Solid C60 Research Articles

Neutron scattering and μSR studies of fullerenes and their derivatives

The results of recent neutron scattering and μSR studies on C60 and C70 fullerenes and their derivatives in the solid state are briefly reviewed. Particular attention is paid to the temperature evolution of the rotational dynamics and the phase transitions accompanying orientational ordering. The potential of muons as spin labels both inside and outside the fullerene cage is also explored. Solid C60 shows a first-order phase transition at 260 K from a face-centred to a simple cubic structure. The isotropic molecular reorientations change abruptly to quasi-random jump motion between nearly-degenerate orientations differing in energy by 11.4(3) meV. A transition to an orientational glass state occurs at 85 K. Static disorder dominates the crystal chemistry of C70. The orientational ordering transitions from the high temperature face-centred cubic structure to the low temperature rhombohedral and monoclinic structures are accompanied by severe hysteresis effects. The rotational dynamic behaviour changes progressively on cooling from isotropic to anisotropic to uniaxial reorientations about the unique molecular axis. High pressure diffraction experiments reveal that C70 has a smaller compressibility than C60. Disorder effects are also present well in the superconducting state for the potassium fulleride K3C60. Small-amplitude librational motion of the C603− units is evident up to 650 K.

Comment on "Microscopic theory of orientational disorder and the orientational phase transition in solid C60"

A Comment on the Letter by K. H. Michel et al. Phys. Rev. Lett. 68, Phys. Lett. 68, 2929 (1992).Received 7 July 1992DOI:https://doi.org/10.1103/PhysRevLett.69.3589©1992 American Physical Society

Superconductivity in barium fulleride

INTERCALATING solid C60 with dopant atoms yields materials with a remarkable range of properties1. Alkali metal atoms, for example, readily form charge-transfer compounds2,3, AxC60 (where A is an alkali metal), which can be metallic, superconducting or insulating depending on the dopant concentration4–9. In all cases, the superconducting phase has a face-centred cubic (f.c.c.) structure and stoichiometry A3C60, which suggests a common mechanism for superconductivity dependent, at least in part, on the external coordination number of the C60 molecules. More recently, it has been shown10 that the alkaline earth metal calcium can also be intercalated with fulleride to form a superconducting phase, again with a f.c.c.-derived structure, near a Ca:C60 ratio of 5:1. Here we report the intercalation of fulleride with barium, in which a pure body-centred cubic phase with a lattice constant of 11.171 A is realized near a stoichiometry of Ba6C60. This phase is also superconducting (with a transition temperature of 7 K), suggesting that the mechanism of superconductivity is related to an intrinsic property of the C60 molecules, rather than the external coordination number.

Intermolecular interactions and the nature of orientational ordering in the solid fullerenes C 60 and C 70

We have proposed an intermolecular potential for C 60 molecules that not only reproduces the correct low-temperature structure, but also correlates a wide range of experimental properties, including the molecular reorientational time in the room-temperature rotator phase, the volume change at the orientational ordering transition, and the librational frequencies in the low-temperature phase. The low- pressure phases in solid C 70 have been explored using constant-pressure molecular dynamics and an intermolecular potential derived from one that gives an excellent account of the properties of solid C 60 . The molecular dynamics calculations predict three low-pressure phases: a high-temperature rotator phase, a partly ordered phase with trigonal symmetry, and an ordered monoclinic phase. The calculations on C 70 were carried out on a cluster of IBM RS/6000s, operating in parallel.

C60 solid state rotational dynamics and production and EPR spectroscopy of fullerenes containing metal atoms

The narrow single-line NMR spectrum of solid C60 at ambient temperature demonstrates that the molecules rapidly reorient within the crystal lattice. The rate of this motion can be quantitatively probed using solid-state NMR by measuring the relaxation rate, 1/T1. The dominant contribution to this rate at ambient temperature arises from the combination of rotation and chemical shift anisotropy (CSA). The contribution, 1/T1CSA, is isolated by measuring the magnetic field dependence of T1, allowing the molecular reorientational correlation time, tau , to be determined from T1CSA and the CSA tensor components. At 283 K tau =9.1 ps, only three times longer than the calculated tau for free rotation, and shorter than the value measured for C60 in solution (15.5 ps). This tau corresponds to a rotational diffusion constant D=1.8*1010 s-1. Below 260 K a second phase with a much longer reorientation time is observed, consistent with reports of an orientational phase transition in solid C60. In both phases tau shows Arrhenius behaviour with apparent activation energies of 1.4 and 4.2 kcal mol-1 for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The authors succeeded in producing quantities of fullerene cages containing metal atoms sufficient to allow the first characterization of such a species by electron spin resonance spectroscopy. The EPR spectrum of La@C82 shows an octet of lines centered in the region characteristic of fullerene anion radicals, with a small hyperfine splitting which indicates that the spin is predominantly associated with the carbon shell, leaving the lanthanum atom in its preferred +3 oxidation state.

Femtosecond pump-probe investigation of electron dynamics in solid C60 films.

We present an investigation of electron dynamics occurring on a femtosecond time scale in thin films of undoped and Rb-doped ${\mathrm{C}}_{60}$. We have studied the dependence of the observed dynamics on laser intensity as well as temperature. We find strong absorption induced by laser pumping. Electron-density-dependent dynamics are observed on a 1-ps time scale. A discussion of the various scattering mechanisms at work is presented.

Orientational ordering in solid C70: Predictions from computer simulation.

The low-pressure phases in solid ${\mathrm{C}}_{70}$ have been explored using constant-pressure molecular dynamics and an empirical intermolecular potential extrapolated from one that gives an excellent account of the properties of ${\mathrm{C}}_{60}$. On cooling the high-temperature rotator phase, we find a transition to a partially ordered phase with trigonal symmetry and then a second transition to a monoclinic structure.

Optical Excitations near the Fundamental Gap of Solid C60

Optical studies of solid C60 employing time-integrated as well as time-resolved photoluminescence spectroscopy are reported. At low excitation intensity, a richly structured luminescence spectrum is observed. Increasing the excitation intensity leads to pronounced carrier accumulation and to the emission of intense and spectrally broad luminescence. It is concluded that multiple-step absorption processes lead to carrier recombination originating from higher conduction or lower valence band states.

Discussion of Solid C60 Upon Lennard-Jones Potential

The properties of solid C60 (fcc C60) are discussed based upon Lennard-Jones potential U(R) = ε[(σ/R)12 - (σ/R)6]. The magnitude of the constant ε is estimated by using the standard result of electrostatics for the interaction of two dipole moments of C60 molecules. The cohesive energy and bulk modulus of the solid fullerenes are calculated by classical theory.

Ground-state structural and dynamical properties of solid C60 from an empirical intermolecular potential.

A simple intermolecular potential is constructed to describe solid ${\mathrm{C}}_{60}$. The structural and elastic properties are calculated and show good agreement with available experimental results. Phonon-dispersion curves and density of states are predicted. The librational modes are found to be between 10 and 20 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the highest vibrational mode is at 45 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The phonon density of states is used to calculate the specific heat, which agrees reasonably well with experiment.

Dielectric function of solid C70 films

Using variable-angle ellipsometry and normal incidence reflection/transmission optical techniques we have measured the dielectric function ε(ω) at room temperature for solid C70 films over the photon energy range 0.5≤E≤5.3 eV. The onset of absorption across the highest-occupied-molecular-orbit (HOMO)–lowest-unoccupied-molecular-orbit (LUMO) gap is measured to be 1.25 eV. Furthermore, structure in the interband absorption at 2.41, 3.10, 3.50, and 4.45 eV is observed. The refractive index at zero frequency is estimated to be n(0)=1.94 as compared to the value n(0)=1.90, which we obtained for solid C60 from our previous study.

Rotational dynamics and orientational melting of C60: A neutron scattering study

Well-defined librational excitations have been observed at energies of 2–3 meV in the low temperature ordered phase of solid C60. These relatively high energies imply a stiff orientational potential below the transition. The sharpness of the peaks indicates that this potential does not depend strongly on the axis of the angular displacement. The modes soften and broaden as the temperature approaches that of the orientational melting transition which occurs when the librational amplitude is a considerable fraction of nearest-neighbor interatomic angles.

Electrical resistance of iodine-doped C60 under high pressure

The resistance of an iodine-doped C60 complex is measured under high pressure up to 36 GPa. It decreses with pressure up to 27 GPa, where it shows a shallow minimum. Above 27 GPa, the resistance increases slightly with pressure. The electrical band gap at 17 GPa is of 0.3 eV, which is close to that of the solid C60 at almost the same pressure.

Discontinuous volume change at the orientational-ordering transition in solid C60.

X-ray and neutron-diffraction measurements have been used to study the evolution of the lattice parameter of solid ${\mathrm{C}}_{60}$ through the orientational-ordering transition. The lattice parameter jumps by +0.044\ifmmode\pm\else\textpm\fi{}0.004 \AA{} on heating, indicating a strongly first-order transition. The average isobaric volume thermal-expansion coefficient both above and below the transition is 6.2\ifmmode\pm\else\textpm\fi{}0.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$. We observe phase coexistence over a 5-K range, but little if any hysteresis.

Intercalation of solid C60 with iodine

METALLIC and superconducting donor-type intercalation compounds of C60 are now well established. These involve electron transfer from a sublattice of alkali-metal dopants to a sublattice of fullerene molecules. Three stoichiometric phases have been identified1–3, and a binary phase diagram describing composition regions of two-phase coexistence has been proposed2. Oxidative intercalation by electron acceptors, meanwhile, has not previously been reported. Ohno et al.4 recently presented photoemission data suggesting that solid C60 will take up only minor amounts of iodine, resulting in a non-metallic product which is not a stoichiometric compound4. Here we report that, by using different reaction conditions, we have prepared a highly crystalline C60: iodine compound of ideal stoichiometry C60I4, with an alternating guest–host layer structure closely resembling that of classic intercalation compounds. The 300-K resistivity exceeds 109 Ωcm and we observe no superconductivity down to 4 K, despite the fact that the inter-fullerene separation lies in the range of that in the superconducting M3 C60 phases (where M is K, Rb, Cs). C60I4 is apparently the first example of a fullerite intercalation compound in which there is no electron transfer between C60 and the intercalate.

Superconductivity at 8.4 K in calcium-doped C60

IT has been demonstrated1–6 that solid C60 can be readily intercalated with group IA alkali metals to give metallic or insulating compounds, depending on the dopant concentration. The metal atoms diffuse into tetrahedral and octahedral interstitial sites of the C60 lattice with little disturbance to the face-centred cubic (f.c.c.) packing7. The A3C60f.c.c. phases (A is K, Rb, Cs and mixtures of these) exhibit superconductivity with a transition temperature that increases with lattice constant8. At higher dopant concentration a body-centred tetragonal A4C60 phase9 and an insulating, body-centred cubic A4C60 phase10 are found. Here we report that the divalent group IIA intercalant calcium can be intercalated into the f.c.c. sites of C60 to form a solid solution, and that, near a Ca:C60 ratio of 5:1, a phase transformation occurs to a simple cubic phase. Measurements of microwave loss, magnetic susceptibility and Meissner effect show that the simple cubic phase becomes superconducting below 8.4 K.

Free energy and orientational phase transition in solid C60

On the basis of a microscopic model, the free energy of the orientationally disordered fcc crystalline phase of C60-fullerite is derived up to fourth order in the rotational density fluctuations. The order parameter variables are symmetry adapted rotator functions which belong to thel=6 andl=10 manifolds. They are basis functions ofT2g representations of the cubic group. The phase transition to thePa3 structure is discussed, and is found to be of first order. The orientationally ordered phase is described in terms of condensed rotator functions.

Identification of a growth defect in solid C60 by electron diffraction

The origin of an anomalous sawtooth-shaped feature in x-ray powder diffraction of solid C60 is explained via electron diffraction analysis. Films sublimed on holey carbon crystallize with close-packed (111) planes parallel to the surface. Rods of diffuse scattering are found along the 〈111〉 axis normal to the surface but not along other 〈111〉 axes. Powder averaging of these rods, coupled with the x-ray form factor of spherical shells with 3.5 Å radius, accounts for the sawtooth feature. We attribute this phenomenon to planar defects parallel to close-packed layers, which form during the growth of solid C60 by sublimation. A possibly related consequence of the growth mode is the observation of strong macroscopic (111) preferred orientation in films sublimed on a variety of substrates.

Classical and Quantum Simulations for Large Systems on Parallel Computers

ABSTRACTMolecular dynamics (MD) method is used to investigate structural transformation and the loss of intermediate range order in SiO2 glass at very large positive pressures and the modification of SiO2 glass network at very large negative pressures. The nature of molecular vibrations in solid C60 has been studied with tight binding molecular dynamics (TBMD) method. Implementations of simulation algorithms on parallel computers are also discussed.In a-SiO2 at high pressures, the height of the first sharp diffraction peak in S(q) is considerably diminished and its position shifts to larger wave vectors. At twice the normal density, Si-O bond length increases, Si-O coordination changes from 4 to 6, and O-Si-O band-angle changes from 109° to 90°. This is clearly a tetrahedral to octahedral transformation, which is observed recently by Meade, Hemley, and Mao in their diffraction experiments using synchrotron radiation.MD simulations of porous silica are carried out in the density range 2.2 - 0.1 g/cm3 Internal surface area, pore surface-to-volume ratio, gyration radius, and fractal dimension are studied as a function of density. Simulations are in good agreement with the experimental results obtained by x-ray scattering. The results reveal a crossover of the structural correlations between short- to intermediate-range (< 8 Å) and fractal- to large-scale-regime (10 ~ 100 Å).Dispersion and density of states (DOS) of inter- and intra-molecular phonons are calculated for orientationally ordered and disordered solid C60 using the TBMD method. Inter-molecular phonon DOS extends up to 7.6 meV and shows libron peaks at 2.4 meV and 3.7 meV in the orientationally ordered phase. Orientational disorder softens libron modes. Intra-molecular phonons below 70 meV also show significant dispersion. Our results are in good agreement with the recent inelastic neutron scattering experiments.MD is a numerical approach which involves the solution of Newton's equations for particles in the system. The multiple-time-step (MTS) approach reduces this computation significantly by exploiting the different time scales for short-range and intermediate-range interactions. Using the linked-list scheme, parallel algorithms are designed to implement on the in-house 8-node iPSC/860, a MIMD (multiple instruction multiple data) machine. Our group has also developed a quantum dynamical simulation scheme for Computational Nanoelectronics based on the quantum molecular dynamics (QMD) method. The QMD algorithm has been implemented on the in-house 8,192-node MasPar, a SIMD (single instruction multiple data) architecture.

C60embedded in γ-cyclodextrin: a water-soluble fullerene

A water-soluble complex of C60 is formed on refluxing a solution of γ-cyclodextrin with solid C60; the lifetime of the triplet excited state of C60 in the complex is 83 µs in an oxygen free solution.