d'interaction entre molécules voisines et un gain de temps de calcul considérable. Nous observons alors un blocage des réorientations moléculaires à basse température, tandis que la symétrie cristalline devient monoclinique. Ceci est interprété en termes de transition de phase dont la température (215 K) et la variation d'énergie (5 kJ mol-1) sont très proches des valeurs expérimentales.

Deuterated thiophene has been studied by powder neutron diffraction spectroscopy from ∼ 60 K to melting temperature. Preliminary DSC measurements have shown that the thermodynamic behaviours of hydrogenated and deuterated thiophene are almost identical, so that the conclusions drawn from the present structural studies of C4D4S are valid for both compounds. The structures of stable phases IV and V have been observed for the first time: phase V lattice is obtained by multiplying by 2 parameter c of orthorhombic phase III and has been assigned space group P21ma; phase IV also corresponds to a superstructure of phase III, but, as expected because of the quasi-fivefold molecular symmetry and of its reorientational dynamics, it can be thought to be an incommensurate phase.

A well-developed surface of a decagonal C 60 crystal grown from n -hexane has been imaged directly by STM with molecular resolution. Fairly regular patterns have been obtained over large surface areas. However, no internal structure of individual carbon molecules could be observed. This means that some orientational motion persist in the molecular packing of the decagonal crystal as also demonstrated in cubic C 60 crystals.

X-ray diffraction and thermodynamic measurements show that crystalline fullerene C70 obtained from toluene solutions is a C70, toluene 1: 1 solvate. Its structure is orthorhombic with space group Pbca (a= 21.075 angstrom, b= 32.990 angstrom, c= 10.844 angstrom). The evolution of this solvate towards the face-centered cubic and hexagonal phases of pure C70 is described and a hierarchy of relative stabilities of these latter phases is tentatively proposed.

Ten-sided prismatic crystals of C60 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.

X-ray diffraction has employed to study the low temperature structural behavior of C60 crystals. The intensity (I) of selected reflections which appear at the face-centered cubic-simple cubic transition was measured as a function of temperature down to 23 K. This orientational ordering transition is found at T0=259±1 K. It displays a hysteresis of about 1 K but no discontinuity of the intensity at T0. Anomalies in d(I)/d(T) around 155 K and 85 K are clearly revealed. They are associated with changes in the C60 molecular reorientations and their origin is discussed.

C60 fullerene may crystallize from benzene solutions as thin yellow plates, whose morphology has suggested the existence of a hcp modification. By means of single crystal X-ray diffraction, we show that these plates crystallize in the cubic system (space group Fm3m, a = 14.155 angstrom). No trace of solvent could be observed by TGA and DSC experiments. Surprisingly, these plates do not exhibit the phase transition near 260 K. Nevertheless, the existence of a hcp variety cannot be discarded since scanning electron microscopy reveals the presence of hexagonal prisms in a sample made of pure C60 powder.

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.

X-ray measurements reveal a tenfold symmetry in a single crystal of fullerene C60 grown from a n-hexane solution.

Decagonal C60 crystals grown from n-hexane solutions correspond to an orthorhombic 1:1 solvate (a=10.249 Å, b=31.308 Å, c=10.164 Å). It forms with negative excess volume (−55.5Å3 per formula unit) and transforms on heating into fcc C60 (desolvation enthalpy of +50.6 kJ per solvate mole, close to the sublimation enthalpy for pure n-hexane) while n-hexane desorption from fcc C60 is accompanied by an enthalpy of +48.6 kJ per solvent mole. Thus solvate formation is preferred to solvent adsorption. Orthorhombic C60·1n-hexane undergoes no degradation when stored in air for 9 years at room temperature in the dark.

X-ray measurements reveal a tenfold symmetry in a single crystal of fullerene C60 grown from a n-hexane solution. Among various hypotheses, that of a quasicrystal is considered.

Thermodynamic, dynamic and structural properties of crystalline thiophene are reviewed. Calorimetric, NMR and dielectric spectroscopies reveal in both phase sequences that there exists an in-plane non-perfect five-fold reorientational motion which persists in all phases, with only slight modifications at phase transitions. This motion ultimately freezes on cooling, thus leading to a glassy crystalline state below ∼40 K in both sequences. Incommensurate phases have been found in the metastable phase sequence: they are related to the same molecular motion. The existence of incommensurability in the stable sequence and the nature of the expected commensurate-incommensurate phase transitions are discussed. It is concluded that the glass transitions are due to a symmetry departure and that the pseudo-pentagonal molecular symmetry is the origin of the dynamics-driven incommensurability.

A proton wide-line N.M.R. study of crystalline thiophene has been undertaken using a high sensitivity F.T. spectrometer. Both stable and metastable phase sequences have been observed and phase transitions have been detected in very good agreement with our previous calorimetric studies [1]. Theoretical second moments have been calculated using the X-ray scattering data of Andre et al. [2]. Given the structural models, we have shown that the many different allowed configurations lead to a wide distribution of theoretical intermolecular second moments, suggesting very different local steric hindrances. All the known crystalline phases, from crystallization temperature down to 120 K and presumably lower, are orientationally disordered. The nature of this disorder was proven to be the in-plane molecular reorientation. The frequency of the motion is commensurate with the local field in a very wide range of temperatures indicating a wide distribution of correlation times. We suggest that this could be a signature for glassy crystalline-forming materials Etude par RMN a raies larges du thiophene cristallin a l'aide d'un spectrometre a haut champ et transforme de Fouria. Observation des sequences stable et metastable, ainsi que des transformations de phases. Calcul des seconds moments theoriques a l'aide des donnees radiocristallographiques de Andre et al.