Electronic spin states in fullerides and endohedral fullerenes [Elektronische Ressource] / vorgelegt von Jürgen Rahmer

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• Publié par: universitat_stuttgart
• Publié le: 01 janvier 2004
• Nombre de lectures: 28
• Langue: English
• Poids de l'ouvrage: 13 Mo

Extrait

Electronic Spin States in Fullerides and
Endohedral Fullerenes
Von der Fakulatt Mathematik und Physik der Universit at Stuttgart
zur Erlangung der Wurde eines Doktors der
Naturwissenschaften (Dr. rer. nat.) genehmigte Abhandlung
Vorgelegt von
Jur gen Rahmer
aus Boblingen
Hauptberichter: Prof. Dr. M. Mehring
Mitberichter: Prof. Dr. D. Schweitzer
Eingereicht am: 12. Mai 2003
Tag der mundlic hen Prufung: 16. Juli 2003
2. Physikalisches Institut der Universit at Stuttgart
20032Preface
The proposal of a spherical shape for the carbon cluster C in the middle of the60
1980s triggered a number of fundamental discoveries. At the outset, these were the
clari cation of the singular structural and electronic properties of the “Buckminster
fullerene” C and the larger members of the family of carbon cage molecules. With60
theavailabilityofgreateramounts offullerenesthankstotheKratsc hmerarcburning
synthesis, the interest shifted from molecular to solid state properties of C . Crys-60
talline C , also referred to as “fullerite”, exhibited new astonishing phenomena, like60
the three-dimensional rotation of C within the crystalline structure and the light-60
or pressure-induced formation of one- and two-dimensional intra-crystalline fullerene
polymers.
TheincorporationofalkaliatomsintothelargeinterstitialspacesinsolidC brought60
yet another new discovery: the formation of fullerene charge-transfer salts, so-called
“fullerides”. Depending on the magnitude of the charge transfer, these materials are
insulating, semiconducting, ormetallic, andevensuperconductivitywithcriticaltem-
peraturesashighas40Khasbeenobserved. Furthermore,reversiblestructuralphase
transitionsbetweenmonomerand(C ) -dimeraswellas(C ) -polymerphaseswere60 2 60 n
discovered. Theuseofcounterionsotherthanthealkalielementsledtofurtherinter-
1esting materials, like the organic ferromagnet TDAE-C with a Curie temperature60
of 16 K.
Onthemolecularlevel,researchsoonfocussedon“endohedralfullerenes”,i.e.,carbon
cages that were found to enclose atoms or clusters of atoms. These materials allow
intrinsic doping of fullerenes, while at the same time the dopants are shielded and
stabilized by the carbon shell of the cage.
The award of the 1996 Nobel prize in chemistry to Kroto, Curl, and Smalley for the
discovery of the fullerenes marked both the peak and the end of this initial phase of
fullerene research. At this time, the focus started to shift towards carbon nanotubes,
which were discovered in 1991 as a by-product of the fullerene production process.
1TDAE is tetrakis(dimethylamino)ethylene [1].
34
Due to their unique electronic and mechanical properties, carbon nanotubes promise
a number of applications in the elds of nano-electronics and materials’ science. The
present phase of fullerene research is therefore characterized by a lower level of activ-
ity, but a drive towards the realization of new unconventional materials by combining
building blocks from classical chemistry with fullerene cages. Typical research aims
are the creation of magnetic materials by combining transition metal elements with
fullerenes, the improvement of the endohedral fullerene production methods towards
high yields to eventually enable the synthesis of endohedral fullerene solids, the syn-
thesis of C cation salts (“fullerenium salts”), and the inclusion of fullerenes into60
carbon nanotubes (“peapods”).
Thisthesisreportsoninvestigationsofclassicfullerenematerialstypicalfortheinitial
research phase as well as new materials. Several fullerene salts and one endohedral
system were studied by means of electron spin resonance (ESR) at standard and
high elds corresponding to X-band (9.5 GHz) and W-band (95 GHz) frequencies.
In part, supplementary nuclear magnetic resonance (NMR), magnetic susceptibility,
x-ray, and Raman measurements were performed.
The alkali fullerides RbC and CsC are classic fullerene materials, which exhibit60 60
a number of structurally di erent phases. Among them, the metallic polymer phase
hasattractedconsiderableattention. Theexistenceofone-dimensionalpolymerchains
in this phase led to the proposal of a quasi-one-dimensional metallic character. In
contrast, band structure calculations suggested a rather isotropic electronic system.
This contradiction motivated ESR investigations at standard and high- elds, which
are presented in chapter 4.
Bis(arene)chromium fullerides belong to the more recently synthesized com-
pounds, which combine C with transition metals in order to create new magnetic60
materials. Chapter 5 presents a complete characterization of bis(toluene)chromium
fullerideincludingtheclari cationofaphasetransitiontoadimerphasebelow250K.
Thischapterfurthermorepresentsinvestigationsofbis(mesitylene)chromiumfulleride
and bis(benzene)chromium fulleride.
An interesting group of fulleride materials which has not received much attention to
date are the mixed alkali/alkaline-earth fullerides. In comparison to alkali ful-
lerides, the higher charge transfers achieved with alkaline-earth dopants allow higher
doping levels without the need to increase the number of dopant atoms. In this way,
structural e ects which depend on the number of counter ions per C can be sepa-60
ratedfromthein uenceofthemagnitudeofthechargetransfertothemolecule. Two
protagonists of this group of fullerides, namely CsBaC and KCsBaC are presented60 605
in chapter 6.
Another modern system are fullerenium salts, which result from the combination
of C with strongoxidants. These materials address the question whether the wealth60
of solid state phenomena observed in the electron-doped fullerenes can also be real-
ized with hole-doped C . ESR measurements of fullerenium arsenic hexa uoride are60
presented in chapter 7.
The last research topic addressed in this work concerns the classic low-yield endo-
hedral system Sc @C . Although the discovery of this substance dates back to3 82
131991, temperature-dependent ESR studies and the availability of C enriched ma-
terial allowed new insights into this fascinating compound. Chapter 9 presents the
investigations of Sc @C in solution. In order to enable a better assessment of the3 60
obtainedresults, chapter8discussesothergroupIII(Sc, Y,La)endohedralmaterials.
Furthermore, by way of a general introduction to fullerenes, chapter 1 reviews molec-
ular properties of C and higher fullerene cages, while chapter 2 gives a brief report60
on relevant solid state properties of C and the fullerene salts.60
Chapter 3 brie y discusses the experimental methods used in this work. Right from
the beginning of fullerene research, electron spin resonance (ESR) played an im-
portant role in the elucidation of electronic properties of fullerene-based materials.
It helped to identify and discriminate the rst paramagnetic endohedral fullerene,
La@C ,fromdiamagneticemptycages. Moreover,solidstatepropertieslikemetallic-82
ity,magnetic-orderphenomena,orsuperconductivitycanbedirectlyaccessedbyESR.
Temperature-dependent studies furthermore supply information about electronic dy-
namics in molecules as well as in the solid state. Finally, hyper ne coupling between
the electronic spin and neighboring nuclear spins provides information on the degree
of delocalization of the electronic wave function.
ESRsignalanalysisisgreatlyfacilitatedbythecomparisonofspectraobtainedatdif-
ferent frequency bands. The availability of a 95 GHz (W-band) ESR spectrometer in
addition to standard 9.5 GHz (X-band) spectrometers was thus a crucial prerequisite
for the thorough ESR analysis of fullerene materials presented in this work.
As mentioned above, in addition to ESR other methods have been applied. Among
those were x-ray di raction, magnetic susceptibility (SQUID) measurements, Raman
spectroscopy, and nuclear magnetic resonance (NMR), all of which are brie y intro-
duced and discussed in view of their role in fullerene research in chapter 3.
As a tribute to new dissertation regulations, a German abstract is appended to this
work in chapter “Zusammenfassung”.6Contents
Preface 3
List of Abbreviations 11
1 Fullerenes – Carbon in 3D 13
1.1 A New Form of Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.2 Buckminsterfullerene C . . . . . . . . . . . . . . . . . . . . . . . . . 1660
1.2.1 Structure and Symmetry . . . . . . . . . . . . . . . . . . . . . 16
1.2.2 Electronic Properties . . . . . . . . . . . . . . . . . . . . . . . 18
1.3 Higher Fullerenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.4 Endohedral Fullerenes . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.4.1 The First “Endohedrals” . . . . . . . . . . . . . . . . . . . . . 22
1.4.2 The Family of Endohedral Fullerenes . . . . . . . . . . . . . . 23
2 Solid State Fullerene Compounds 25
2.1 Fullerite – C in the Solid State . . . . . . . . . . . . . . . . . . . . . 2560
2.1.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.1.2 Electronic Properties . . . . . . . . . . . . . . . . . . . . . . . 27
2.1.3 Polymerized Fullerite . . . . . . . . . . . . . . . . . . . . . . . 28
2.2 Fullerides – C Forms Salts . . . . . . . . . . . . . . . . . . . . . . . 2960
2.2.1 Alkali Fullerides . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2.2 Mono-Alkali Fullerides AC (A=K,Rb,Cs) . . . . . . . . . . . 3060
2.2.3 A C – A Medium T Supercondu