Fulleride Polymer Research Articles

Easy synthesis of Pd fullerene polymer structures from the molten state of tris(dibenzylideneacetone)dipalladium(0)

AbstractPd fullerene composites were first synthesized and studied in the early 90s by for example Nagashima et al. In this study we present a novel and rapid approach to synthesize Pd fullerides based on direct reaction of C60 with Pd2dba3. We show that the Pd fullerene polymer phase forms at temperatures around the melting point of Pd2dba3 (150 °C) and that it proceeds upon further annealing while releasing dba. The synthesis reactions were studied in TGA/DSC. TEM revealed that the material easily collapses under the electron beam into nanoparticles. Under very low doses almost no particles can be found. Similarly, Raman spectroscopy confirmed the formation of Pd fulleride polymers but also supported the collapse of the Pd fulleride phase when irradiated by high laser power. CVD experiments have been conducted on directly coated Si substrates showing similar results to previous reports, namely that Pd2C60 is an efficient catalysts for the growth of helical carbon nanofibers. Our study gives both insights into the formation of nanoparticles as well as the synthesis of C60 polymers. The method is also compatible with direct coating processes making it useful for a broad spectrum of CVD and catalysis applications.

Effect of high pressure on electrical transport in theLi4C60fulleride polymer from 100 to 400 K

In situ resistance measurements have been carried out on ${\text{Li}}_{4}{\text{C}}_{60}$ under pressures up to 2 GPa at temperatures from below 100 to 400 K. In agreement with recent reports we find an Arrhenius law behavior for the conductivity, which can be interpreted in terms of ${\text{Li}}^{+}$ ionic conduction with an activation energy near 225 meV. The activation energy decreases with increasing pressure at an initial rate of about $\ensuremath{-}11\mathrm{%}/\text{GPa}$ and the room-temperature conductivity increases by a factor of about 6 from 0.1 to 2 GPa. We also observe conductivity terms with a lower excitation energy, most probably associated with conduction by electrons excited from defect-induced states in the main band gap. We discuss this conduction behavior in the context of recent measurements on both ${\text{Li}}_{4}{\text{C}}_{60}$ and other alkali-metal intercalated phases such as ${\text{Rb}}_{4}{\text{C}}_{60}$, ${\text{Na}}_{2}{\text{C}}_{60}$, and ${\text{Na}}_{4}{\text{C}}_{60}$. After heating to 400 K at 2 GPa the conduction behavior changes drastically, manifested by a change in the slopes of $R$ versus $T$ curves signifying newly created gap states. Postexperimental characterization by Raman spectroscopy and x-ray diffraction indicate the loss of Li especially from the grain surfaces. Finally, high-pressure Raman studies suggest a possible metallization transition above 9 GPa.

Superionic Conductivity in theLi4C60Fulleride Polymer

We report on the extraordinary superionic conductivity in the fulleride polymer Li4C60, a crystalline material with no disorder. 7Li, NMR, and dc frequency dependent conductivity show uncorrelated ionic hopping across small energy barriers (DeltaE_{a} approximately 200 meV) and an ionic conductivity of 10;{-2} S/cm at room temperature, higher than in "standard" ionic conductors. Ab initio calculations of the molecular structure find intrinsic unoccupied interstitial sites that can be filled by Li+ cations in stoichiometric Li4C60 even at low temperatures. The low energy required for the occupation of these sites allows a sizable Li+ diffusion above 130 K. The results suggest novel application of lithium intercalated fullerides as electrodes in Li ions batteries.

Structure and properties of the stable two-dimensional conducting polymerMg5C60

We present a study on the structural, spectroscopic, conducting, and magnetic properties of ${\mathrm{Mg}}_{5}{\mathrm{C}}_{60}$, which is a two-dimensional (2D) fulleride polymer. The polymer phase is stable up to the exceptionally high temperature of $823\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The infrared and Raman studies suggest the formation of single bonds between the fulleride ions and possibly $\mathrm{Mg}\text{\ensuremath{-}}{\mathrm{C}}_{60}$ covalent bonds. ${\mathrm{Mg}}_{5}{\mathrm{C}}_{60}$ is a metal at ambient temperature, as shown by electron spin resonance and microwave conductivity measurements. The smooth transition from a metallic to a paramagnetic insulator state below $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is attributed to Anderson localization driven by structural disorder.

ESR spectrometer with a loop-gap resonator for cw and time resolved studies in a superconducting magnet

The design and performance of an electron spin resonance spectrometer operating at 3 and 9 GHz microwave frequencies combined with a 9-T superconducting magnet are described. The probehead contains a compact two-loop, one gap resonator, and is inside the variable temperature insert of the magnet enabling measurements in the 0–9 T magnetic field and 1.5–400 K temperature range. The spectrometer allows studies on systems where resonance occurs at fields far above the g ≈ 2 paramagnetic condition such as in antiferromagnets. The low quality factor of the resonator allows time resolved experiments such as, e.g., longitudinally detected ESR. We demonstrate the performance of the spectrometer on the NaNiO 2 antiferromagnet, the MgB 2 superconductor, and the RbC 60 conducting alkaline fulleride polymer.

A longitudinally detected high-field ESR spectrometer for the measurement of spin–lattice relaxation times

We describe a high-field longitudinally detected electron spin resonance (LOD-ESR) spectrometer operating at 35 and 75 GHz. The lack of resonant microwave circuits facilitates operation at different microwave frequencies without changing the probehead. A very low noise radio frequency detection compensates partially the resulting low sensitivity. The major elements of the LOD-ESR spectrometer are commercially available and may be adapted to usual high frequency spectrometers. The instrument allows field and frequency dependent spin lattice relaxation time ( T 1) studies. T 1 in the range of 2–80 ns can be determined from the phase sensitively detected LOD-ESR spectra. We demonstrate the performance of the apparatus by the measurement of T 1 in the normal state of RbC 60, an electrically conducting alkaline fulleride polymer.

Polymeric sheets in Mg 4C 60

We present preliminary results on the preparation and structure of Mg 4C 60, a new fulleride polymer. A series of Mg x C 60 compositions (0< x<6) were prepared by solid state reaction. While most samples were multiphase, a single phase material was obtained at the nominal composition Mg 4C 60. The X-ray diffraction pattern of Mg 4C 60 was successfully indexed and fitted to a rhombohedral structure based on two-dimensional polymeric sheets of C 60 ions. This result extends the family of fulleride polymers to the salts of multiply charged cations and thus opens a new direction in this field.

Comment on "Low temperature magnetic instabilities in triply charged fulleride polymers".

A Comment on the Letter by Denis Arcon et al., Phys. Rev. Lett. 84, 562 (2000). The authors of the Letter offer a Reply.Received 9 August 2000DOI:https://doi.org/10.1103/PhysRevLett.87.129703©2001 American Physical Society

Low Temperature Magnetic Instabilities in Triply Charged Fulleride Polymers

The electronic properties of the C3-60 polymer in Na2Rb0.3Cs0.7C60 were studied by X-band and high field (109.056 GHz) ESR. They are characteristic of a strongly correlated quasi-one-dimensional metal down to 45 K. On further cooling, a pseudogap of magnetic origin opens at the Fermi level below 45 K with three-dimensional magnetic ordering occurring below T(N) approximately 15 K, as confirmed by the observation of an antiferromagnetic resonance mode. The Na2Rb1-xCsxC60 family of polymers offers a unique way to chemically control the electronic properties, as the opening of the gap in this system of predominantly itinerant electrons is an extremely sensitive function of the interchain separation.

Effect of charge state on bonding in fulleride polymers

Both two- and one-dimensional single bonded fulleride polymer phase have been investigated theoretically using the Hückel model. Parameters for the sp 3 hybridized carbon atoms forming the bonds have been found by fitting the energy spectrum and bond orders of the bonding atoms of a single bonded C 60 dimer to the results of VEH calculations on model systems of the polymers. Our calculations strongly support previous results, that the two-dimensional polymeric phase is stabilized when a charge of four electrons per C 60 molecule is applied. The one-dimensional single bonded chain is stable when the C 60 molecules are at least doubly charged, which is in agreement with experimental observations for Na 2 RbC 60(three electrons per C 60 molecule).

Na4C60: An Alkali Intercalated Two-Dimensional Polymer

The long-missing stoichiometric phase of ${\mathrm{Na}}_{4}{\mathrm{C}}_{60}$ has been identified by Rietveld analysis of synchrotron powder diffraction data. Its monoclinic structure is based on polymer planes of ${\mathrm{C}}_{60}$ where each molecule forms four ``single'' bonds within the plane. This compound is not only the first fulleride polymer with such bonds, but also the first two-dimensional polymer which is naturally intercalated with alkali ions and can be synthesized at ambient pressure. ${\mathrm{Na}}_{4}{\mathrm{C}}_{60}$ is a metal and is expected to be a prototype structure where electronic overlap between the planes can be tuned by the alkali cation size.