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Exchange interactions between isolated pairs of spin centers in diamond have been calculated, based on an accurate atomistic electronic structure for diamond and any impurity atoms, for spin-center separations up to 2~nm. The exchange interactions exceed dipolar interactions for spin center separations less than 3~nm. NV$^-$ spin centers, which are extended defects, interact very differently depending on the relative orientations of the symmetry axis of the spin center and the radius vector connecting the pair. Exchange interactions between transition-metal dopants behave similarly to those of NV$^-$ centers. The Mn\---Mn exchange interaction decays with a much longer length scale than the Cr\---Cr and Ni\---Ni exchange interactions, exceeding dipolar interactions for Mn\---Mn separations less than 5~nm. Calculations of these highly anisotropic and spin-center-dependent interactions provide the potential for design of the spin-spin interactions for novel nanomagnetic structures., 5 pages

Local symmetry properties of two quasicrystals, an icosahedral Al70Pd20Mn10 and a decagonal Al70Co15Ni15 , are investigated using the technique of secondary-electron imaging (SEI). An existing structure model is confirmed for Al70Pd20Mn10 , where a new basic structure unit, consisting of an alternate stacking of pentagons and center-decorated decagons, is proposed for Al70Co15Ni15 . A link between the icosahedral and the decagonal phase is suggested.

The electronic structure of crystalline AlPd and quasicrystalline ${\mathrm{Al}}_{70}$${\mathrm{Pd}}_{20}$${\mathrm{Mn}}_{10}$ has been investigated by x-ray photoelectron and electron energy-loss spectroscopy. The spectral shape of the Pd-derived 4d states in the valence-band region and the 3d core-level shifts are identical in both alloys, suggesting similar local environments for Pd atoms, both chemically and structurally. Although the macroscopic crystal structure is essentially different, these alloys have remarkably similar electronic properties determined by the Al-Pd interaction. In both materials, the spectra display a satellite at an energy of approximately 20 eV, accompanying all excitations, which is localized at the Pd site, whereas plasmon losses characteristic of pure Al are strongly suppressed. The Al-Mn interaction in the quasicrystal causes minor changes in the Al core levels and in the valence band by increasing the density of states at the Fermi level.

The electronic structure of an icosahedral Al 70 Pd 20 Mn 10 quasicrystal has been investigated by X-ray photoelectron and electron energy-loss spectroscopy. The spectral form of the valence band, the core-level intensities and the core-level shifts all strongly indicate a close resemblance to crystalline AlPd. The density of states at the Fermi level of the quasicrystal is found to be significantly lower than that of pure Al, Pd and Mn. XPS spectra show a satellite that accompanies the main Pd lines at 18 eV. The same broad feature is observed in EELS, while plasmon losses characteristic of pure Al are suppressed. Hence, the concentration of nearly-free electrons is significantly low, in agreement with the relatively high resistivity of the material. Other than that, no unusual spectroscopic features are observed that could be attributable to the quasicrystalline structure of the Al 70 Pd 20 Mn 10 alloy.

We demonstrate that a scanning tunneling microscope tip can be used to manipulate the tightly bound core (d-electron) state of single Fe ions embedded in GaAs. Increasing tip-sample voltage removes one d electron from the core of a single Fe, changing the dopant from the (Fe2+)(-) ionized acceptor state to the (Fe3+)(0) isoelectronic state, which alters the spin moment and dramatically modifies the measured local electronic contrast in topographic maps of the surface. Evidence of internal transitions among the d states of the Fe core is also seen in topographic maps where dark anisotropic features emerge from the interference between two paths: the direct tip-sample tunneling and tunneling which excites a d-state core exciton of the Fe dopant.

INTERCALATED fullerides exhibit remarkable physical properties, including superconductivity at record high temperatures for organic compounds1–6. The donor intercalants have so far been limited to alkali and alkaline-earth metals. Here we describe the synthesis, structure and magnetic properties of a rare-earth-doped fulleride, Yb2.75C60, which exhibits superconductivity below 6 K. Ytterbium cations occupy off-centred interstitial sites in the face-centred cubic C60 lattice, and leave eight tetrahedral sites vacant in an orthorhombic unit cell which is thereby doubled in each direction. These cation-vacancy sites exhibit long-range ordering, generating a superstructure which seems to be stabilized by a short-range interaction between charge-deficient single bonds in C60 and exclusively divalent cations. Our results demonstrate that superconductivity is not limited to C60 compounds of high structural symmetry.

Changes in surface roughness have been studied as a function of bulk compressive and tensile strains (biaxial in the plane of the sample surface) in thin films of compositionally uniform and dislocation-free ${\mathrm{Ge}}_{0.5}$${\mathrm{Si}}_{0.5}$. A pronounced surface roughness is observed only for films under compressive strains exceeding 1.4%. Molecular dynamics simulations show that this striking result has its origin in the strain-induced lowering of surface step free energies.

We report synthesis, structures and properties of strontium intercalated fullerite, SrxC60 compounds. This is identified as the only fulleride system where fcc and bcc phases compete in the same compositional range. In the vicinity of x=3, a bcc A15 phase coexists with a fcc phase with lattice constants 11.140 and 14.144 A, respectively. For Sr6C60, an Im 3 bcc phase with ao=10.975 A is observed. This phase becomes superconducting at T=4 K. These structural and electronic properties, being intermediate to grossly different calcium and barium intercalated fullerides, provide a unifying picture of all alkaline-earth intercalated fullerides.

A new stable compound ${\mathrm{Ba}}_{3}$${\mathrm{C}}_{60}$ is reported in the Ba-${\mathrm{C}}_{60}$ phase diagram. Rietveld refinement of x-ray powder diffraction data shows that this compound has the A15 structure with a lattice constant of 11.34 \AA{}. The Pm3\ifmmode\bar\else\textasciimacron\fi{}n space group implies a perfect alternating orientational order for the ${\mathrm{C}}_{60}$ molecules, not previously observed in doped fullerite structures. The relative stability of the A15 phase over the fully intercalated fcc structure can be explained by a simple model involving the Madelung energy differences, orientation dependence of the ${\mathrm{C}}_{60}$-cation interaction energy, and distortion-induced relaxational energy gains.

Ideal quasicrystalline form of matter is defined by an atomic structure that has a perfect orientational long range order, a non-periodic long range translational order, and a non-crystallographic point group symmetry. In view of these exotic structural properties, perhaps not so surprisingly "quasicrystals" were also called "impossible crystals". These exotic structures do exist in nature and were first discovered in 1984 in an AI-Mn alloy. This particular alloy had a grain size of several microns, and was thermodynamically metastable. In eight short years, and after the discovery of many other quasicrystalline compounds, the quasicrystalline single grain size we could grow increased from microns to a few centimeters. Today, we can grow nearly defect-free and stable two- and three-dimensional quasicrystals with macroscopic dimensions. All quasicrystals discovered to date exhibit a universal non-congruent melting property which we exploit in our crystal growth techniques. This paper briefly reviews the remarkable progress made in growing single grain quasicrystals.

We describe a high-temperature aerosol apparatus for the synthesis of 3-8-nm, surface-oxidized Si crystallites. The particles are made by homogeneous gas-phase nucleation following pyrolysis of dilute disilane in He. The particles are collected as a robust ethylene glycol colloid, and characterized by transmission electron microscopy, X-ray powder Brag scattering, IR, high-pressure liquid chromatography, and optical spectroscopy. The particles exhibit a shell structure, with a crystalline Si core, of bulk lattice constant, capped with an approximately 1.2-nm shell of silicon dioxide

Photoemission spectra of vacuum deposited layers of C 60 , before and after exposure to K vapor, show that the K donates its conduction electron into the band derived from the lowest unoccupied molecular orbital. A compound with composition of K 3 C 60 , corresponding to the maximum conductivity, has been prepared. In it the potassium atoms presumably occupy both the octahedral and the two tetrahedral interstitial sites of the face-centered-cubic (fcc) C 60 structure.

BULK C60 (fullerite) is a solid composed of extremely hard pseudospherical molecules bonded by weak van der Waals interactions. We expect that pressurization will probe this diversity as the intermolecular distance is reduced. Here we present experimental measurements of the room-temperature pressure–volume equation of state of C60 fullerite to pressures of 20 GPa. The face-centred cubic phase remains stable under hydrostatic compression to this pressure, with an atmospheric-pressure isothermal bulk modulus of K0 = 18.1 ± 1.8 GPa and dK0/dP = 5.7 ± 0.6, characteristic of isotropic van der Waals intermolecular bonding. X-ray diffraction intensity changes are consistent with the compressibility of the C60 molecule being significantly lower than that of the bulk fullerite solid. Under non-hydrostatic compression, a transition to a crystallographic structure of lower symmetry is observed at 16±1 GPa.

Recent interest in closed cage carbon clusters fullerenes is due to their remarkable physical and chemical properties, and is facilitated by a novel synthesis process that made macroscopic quantities of this novel material readily available. The condensed form of C 60 , fullen te, is doped with alkali or alkaline-earth metals and electronic properties are significantly modified. In donor dopants case, electrons are donated to C 60 molecules to populate the molecular orbital derived bands. At half-filling of the conduction bands, systems become superconductors with remarkably high transition temperatures. Here we present a brief overview of the synthesis, crystal growth and doping of fullerites.

The formation energy, [Delta]H[sub f], of fullerite intercalation compounds A[sub n]C[sub 60] with A = Na, K, Rb, and Cs and n = 6 is measured using differential scanning calorimetry. The alkali metals, K, Rb, and Cs react with C[sub 60] powders readily and directly form the bcc A[sub 6]C[sub 60]. [Delta]H[sub f] values are found to be [minus]28, [minus]39, [minus]40, and [minus]47 kcal/mol per alkali metal for Na, K, Rb, and Cs, respectively. The activation energy of diffusion of the alkali metals Rb and Cs in the corresponding A[sub 6]C[sub 60] compound is evaluated to be approximately 30 kcal/mol. 25 refs., 4 figs., 1 tab.

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.

Recent interest in closed cage carbon clusters fullerenes is due to their remarkable physical and chemical properties, and is facilitated by a novel synthesis process that made macroscopic quantities of this novel material readily available. The condensed form of C 60 , fullen te, is doped with alkali or alkaline-earth metals and electronic properties are significantly modified. In donor dopants case, electrons are donated to C 60 molecules to populate the molecular orbital derived bands. At half-filling of the conduction bands, systems become superconductors with remarkably high transition temperatures. Here we present a brief overview of the synthesis, crystal growth and doping of fullerites.

The dielectric constant ∈ and conductivity σ of undoped C60 single crystals have been measured as a function of temperature, 10 K < T < 330 K, and frequency, 0.2 kilohertz < f < 100 kilohertz. On cooling below the first-order structural phase transition at 260 K, a Debye-like relaxational contribution to the dielectric response is observed, which requires the presence of permanent electric dipoles. The relaxation rate is thermally activated with a broad distribution of energies centered at 270 millielectron volts. The existence of a dipole moment in C60 is unexpected, because it is precluded by symmetry for the pure ordered cubic phase. These data suggest that the high degree of frozen-in orientational disorder of the C60 molecules is responsible for the existence of electric dipolar activity.

We report the studies of the elasticity of ${\mathrm{O}}_{60}$ single crystals. For sublimed fcc crystals, Young's modulus has an 8% jump at the first-order transition at 260 K. At \ensuremath{\sim}160 K there is a frequency-dependent elastic anomaly resulting from time dependent stress relaxation. Comparison with rotation rates seen in NMR suggests that the dynamics below 260 K is more complex than jumps between equivalent molecular configurations. Solvent grown monoclinic crystals have a second-order transition at 242 K, while the 160-K glass transition remains unchanged.

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.

The temperature-dependent resistivity of the two-dimensional quasicrystals ${\mathrm{Al}}_{65}$${\mathrm{Cu}}_{15}$${\mathrm{Co}}_{20}$ and ${\mathrm{Al}}_{70}$${\mathrm{Ni}}_{15}$${\mathrm{Co}}_{15}$ is studied along the periodic direction (${\mathrm{\ensuremath{\rho}}}_{\mathit{p}}$) and in the quasiperiodic plane (${\mathrm{\ensuremath{\rho}}}_{\mathit{q}}$) from 4.2 to 600 K. A transport anisotropy of 〈${\mathrm{\ensuremath{\rho}}}_{\mathit{q}}$/${\mathrm{\ensuremath{\rho}}}_{\mathit{p}}$〉\ensuremath{\simeq}8 is measured. In both materials ${\mathrm{\ensuremath{\rho}}}_{\mathit{p}}$ is metallic and can be described by a semiclassical electron-phonon scattering model. For ${\mathrm{\ensuremath{\rho}}}_{\mathit{q}}$ a nonmetallic behavior is found, suggesting a phonon-assisted tunneling mechanism.

Crystals of C60 or C70 cocrystallized with n-pentane grow as elongated, ten-sided columns. X-ray diffraction shows ordering of C60 or C70 molecules along the column and a remarkable tenfold symmetry normal to the column. The ratio of the x-ray-diffraction vectors of the two lowest-order diffraction spots is nearly equal to the “golden ratio,” τ. Despite these similarities with decagonal, quasicrystalline order, the diffraction indicates crystalline order with a twinned unit cell.

We report a successful epitaxial growth of an intermetallic compound Fe 3 (Al,Si) on GaAs(100). Fe 3 (Al,Si) has a BiF 3 (DO 3 ) structure with a lattice constant which can be adjusted to achieve a perfect lattice match with GaAs(100) face by tuning the relative concentration of Al to Si. The crystal growth was carried out in an MBE system consisting of dual growth chambers, one for III–V compound semiconductors and the other for growing metals ot group IV like Si. Sharp, elongated streaks were observed in the reflection high energy electron diffraction (RHEED) pattern after the deposition of one monolayer (ML) of Fe 3 (Al,Si), indicating the attainment of an atomically smooth surface. The streaky RHEED pattern sharpened further until a 34 A (12 MLs) thickness was reached, and retained similar quality in thicker films. The crystal structure of the films was also characterized by high-resolution X-ray diffraction and Rutherford backscattering/channeling analysis. A rocking curve as narrow as 0.03° full width half maximum along (400) Bragg reflection was obtained for an Fe 2.9 (Al 0.4 Si 0.7 ) film 1000 A thick.

Composite semiconductor crystallites involving CdSe grown on an ZnS seed, and vice versa, have been synthesized and capped with organic ligands in inverse micelle solutions. These composite particles, as well as capped seed crystallites of CdSe and ZnS, are isolated, purified, and characterized for relative atomic composition, structure, and electronic properties. The Debye X-ray scattering equation, when solved for these layered particles, shows that powder X-ray scattering is insensitive to a small foreign inclusion. A simple theoretical model for the LUMO and HOMO of layered crystallites shows that a small (< 15-{angstrom} diameter) interior foreign seed causes only small shifts of the lowest excited state, to either higher or lower energies. The capped CdSe seed and the capped CdSe portion of the layered particle grown on a ZnSe seed undergo low-temperature (169{degree}C) annealing to give near-single-crystal X-ray scattering. However, CdSe annealing is blocked by a surface ZnS layer which is ca. 4 {angstrom} thick. While growth to make composite particles does occur, neither particle shows evidence for epitaxial growth.

Composite As2Se3/polyethersulfone fibers are fabricated that have As2Se3 core diameters between 0.5 and 3 microns surrounded by a PES cladding 50 to 150 microns in diameter. The small core areas along with the large nonlinear coefficients found for As2Se3 make these fibers excellent candidates for compact low-cost nonlinear devices.

We have characterized the structure of epitaxial Al2O3 films deposited on Si (111) substrate using electron beam evaporation from a high-purity single crystal sapphire source in a molecular beam epitaxy (MBE) approach. The structural studies were carried out mainly by single crystal x-ray diffraction with the initial epitaxial growth observed by in-situ reflection high energy electron diffraction. The Al2O3 films grow in the cubic γ-phase with a very uniform thickness, and a high structural perfection. The axes of the film and the Si substrate are well aligned. A mosaic scan of the Al2O3 (222) peak (with no in-plane component) finds a 0.3 degree (or 18') spread. All three unit cell vectors of the film and the substrate are parallel, but the in-plane cone scans of the {004} and {044} diffraction peaks about the surface normal find a ±3 degree film in-plane rotation with respect to the substrate surface orientation.

We have characterized the structure of the epitaxial Sc2O3 films grown on a α-Al2O3 (111) substrate using molecular beam epitaxy (MBE) techniques. The Sc2O3 films grow in the bulk bixbyite phase with a very uniform thickness, and a high structural perfection. They grow with their cubic (111) axis parallel to the rhombohedral (111) axis of the sapphire substrate. The in-plane orientation of the films, however, is rotated by ±30 degrees with respect to the substrate rhombohedral axes. This is explained by the presence of two equivalent orientations of the 3-fold axis of the film on the quasi 6-fold surface of the substrate.

We report the results of dc magnetization and electrical resistivity studies of a single‐domain Al70Ni15Co15 decagonal quasicrystal. The temperature dependence and anisotropy of the electrical resistivity are in excellent agreement with previous results for a similar sample. The magnetic properties were determined by superconducting quantum interference device magnetometry over the temperature range 1.8–300 K for magnetic fields between 10 Oe and 10 kOe. The magnetic behavior can be characterized by a weak ferromagnetic component M0(H) which saturates in moderate fields (H≊1 kOe) and a differential susceptibility, χ0=ΔM/ΔH, obtainable from the high‐field data. The observed magnetization M=χ0H+M0 is slightly anisotropic and essentially temperature independent below room temperature. For H parallel to the quasicrystal axis, we observe χ0∥=7.1×10−7 emu/g and M0∥=2.0×10−3 emu/g. For H perpendicular to the quasicrystal axes, we obtain χ0⊥=8.2×10−7 emu/g and M0⊥=1.7×10−3 emu/g. An anisotropic and weak supercond...

We have measured the pressure dependence of the superconducting transition temperature of Ca 5 C 60 . In striking contrast to the alkali doped C 60 superconductors, where a large negative derivative is observed, we find a small positive derivative of 0.11 (±0.05) K/kbar. This result is in disagreement with a simple extension of current models.

The formation energy, [Delta]H[sub f], of fullerite intercalation compounds A[sub n]C[sub 60] with A = Na, K, Rb, and Cs and n = 6 is measured using differential scanning calorimetry. The alkali metals, K, Rb, and Cs react with C[sub 60] powders readily and directly form the bcc A[sub 6]C[sub 60]. [Delta]H[sub f] values are found to be [minus]28, [minus]39, [minus]40, and [minus]47 kcal/mol per alkali metal for Na, K, Rb, and Cs, respectively. The activation energy of diffusion of the alkali metals Rb and Cs in the corresponding A[sub 6]C[sub 60] compound is evaluated to be approximately 30 kcal/mol. 25 refs., 4 figs., 1 tab.

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.

We report optical reflectivity data for single crystals of C 60 in the energy range 1.8–4.8 eV. A Kramers—Kronig analysis of the reflectivity spectrum yields an absorption spectrum similar to that of thin films of polycrystalline C 60 . Based on the general features of the linear absorption spectrum, it would appear unlikely that crystalline C 60 manifests exceptionally large second- or third-order optical nonlinearities.

Measurements of the superconducting ${\mathit{T}}_{\mathit{c}}$ and Raman spectra in isotopically modified ${\mathrm{Rb}}_{3}$${\mathrm{C}}_{60}$ are reported. A decrease in ${\mathit{T}}_{\mathit{c}}$ of 0.65\ifmmode\pm\else\textpm\fi{}0.10 K in samples with (75\ifmmode\pm\else\textpm\fi{}5)% substitution of $^{13}\mathrm{C}$ for $^{12}\mathrm{C}$ yields an exponent of \ensuremath{\alpha}=0.37\ifmmode\pm\else\textpm\fi{}0.05. This interpreted as evidence for phonon-mediated pairing.

FCC C{sub 60} crystals exposed to pure O{sub 2} are converted to carbon-oxygen adducts at {approximately}200{degrees}C. The carbon-oxygen species decompose into CO and CO{sub 2} gases at a higher temperature, {approximately}400{degrees}C. The total heats of oxidation of C{sub 60} are determined by calorimetry to be {approximately}53 kcal/mol. The activation energy for the oxidation is 58.2 kcal/mol. The C{sub 60} powders in inert gases such as N{sub 2} and Ar sublime at {approximately}600{degrees}C. A method is derived to determine the heats of sublimation from thermogravimetric analysis. The heat of sublimation of C{sub 60} powders in pure N{sub 2} or Ar is found to be {approx_gt}39 kcal/mol. 16 refs., 4 figs., 1 tab.

In pure oxygen at moderate temperatures of 200 °C, an fcc C60 transforms into amorphous carbon‐oxygen compounds and the icosahedral C60 molecular structure is destroyed. The maximum oxygen uptake of pure C60, O/C60, is 12. Isothermal TGA transformation curves are sigmoid‐shaped with the kinetic exponent n∼5/2 which conforms with a two‐dimensional nucleation and growth mode. The heat of formation for the carbon‐oxygen compounds is 90 kcal/mol O, and the formation energy for the reaction: 60C (graphite)→C60 molecule is estimated to be ∼600 kcal/mol.

Thermal sublimation of pure C60 and C70 has been used for depositing well‐characterized fullerene films on a variety of substrates. Film purity is determined by infrared absorption spectra and the extent of crystallinity of the face‐centered cubic structure by x rays. Thickness‐dependent optical and electrical measurements reveal uniform films over the thickness range 200–1000 A. We obtain optical absorption coefficients having values between those of Si and Ge and a relative permittivity having a value close to that of amorphous SiO2.

Photoemission spectra of vacuum deposited layers of C(60), before and after exposure to K vapor, show that the K donates its conduction electron into the band derived from the lowest unoccupied molecular orbital. A compound with composition of K(3)C(60), corresponding to the maximum conductivity, has been prepared. In it the potassium atoms presumably occupy both the octahedral and the two tetrahedral interstitial sites of the face-centered-cubic (fcc) C(60) structure.

Meissner-effect and microwave-absorption measurements on bulk samples show that ${\mathrm{Rb}}_{\mathit{x}}$${\mathrm{C}}_{60}$ is superconducting with a maximum transition temperature of 28 K. This is a 10-K (60%) increase over the K-doped material. Only ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ and the cuprate superconductors have higher transition temperatures.

BULK C60 (fullerite) is a solid composed of extremely hard pseudospherical molecules bonded by weak van der Waals interactions. We expect that pressurization will probe this diversity as the intermolecular distance is reduced. Here we present experimental measurements of the room-temperature pressure–volume equation of state of C60 fullerite to pressures of 20 GPa. The face-centred cubic phase remains stable under hydrostatic compression to this pressure, with an atmospheric-pressure isothermal bulk modulus of K0 = 18.1 ± 1.8 GPa and dK0/dP = 5.7 ± 0.6, characteristic of isotropic van der Waals intermolecular bonding. X-ray diffraction intensity changes are consistent with the compressibility of the C60 molecule being significantly lower than that of the bulk fullerite solid. Under non-hydrostatic compression, a transition to a crystallographic structure of lower symmetry is observed at 16±1 GPa.

THE recent syntheses1,2 of macroscopic quantities of C60 have suggested possible applications in host–guest and organic chemistry, tribology, electrochemistry and semiconductor tech-nology. Here we report the preparation of alkali-metal-doped films of C60 and C70 which have electrical conductivities at room temperature that are comparable to those attained by n-type doped polyacetylene. The highest conductivities observed in the doped films are: 4Scm−1 (Cs/C60), 100 (Rb/C60), 500 (K/C60), 20 (Na/C60), 10 (Li/C60), 2 (K/C70). The doping process is reversed on exposure of the films to the atmosphere. At high doping levels, the films become more resistive. We attribute the conductivity induced in these films to the formation of energy bands from the π orbitals of C60 or C70, which become partially filled with carriers on doping. The smaller alkali metal ions should be able to fit into the interstices in the lattice without disrupting the network of contacts between the carbon spheroids. In the case of C60, this would allow the development of an isotropic band structure, and we therefore propose that these materials may constitute the first three-dimensional 'organic' conductors.

Sputtering with Ar+ ions induces structural phase transitions at the pentagonal surface of the icosahedral quasicrystal Al70Pd20Mn10. Sputtering at different temperatures changes the surface composition, thereby stabilizing different structures. At room temperature, the structure changes to body-centered cubic but, at elevated temperatures, it displays decagonal symmetry. In both cases, annealing the sample restores both the bulk composition and the icosahedral symmetry of the original surface.

Non-oxide glasses1-5 find an increasing use in fiber optic applications. Most of the current research activity is in the halide glass fibers used as active hosts for rare-earth ion doping. These include, 1.3 and 1.5 μm optical amplifiers, frequency upconversion devices, fiber lasers, permanent gratings, etc. Other non-oxide glasses, chalcogenides and mixed chalcogenide halide glasses have potential in fiber applications in IR laser power deliverers, image-guides and sensors. Some of the important issues that require further research include, the glass purification, stable glass compositions, hermetic coatings for fibers, better methods for preform and fiber fabrication.

Near-edge and extended x-ray absorption fine structure (NEXAFS and EXAFS) measurements have been made as a function of Pb dopant concentration to study the changes in local geometric and electronic structure in several heavy-metal fluoride glasses (Zr/Ba/La/Al/Na, or ZBLAN). A strong correlation is found between the observed glass instability for Pb concentrations > 10% and the structural changes that are measured exclusively around the Ba atoms. Implications of these novel findings are examined in light of the present understanding of these complex glass systems.

Although fullerenes and their compounds are very new and exhibit new phenomena, pressure has already been an important factor in the characterization and study of these materials. In order to illustrate this, we will review collaborative studies on: i) the effect of pressure on alkali and alkaline earth doped C60 superconductors, ii) the effect of pressure and pressure medium on the ordering temperature of C60, and iii) the role of pressure in the study of the feasibility of using C60 as ‘‘lattice sieves’’ for separation of gases.

Recent discoveries made in synthesizing stable quasicrystals allows us to apply conventional techniques in growing macroscopic single grains and use conventional probes to study the complex structure of this novel form of matter. The decagonal quasicrystals in particular appears to have a far less complicated phase diagram and an extreme phase stability compared to their three dimensional icosahedral counterparts. We have grown several millimeter size single grains and studied the structure of stable A165Co20Cu15 Decagonal quasicrystals, using High Resolution X-ray Diffraction (HRXD) and Scanning Tunneling Microscopy (STM). Unlike diffraction probes which provide large volume reciprocal space information, STM is a local probe and provides direct real-space information with atomic resolution. The two techniques are therefore complimentary and when combined becomes very powerful in structure determinations of complex systems like quasicrystals. We have been able to prepare clean single grain surfaces in UHV and resolve individual atoms in quasiperiodic ordering with STM. Images of 10-fold quasiperiodic surfaces exhibit very well defined five sets of mass density lines, separated by multiples of 72 degree rotations. Near perfect quasiperiodic order seem to extend beyond thousands of angstroms, and can be modeled by a Penrose lattice. All atomic layers observed have identical local structure unlike the two dissimilar layers of the approximant Al13Fe4 phase. High resolution x-ray scattering from single grains finds 2000 angstroms size defect free quasicrystalline layers stacked periodically.