R.A.EVARESTOV, A.V.LEKO and V.A.VERYAZOV St.Petersburg State University, 199034 St.Petersburg, Russia YU.S.GRUSHKO, S.G.KOLESNIK and S.N.KOLESNIK St.Petersburg Nuclear Physics Institute, 188350 Gatchina, Russia
A joint study of phonon and electron subsystems of C60I4-x and
C60Br24 crystals has shown a different character of chemical bonding
in these materials: C60I4-x is a compound with weak van-der-Waals
interaction between C60 and I2 molecules whereas C60Br24 is a
covalent crystal with a charge transfer between nearest C and Br
atoms.
1. Introduction
The present paper is devoted to experimental and theoretical stu- dies of the influence of different halogen doping of fullerenes on their phonon and electron states. We investigated two extreme cases: a) C60I4-x compounds formed by C60 molecule sublattice and I2 molecule sublattice which weakly interact via van-der-Waals forces. b) C60Br24 compounds with covalent bonds between C and Br atoms.
We used the group theory analysis and Raman scattering (RS) experiments for investigations of phonons.
Calculations of the electronic structure were made within the
CNDO (Complete Neglect of Differential Overlap) approximation with
taking into account the translational symmetry of crystals.
2. Preparation of the samples
The purified solid C60 (above 99.8%) was washed with ether and
then was sublimed in high vacuum to remove incorporated solvent. The
brominated C60 was prepared by modified procedure described in
[1]. A mixture of fine powdered C60 and liquid bromine (10 mg/ml)
was stirred under nitrogen at room temperature for 5 days. The solid
product was isolated by evaporation of the excess of bromine in a flow
of dry nitrogen at 50 C.
3. Group symmetry analysis of normal vibrations
3.1. C60I4 crystals
The space group of C60I4 is C3i1(P3-) (hexagonal Bravais lattice) with 10 sets of C atoms (C1-C10) occupying 6g positions (site symmetry C1) and I atoms occupying 2d(C3) and 3f(Ci) positions with fractional occupation numbers [2].
Using the method of band induced representations of space groups [3] we determined the symmetry of normal vibrations of C60I4 crystals at the symmetry points of the Brillouin zone (BZ).
The full vibrational representation at the ƒ-point of the BZ is
where the contribution of iodine atom vibrations is given byƒ = 31Ag + 31(1Eg + 2Eg) + 34Au + 34(1Eu + 2Eu) (1)
We determined that only iodine atoms in 2d position contibute into the Raman-active modes Ag and Eg=1Eg + 2Eg.ƒ(I) = Ag + (1Eg + 2Eg) + 4Au + 4(1Eu + 2Eu). (2)
3.2. C60Br24 crystals
The symmetry of C60Br24 crystals is low C3i1(P3-) (hexagonal Bravais lattice) [1] but the intrinsic molecular symmetry is close to the idealized maximal symmetry Th3 (Fm3) (fcc lattice).
In the hexagonal lattice 10 sets of carbon atoms (C1-C10) and 4 sets of Br atoms (Br1-Br4) occupy 6g positions (site symmetry C1).
In the fcc lattice 12 carbon atoms (C1) occupy 12h position (Cs) and two sets of carbon atoms (C2,C3) and Br atoms occupy 24i positions (C1).
For the fcc lattice, the full vibrational rep at the ƒ-point is
ƒ = 11Ag + 11(1Eg+2Eg) + 31Fg + 10Au + 10(1Eu+2Eu) + 32Fu (3)
where the contribution of Br-atom vibrations is given by
ƒ(Br) = 3Ag + 3(1Eg+2Eg) + 9Fg + 3Au + 3(1Eu+2Eu) + 9Fu. (4)
The real structure can be obtained from the idealized one by a small perturbation which reduces the symmetry. This leads to a splitting of Fg and Fu lines Fg,u -> Ag,u+[1Eg,u+2Eg,u]. As a result the full vibrational rep is
4. Experimental study of phonon subsystemƒ= 42[Ag + (1Eg + 2Eg)+ Au + (1Eu + 2Eu)]. (5)
Raman scattering spectra were studied using Dilor Z-24 triple spectrometer and Spectra-Physics-2020 Ar-laser, l=514.5nm. Fig.1 presents the RS spectra of C60, C60I2.4, and C60Br24 crystals.
RS spectra of C60 were investigated in a number of works (see,
e.g., [4-6]). According to group-theory analysis the C60 molecule has
10 Raman-active modes 2Ag+8Hg. For C60 crystals, the most intense
10 lines are due to these intramolecular modes.
4.1. C60I2.4 crystals
In RS spectra of C60I2.4 these lines are also observed. Additionally, a broad band 165-215 cm-1 is seen in the low frequency region of C60I2.4 spectra. The totally-symmetrical vibration frequency of a free I2 molecule is 213 cm-1 [7]. Therefore, we can assign this band to the only Ag vibration induced by I atoms in C60I2.4 crystals, see eq.(2).
Taking into account that the other lines in RS spectra nearly do
not shift with respect to C60 spectra, we can conclude that C60 and
I2 molecules are bound by weak van-der-Waals forces.
4.2. C60Br24 crystals
The RS spectra of C60Br24 differ significantly from that of C60 (Fig.1). The most of the observed lines appear to be assigned to totally-symmetric Ag modes which, as a rule, are the most intense lines in the spectra. In C60Br24 spectra, the line with the frequency of a free Br2 molecule (317 cm-1, [7]) is absent.
Thus, a formation of C-Br bonds influences strongly on all the
vibrations of fullerenes including those in which the Br atoms are
involved (3Ag, see eq.(4)) and other vibrations in which the Br
atoms do not participate directly.
5. Chemical bonding and charge transfer calculations The electronic structure and chemical bonding in crystalline fullerenes C60, C60I2, and C60Br24 were studied by CNDO - band model technique. The results of these calculations are presented in Table 1.
| Bond type | C60 | C60I2 | C60Br24 ideal | C60Br24 real |
| a(C-C) | 1.49 | 1.48-1.49 | 1.75 | 1.78 |
| b(C-C) | 1.10 | 1.10 | 0.99 | 0.95-0.98 |
| c(C-C) | 1.10 | 1.10 | 0.99-1.03 | 0.98 |
| d(C-C) | 1.49 | 1.48-1.49 | 1.68 | 1.78 |
| C-Hal | - | 0.002 | 0.83 | 0.84 |
| Hal-Hal | - | 0.99 | 0.02 | 0.01 |
5.1.C60I4-x crystals
The structure of I-doped fullerene is based on data from [2] for
C60I4, however we considered only one iodine molecule per unit
cell, namely C60I2. We found very weak interaction between C60
ball and iodine molecule I2. The Wiberg indexes of C-C bonds in
C60I2 are close to that in undoped fullerene C60.
5.2.C60Br24 crystals
Crystal structure studies of C60Br24 [1] show that the C60 balls
are distorted. To distinguish a role of this distortion and
partial saturation of C-C bonds we performed calculations for
both idealized (with unperturbated C60) and real structure of
C60Br24. It was found the Br atoms to form single covalent bonds
with carbon atoms. The comparison of calculated Wiberg indexes
of C-C bonds shows that delocalized conjugated bonds in six-fold
rings in perfect C60 transform into single or double bonds in
C60Br24 (see Fig.2). We can conclude that even in idealized
geometry the appearance of different types of bonds takes place,
however the charge distribution in idealized and real structures
of C60Br24 is different. Atomic charges on Br atoms are -0.1;
atomic charges on neighbouring carbon atoms are 0.16 and 0.10 in
idealized and real structures, respectively.
6. Conclusions
The study of phonon and electron states in C60I4-x and C60Br24 shows a significant difference in their properties.
C60I4-x can be considered as a quasi-molecular crystal formed by two sets of weakly interacting molecules C60 and I2. The vibrational spectrum of this crystal is a superposition of C60 and I2 spectra. In this compound, the charge transfer is nearly absent.
In contrast, the vibrational spectrum of C60Br24 differs both from C60 and Br2 spectra. The C60Br24 compound is characterized by a strong covalent bonding C-Br and by a significant redistribution of the electron density at C60 molecules. Instead of conjugated bonds in hexagons, the well-defined single and double bonds are formed. The charge transfer between neighbouring C and Br atoms takes place. The charge values at the atoms are of the order of 0.1e.
This work was supported by the Foundation for Intellectual Collaboration (St.Petersburg, Russia).
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FIGURE CAPTIONS
Fig.1. Raman spectra of C60, C60I2.4, and C60Br24 compounds at T=300K.
Fig 2 Different type of bonds in the C60Br24 molecule in the crystal (a).
Notation of bonds is shown on a fragment(b).
Double bonds on a visible part of fullerene ball are shown by
thick lines.