High-pressure infrared studies of Jahn-Teller-active and orientationally disordered C60-based compounds

  • This dissertation presents the high pressure infrared investigation of fullerene-based compounds. The transmission/reflection measurements were carried out over a very broad frequency region (300-20000 cm-1) for pressures up to 9 GPa. Within this project, (Ph4P)2IC60 was synthesized by electrochemical crystallization and characterized by ATR-FTIR spectroscopy in the mid- and near-infrared region. Alkali fullerides namely K4C60 and Rb4C60 were synthesized by solid-state reaction and characterized by powder x-ray diffraction. Addition of an electron to the C60 molecules in (Ph4P)2IC60 causes splitting of the LUMO level and induces Jahn-Teller distortion. (Ph4P)2IC60 has several vibrational modes due to symmetry reduction of the C60 anions and due to the cation modes in the far- and mid-infrared region. The near infrared region exhibits the electronic and vibronic transitions. The pressure dependence of the vibrational modes and the electronic/vibronic transitions is discussed inThis dissertation presents the high pressure infrared investigation of fullerene-based compounds. The transmission/reflection measurements were carried out over a very broad frequency region (300-20000 cm-1) for pressures up to 9 GPa. Within this project, (Ph4P)2IC60 was synthesized by electrochemical crystallization and characterized by ATR-FTIR spectroscopy in the mid- and near-infrared region. Alkali fullerides namely K4C60 and Rb4C60 were synthesized by solid-state reaction and characterized by powder x-ray diffraction. Addition of an electron to the C60 molecules in (Ph4P)2IC60 causes splitting of the LUMO level and induces Jahn-Teller distortion. (Ph4P)2IC60 has several vibrational modes due to symmetry reduction of the C60 anions and due to the cation modes in the far- and mid-infrared region. The near infrared region exhibits the electronic and vibronic transitions. The pressure dependence of the vibrational modes and the electronic/vibronic transitions is discussed in detail. Several C60 monoanion and the cation vibrational modes exhibit anomalies around 2 GPa, which are interpreted as the dynamic-to-static Jahn-Teller transition. The driving mechanism of the transition is the steric crowding of the lattice by C60 monoanions and the cations. The symmetry of the dynamic distortion is deduced as D3d/D5d and that of static distortion is either D3d or lower symmetry. High pressure response of A4C60 (A = K, Rb) which are classified as the Mott-Jahn-Teller insulating compounds were studied. These compounds are highly air sensitive and all preparations had to be carried out in the glove box. These compounds undergo a pressure induced insulator-to-metal transition but the underlying mechanism of the transition is not well understood. From the spectral signature of the transition, the underlying mechanism is discussed in detail. The Drude like behavior in the far-infrared spectrum shows that the insulator-to-metal transition is governed by the band width changes. Since there are no additional phonons observed up to the highest measured pressure, the phase separation mechanism is not favored from these data. Finally the neutral C60-based compound namely C60.C8H8 was investigated with the best pressure transmitting medium. The orientational ordering is observed from the triplet splitting of the vibrational modes at a critical pressure of 3 GPa. This transition is driven by both nearest neighbor C60-C60 and C60-C8H8 interactions. From the triplet splitting of the vibrational modes, the symmetry of the ordered C60 molecules are deduced as D2h.show moreshow less

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Metadaten
Author:Estaline Amitha Francis
URN:urn:nbn:de:bvb:384-opus4-21780
Frontdoor URLhttps://opus.bibliothek.uni-augsburg.de/opus4/2178
Advisor:Christine A. Kuntscher
Type:Doctoral Thesis
Language:English
Publishing Institution:Universität Augsburg
Granting Institution:Universität Augsburg, Mathematisch-Naturwissenschaftlich-Technische Fakultät
Date of final exam:2012/11/30
Release Date:2013/04/10
Tag:infrared spectroscopy; high pressure technique; fullerene; Jahn-Teller effect; orientational ordering; insulator-to-metal transition
GND-Keyword:Fullerene; Stoffeigenschaft; Metall-Isolator-Phasenumwandlung; Jahn-Teller-Effekt; Infrarotspektroskopie; Hochdrucktechnik
Institutes:Mathematisch-Naturwissenschaftlich-Technische Fakultät
Mathematisch-Naturwissenschaftlich-Technische Fakultät / Institut für Physik
Dewey Decimal Classification:5 Naturwissenschaften und Mathematik / 53 Physik / 530 Physik
Licence (German):Deutsches Urheberrecht