Gavin B. M. Vaughan

Orientational disorder in solvent-free solid c70.

The high-temperature structure of solvent-free C70 has been determined with high-resolution x-ray powder difraction and electron microscopy. Samples crystallized from solution form hexagonal close-packed crystals that retain an appreciable amount of residual toluene, even after prolonged heating. Samples prepared by sublimation, which contain no detectable solvent, are primarily face-centered cubic with some admixture of a hexagonal phase. The relative volume of the hexagonal phase can be further reduced by annealing. The structures of both phases are described by a model of complete orientational disorder. The cubic phase contains an appreciable density of stacking faults along the [111] direction.

Compressibility of M3C60 Fullerene Superconductors: Relation Between Tc and Lattice Parameter

X-ray diffraction and diamond anvil techniques were used to measure the isothermal compressibility of K3C60 and Rb3C60, the superconducting, binary alkali-metal intercalation compounds of solid buckminsterfullerene. These results, combined with the pressure dependence of the superconducting onset temperature Tc measured by other groups, establish a universal first-order relation between Tc and the lattice parameter a over a broad range, between 13.9 and 14.5 angstroms. A small secondorder intercalate-specific effect was observed that appears to rule out the participation of intercalate-fullerene optic modes in the pairing interaction.

X-Ray Diffaction Evidence for Nonstoichiometric Rubidium-C60 Intercalation Compounds

Powder x-ray diffraction at 300 K on equilibrated samples of several nominal compositions χ in RbχC60 is reported. In addition to the face-centered cubic (fcc) (χ = 3, superconducting), body-centered tetragonal (χ = 4), and body-centered cubic (bcc) (χ = 6) stoichiometric phases, direct evidence for a dilute fcc doped phase, 0 x c ≤ 1, and for a substoichiometric bcc phase, χ ∼ 5, is presented. In contrast, χ = 3 and χ = 4 appear to be line phases with nearly zero solubility of Rb vacancies and interstitials at 300 K. These results are summarized in a provisional binary phase diagram.

Structural phase transitions and orientational ordering in C70

Abstract The thermal behavior of solid C70 has been studied by synchroton X-ray powder diffraction and differential scanning calorimetry. The equilibrium solid state structures formed by C70 were solved by full profile refinement techniques in which orientational and packing disorder were explicitly accounted for. Above 345 K, C70 forms a plastic crystal, with an equilibrium face-centered cubic structure. At lower temperatures, orientational freezing occurs in two stages. Between 295 and 345 K, disorder persists only about the long axis of the molecule, and the lattice undergoes a rhombohedral distortion. Below 295 K, the rhombohedral lattice undergoes a further distortion, resulting in a previously unobserved monoclinic structure, in which the molecules are presumed to be essentially static. At all temperatures, however, the structure of C70 retains an ABC packing sequence.

The orientational phase transition in solid buckminsterfullerene epoxide (C60O)

Abstract Crystalline C 60 O, the first fullerene epoxide, has been studied using calorimetry and high resolution powder X-ray diffraction. At room temperature, C 60 O is orientationally disordered with a face-centered cubic lattice, a = 14.185 A. Upon cooling an orientational ordering transition at 278 K leads to a low temperature simple cubic phase. At 19 K, this phase is qualitatively similar to the orientationally ordered Pa 3 phase of C 60 , with a =14.062 A, but with additional randomness due to a distribution of orientations of the oxygen bonds.

Molecular orientational dynamics in solid C70: Investigation by one‐ and two‐dimensional magic angle spinning nuclear magnetic resonance

We present the results of 13C nuclear magnetic resonance (NMR) measurements that probe molecular orientational dynamics in solid C70 in the temperature range 223–343 K. Orientational dynamics affect the NMR line shapes and spin–lattice relaxation rates by modulating the 13C chemical shift anisotropy (CSA). Motionally averaged CSA line shapes, determined from both one‐dimensional and two‐dimensional magic angle spinning NMR spectra, and relaxation rates are determined for each of the five inequivalent carbon sites in the C70 molecule. Comparisons of the results for the five sites provide evidence for rapid uniaxial molecular reorientation in the monoclinic (T≤280 K) and rhombohedral (280≤T≤330 K) phases and rapid isotropic reorientation in the face‐centered cubic (T≥330 K) phase. The orientational correlation time is roughly 2 ns at 250 K and of the order of 5 ps at 340 K.

Hexagonal order in some mesophases of hexacyclen derivatives

High-resolution X-ray diffraction measurements on the hexacyclen derivatives 1a–c have provided the first direct experimental evidence of hexagonal order in a hexacyclen mesophase.

Highly Symmetric AB2 Framework Related to Tridymite in the Disordered Nitridosilicate La24Sr14–7x[Si36N72](O1–xFx)14 (x = 0.489)

La24Sr14-7x[Si36N72](O1-xFx)14 with x = 0.489 was obtained as a microcrystalline product by metathesis at 1500 °C in a radio-frequency furnace starting from Si(NH)2, La(NH2)3, SrH2, LaF3, and CeF3. The structure of the new nitridosilicate oxide fluoride was determined by combining transmission electron microscopy (TEM) and single-crystal X-ray diffraction using a microfocused synchrotron beam. The structure model with pronounced disorder [P63/mmc, Z = 1, a = 16.2065(3), c = 9.4165(1) Å, R1(obs) = 0.0436] was confirmed by electron diffraction and aberration-corrected Z-contrast scanning TEM. The highly symmetric AB2 framework, which was theoretically predicted but not yet realized, consists of all-side vertex-sharing SiN4 tetrahedra that form channels along [001] filled with La, Sr, O, and F atoms. The connectivity pattern is related to that of tridymite. X-ray spectroscopy and bond-valence-sum calculations were further taken into account for assignment of the N, O, and F atoms.