Browsing by Author "Rahmer, Jürgen"

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    Electronic spin states in fullerides and endohedral fullerenes
    (2003) Rahmer, Jürgen; Mehring, Michael (Prof. Dr.)
    This PhD thesis presents a study of electronic properties and spin states of fullerene materials using high-field electron-spin resonance (ESR) operating at frequencies of 95 GHz (W band) and standard-field ESR operating at 9 GHz (X band). Investigated carbon-cage materials range from solid fullerene salts like alkali fullerides (1), bis(arene)chromium fullerides (2), and mixed alkali/alkaline-earth fullerides (3), over fullerenium salts (4) to molecular systems like the metallofullerene Sc3@C82 (5). 1) Investigations of the alkali fullerides RbC60 and CsC60 show that the one-dimensional fullerene polymer chains, which exist in the orthorhombic room-temperature phase of these metallic solids, do not lead to a quasi-one-dimensional conductivity along the chain direction. Electronic scattering rates inferred from conduction-electron-spin-resonance experiments (CESR) and the structure of the electronic g tensor determined from high-field ESR measurements of crystal samples suggest a rather isotropic electronic system. 2) Bis(toluene)chromium fulleride, Cr(C7H8)2C60, is a novel fullerene salt that incorporates organometallic complexes. Using ESR and supplementary x-ray diffraction, magnetic susceptibility, and 13C NMR measurements, a phase transition from a cubic room-temperature phase to a triclinic (C60)2 dimer phase below 250 K was identified. While in the cubic phase the spins of the cationic chromium complexes are exchange-coupled to the fullerene anion spins, the fullerene dimers in the triclinic structure form spin singlets, so that magnetism in the low-temperature phase is characterized by the paramagnetic bis(toluene)chromium complexes only. Investigations of the related bis(mesitylene)chromium fulleride Cr(C9H12)2C60 reveal a similar, but less pronounced phase transition, which, however, extends over a much broader temperature range. 3/4) Further investigations of solid fullerene materials refer to the mixed alkali/alkaline-earth fullerides KCsBaC60 and CsBaC60 as well as fullerenium salts, i.e., materials containing fullerene cations. For the latter, the existence of fullerene cations after oxidation with AsF5 has been proven by the characteristic g value of the fullerenium ESR signal. 5) Sc3@C82 belongs to the family of endohedral metallofullerenes, i.e., the scandium trimer is located inside the fullerene cage. Overlap of the molecular electronic wave function with the three scandium nuclei with spin I=7/2 leads to a hyperfine-splitted ESR spectrum. At temperatures below roughly 200 K, ESR studies in frozen solution reveal a new spin state with strongly reduced hyperfine coupling. Furthermore, hyperfine coupling studies of 13C enriched molecules find an inhomogeneous distribution of electronic spin density on the cage, with increased density on few distinct carbon positions on the upper cage hemisphere.