M.A. Verheijen

The structure of different phases of pure C70 crystals

Abstract Single crystal of pure C70 are grown from the vapour phase and the structure and morphology of these crystals is studied. By means of X-ray diffraction and TEM measurements five different phases are observed. The observed phases are (from high to low temperatures) fcc, rhombohedral, ideal hcp (c/a=1.63), deformed hcp (c/a=1.82) and a monoclinic phase. The occurrence of these different phases and the phase transitions is accounted for in a simple model. For the monoclinic structure a model for the stacking of the orientationally ordered molecules in the lattice is proposed. For both the hcp and fcc phases a Lennard-Jones type interaction potential is used to calculate bond strengths, lattice energies and the theoretical morphology.

Growth and morphology of c60 crystals

Abstract Single crystals of pure C 60 are grown from the vapour phase. The crystal morphology is compared to the theoretical equilibrium form, predicted on the basis of a periodic bond chain analysis and a statistical mechanical approach. Some interesting growth phenomena are observed and discussed.

Raman scattering in single crystal C60

Raman spectra (using 514 and 740 nm excitation) of high-purity single crystal C60 are presented for the high- and low-temperature phases, showing activity of all “gerade” modes. Within the experimental accuracy, the spectra are found to be consistent with the selection rules for Raman scattering in both the high-temperature fcc and low-temperature 2a0-fcc phases of solid C60. A complete assignment of the observed peaks is proposed.

Hexagonal close-packed c60

CeO crystals were grown from purified powder material with a multiple sublimation technique. In addition to crystals with a cubic close-packed (ccp ) arrangement, crystals were found with a hexagonal close-packed (hcp ) structure. Detailed crystallographic evidence is given, including complete refinements, of the room-temperature structures of both polytypes. The radius of the CsO molecule was determined as 3.54 1 ( 1) A, and was found to be equal for both ccp and hcp crystals.

STRUCTURAL PHASE-TRANSITIONS IN C-70

Cubic as well as hexagonal single crystals of C70 have been grown and investigated by electron diffraction, electron microscopy and x-ray diffraction. Several phase transitions have been detected and crystallographic models are proposed. Hexagonal crystals, stable at room temperature with c/a = 1.63 will undergo two transitions, upon cooling. First the c/a ratio will increase to 1.82 owing to c-axis alignment of the molecules; at a lower temperature the molecules will orientationally order, resulting in a monoclinic structure.

LOW-TEMPERATURE STRUCTURE OF SOLID C-70

The structure of the low-temperature phase of hexagonal close-packed (hcp) grown C70 is determined from single-crystal X-ray diffraction at 220 K and 100 K. An ordering of the molecules is found on the orthohexagonal supercell of a hcp structure with symmetry Pbnm. It involves alignment of the molecules with their long axis parallel to the original hexagonal axis, and a shift out of the original hcp positions. Intermolecular contacts are shown to be different from those found in C60. The crystal is determined to contain stacking faults resulting in the presence of AC and BC deformed hcp arrangements in addition to the original AB stacking. The stacking disorder is shown to be independent of the phase transitions, and presumably is an intrinsic property of hexagonal C70 crystals.

Growth phenomena on C60 crystals

Abstract The {111} and {100} faces of C 60 crystals grown from the vapour phase have been studied by phase sensitive optical microscopy. On these faces growth steps, dislocation outcrops, epitaxial dendrites, slip lines and cracks have been observed, their occurrence depending on the crystal face and on the growth conditions.

Triplet excitations of crystalline C60. An optically detected magnetic-resonance study in zero field

Upon excitation of a single crystal of C60 in the visible, microwave transitions have been detected optically in zero field. The transitions derive from triplet states and their frequencies verify that these states are delocalized over more than one C60 molecule, as previously inferred from EPR experiments. Different triplet states seem to correspond to excitations that are trapped in different domains of the crystal which indicates that structural or impurity-induced defects may be involved.

Structure and lattice dynamics of the ordered phase of solid C70

Abstract Harmonic lattice dynamics calculations have been performed on the low temperature structure of solid C 70 . The theoretically obtained lattice structure agrees with the experimentally determined low temperature structure and the symmetry is found to be P2 1 /m. The orientationally ordered equilibrium is obtained by using van der Waals interactions only, i.e. without including Coulomb interactions. The calculations yield additional information on the orientational ordering. The calculated frequencies of the 12 Raman active lattice modes agreee quantitatively with the recently observed Raman spectrum. The frequencies of the nine complementary IR active lattice modes are predicted.

Structures and phase transitions in C60 and C70 fullerites

Abstract The room-temperature face-centered cubic (FCC- a 0 ) phase as well as the low-temperature simple cubic (SC) phase of C 60 are studied by electron microscopy and electron diffraction. The micro-structure of the room-temperature FCC- a 0 phase is very similar to that of a low stacking fault energy FCC alloy; micro twins and stacking faults on the {111} planes are the main defects. High-resolution observations of these defects are presented here. In some parts the high-resolution images suggest a reconstructed lattice at the surface, possibly due to the presence of oxygen. The phase transition FCC- a 0 ⇒ simple cubic (SC) at 255 K is confirmed and the observed reflections in the SC phase are only compatible with the space group Pa 3 . A second phase transition SC ⇒ FCC-2 a 0 is reported. It occurs presumably at a slightly lower temperature. It is suggested that in the SC phase the molecules still have some rotational degree of freedom about their respective 〈111〉 rotation axis. In the FCC-2 a 0 phase the rotation angle is assumed to be frozen in and to alternate between + ϕ and - ϕ along the 〈100〉 directions. C 70 can grow either hexagonally close-packed (HCP) or cubic close-packed (FCC). On cooling the HCP crystals undergo a phase transformation whereby the c / a ratio increases from 1.64 to 1.82. At lower temperatures the C 70 molecules orient themselves inside the close-packed planes to induce a monoclinic C-centered superstructure. Above room temperature a shear transformation from ABAB… to ABCABC… occurs. Due to the ellipsoidal shape of the molecule, however, the structure is only rhombohedral with α = 88°-89°. Only at higher temperatures does the structure gradually become FCC.