Mg5C60: A stable two dimensional conducting polymer

0.29 MB·English

by  D. Quintavalle

Mg C : A stable two dimensional conducting polymer
5 60
D. Quintavalle,1,∗ F. Borondics,2,† G. Klupp,2 A. Baserga,3 F. Simon,1 A. J´anossy,1 K. Kamara´s,2 and S. Pekker2
1Budapest University of Technology and Economics,
Institute of Physics and Condensed Matter Physics Research Group of the
Hungarian Academy of Sciences, H-1521, Budapest P.O.Box 91, Hungary
2Research Institute for Solid State Physics and Optics,
Hungarian Academy of Sciences, P.O. Box 49, Budapest, Hungary H-1525
3NEMAS-Center for NanoEngineered MAterials and Surfaces,
Dipartimento di Ingegneria Nucleare, Politecnico di Milano, I-20133 Milano, Italy
8 We present a study on the structural, spectroscopic, conducting and magnetic properties of
0 Mg5C60, a two dimensional (2D) fulleride polymer. The polymer phase is stable up to the ex-
0 ceptionally high temperatureof 823 K.Infrared andRaman studiessuggest theformation ofsingle
2 bonds between fulleride ions and possibly Mg - C60 covalent bonds. Mg5C60 is a metal at ambient
temperature as shown by electron spin resonance and microwave conductivity measurements. The
n
smooth transition from a metallic to a paramagnetic insulator state below 200 K is attributed to
a
J Anderson localization driven by structural disorder.

5
PACSnumbers: 61.48.+c,76.30.Pk,76.30.-v,78.30.-j
2

]
l I. INTRODUCTION In this paper, we present a study of the structure and
e
physical properties of the recently synthesized fulleride
-
r polymer Mg C . In a previous study of Mg C , a sto-
t The unusual physical and structural properties of al- 5 60 x 60
s ichiometric compound13 was reported for x = 4. In the
kali metal intercalated fulleride polymers have received
.
t present work we improved the synthesis and conclude
a considerable attention since the discovery in 1994 of
m the linear AC (A=K, Rb, Cs) conducting polymers.1 fromaseriesofsampleswithvaryingMgcontentthatthe
60
homogeneous phase lies in a range of Mg concentrations
Chemical reactions between charged fulleride ions in
-
d solids are rather common. The polymer phases usu- between x=5 and x=5.5. Mg5C60 is the only example
n ally form spontaneously or under mild pressure from of an alkaline earth fulleride polymer. Previous stud-
o the monomeric crystalline salts. Polymers of Cn− an- ies of alkaline earth fullerides focused mostly on CaxC60
c 60 andBa C (Refs. 14,15)superconductorswhicharenot
ions are usually unstable above temperatures of about x 60
[
400 K. The depolymerization is reversible, in contrast polymers. An earlystudy of Mg dopedfulleride films re-
1 to photopolymerization2 and pressure polymerization of portedaninsulatingbehaviorforallMgconcentrations16
v in disagreement with our present results.
neutral C (Refs. 3,4). According to quantum chemi-
2 calcalcula6t0ions,5 [2+2]cycloadditionisfavoredinA C WefindthatMg5C60 isa2Dpolymerwhichismetallic
9 n 60
at high temperature and undergoes a gradual transition
8 for low values of n, and configurations with single inter-
3 fullerene bonds are more stable for n ≥ 3. In agree- to an insulating ground state as the temperature is low-
. ment with these expectations, the stable form of AC− ered. It is stable up to the remarkably high temperature
1 60
is a cycloaddition polymer,6 while single intermolecu- of 823 K.
0
8 lar bonds occur between Cn60− anions in Na4C60 (Ref.
0 7) and Na2RbC60 (Ref. 8). Single and double bonds
: can appear simultaneously, as has been recently demon- II. EXPERIMENTAL
v
stratedinLi C (Refs. 9,10)wheresinglebondsconnect
i 4 60
X polyfulleride chains held togetherby [2+2]cycloaddition Samples of Mg C with nominal concentrations x =
x 60
r bonds. 4 to 6, in steps of ∆x = 0.5, were prepared by solid
a
Fulleride polymers have small electronic bandwidths state reaction between C60 and pure Mg powders under
and large on-site electron repulsion and thus are argon atmosphere in a dry box. The powder mixture
strongly correlated electron systems. Small differences was placed in carbon steel containers to avoid reaction
in the lattice parameters and/or variations in the chain between Mg grains and quartz vessels. Mg grain surface
orientation11 can change the ground state profoundly. activation at 753 K was followed by several annealing
For example, the linear polymers KC and Na RbC steps at temperatures from 653 K to 723 K. To improve
60 2 60
havemetallicgroundstateswhileRbC andCsC with thehomogeneityofthesample,weusedMgpowderwith
60 60
thesametypeofpolymerchainsbutdifferentchainorien- smaller grain sizes than in the previous preparation13.
tationsundergoametal-insulatortransitiontoanantifer- Powders were reground before each annealing step.
romagnetic spin density wave ground state.12 Electron- X-ray powder diffraction was performed using a HU-
electron correlations play an important role in 2D ful- BER G670 Guinier image plate camera in transmission
leride polymers as well: Na C is a strongly correlated mode and highly monochromatic CuKα radiation. In-
4 60 1
metal7whileLi C (Ref. 10)isanonmagneticinsulator. fraredspectrawererecordedonpressedKBrpelletswith
4 60
2
Mg x10
y
sit Raman
n
e
nt
nsity x = 6.0 man i IR
nte x = 5.5 Ra
I n /
o
x = 5.0 si
s
mi
s
x = 4.5 n
a
r
T
x = 4.0
16 18 20 22 35 36 37
2 / deg 500 1000 1500
-1
Frequency (cm )
FIG.1: X-raypowderdiffractiondataforaseriesofMgxC60
polymers, with nominalconcentrations x varyingfrom 4.0 to FIG. 2: Infrared and Raman spectra of Mg5C60.
6.0. Dotted line above36◦ corresponds to Mg metal.
trometer. The spin susceptibility was obtained by inte-
a BrukerIFS 28 FTIR spectrometerunder dynamic vac- grating the ESR spectra. MgO doped with 1.5 ppm of
uum. Raman spectra were collected using a T64000 Mn2+ ions wasthe ESRintensityreferenceto obtainthe
Jobin-Yvon spectrometer in triple grating configuration susceptibility between 100 and 300 K. Below 100 K the
withspectralresolutionbetterthan3cm−1. The532nm Mn:MgOESRsaturatesandfineorthorhombicpolymeric
line of a frequency-doubled Nd-YAG laser was the exci- KC60 powder (a low conductivity metal with a tempera-
tation source. The diameter of the spot was100 µm and ture independent susceptibility)wasusedas anintensity
thenominalirradianceabout150W/cm2,corresponding reference.
to ∼ 0.1 mW power on the sample.
Microwave conductivity was measured at 10 GHz on
fine Mg C powders mixedwith highpurity SnO pow- III. RESULTS AND DISCUSSION
5 60 2
der to electrically isolate the grains. A 10 GHz copper
cylindricalTE011resonantcavitywasusedforthecavity A. X-ray diffraction
perturbation technique conductance measurements17,18.
In this method changes in the quality factor are mea- Powder diffractograms of a series of Mg C salts
x 60
suredusinglock-indetectionofthe frequencymodulated (Fig. 1) show that the samples are single-phase for nom-
response. The sample is placed at the cavity center inal concentration values. The peak above 36◦ demon-
wherethemicrowavemagneticfieldismaximumandmi- stratesthatabovex=5.5nominalcompositionsomeMg
crowave currents encircle the grains. This geometry is metal is present. The single-phase material has been in-
well adapted for fine powders as the microwave electric dexedinthepreviouspaper13asrombohedralwithlattice
field is not shielded by depolarizationeffects.18 The typ- parameters a = b = 9.22A˚,c = 25.25A˚,γ = 120◦. These
ical grain size was much less than the microwave pene- values indicate the formation of two-dimensional hexag-
tration depth. The temperature dependence of the con- onal C polymeric sheets in the ab plane. The material
60
ductivityσ(T),isproportionalto1/Q−1/Q0. QandQ0 belongstotheR3mspacegroup. C60 moleculesareposi-
, the quality factors of the cavity with and without the tionedatthefractionalcoordinates(0,0,0),(2/3,1/3,1/3)
sample,respectively,weremeasuredintwoseparateruns. and (1/3,2/3,2/3), while the Mg positions are (0,0,0.23)
Q0 was about 16000 and it changed little with tempera- and (0,0,0.43). This structure corresponds to the nomi-
ture. Ohmiclossesofasinglesphericalgrainwithradius nal composition x = 4. We regard the magnesium con-
r andconductivityσ areproportionaltor5σ. Thedeter- tent presented in Fig. 1 more reliable than the previous
mination of the absolute value of the conductivity is not one13, due to the improved synthesis method using fine-
possiblewithoutadetailedknowledgeofthedistribution powered magnesium metal. The x-ray results also indi-
of r. cate that Mg C is not a line phase but the structure
5 60
Electron spin resonance (ESR) spectroscopy was per- presented here and in Ref. 13 comprises a range of sto-
formed at 9 and 225 GHz on samples sealed in quartz ichiometries between x=5 and 5.5. From the structural
tubes under200mbarHepressure. At9GHz acommer- data, however, we cannot determine the positions of the
cial Bruker ELEXSYS 500 spectrometer was used, the additional magnesium ions. A range of alkali concentra-
225 GHz spectra were recorded by a home-built spec- tionswithoutchangesinthestructureofthepolymerhas
3

1.3
1.2
nits) Mg5C60 polymer 1.1
U
b.
n (Ar RbC60 dimer 5 K)1.0
ssio (290.9
ansmi Na4C60 polymer (T)/0.8
r
T 0.7
Na2RbC60
0.6
500 750 1000 1250 1500
0.5
-1
Frequency (cm )
100 150 200 250 300 350 400
Temperature (K)
FIG.3: Comparison of theinfrared spectraof varioussingly
bonded fulleride oligo- and polymers.
FIG. 4: Microwave conductivity at 10 GHz of Mg5C60.
been observed in Li C (3 ≤ x ≤ 5),10 with the struc-
x 60
turalparametersgivenbyaRietveldfitcorrespondingto A (2)moderenderedinfraredactivebysymmetryreduc-
g
x = 4. Subsequent photoemission data on LixC60 thin tion and amplified by the ”vibrationalphase relaxation”
films with the same structure provedthe charge transfer mechanism.27 Prerequisites for this mechanism are 1. a
tobeincompleteandconstantthroughoutthehomogene- free-electron continuum, 2. low-frequency vibrational or
ity range.19 translationalmodeswhichmediatethecoupling. Accord-
ingtoourESRandmicrowavedata(seebelow),conduct-
ingelectronsarepresentinMg C anditisknownfrom
5 60
B. Infrared and Raman spectroscopy Raman spectroscopy of C60 photopolymers that inter-
molecular vibrations appear around 100 cm−1 (Ref. 2),
thus both criteriaare met. In the case of Mg C shown
TheinfraredandRamanspectraofMg C areshown 5 60
5 60
in Fig. 2 the mode is both infrared and Raman-active,
in Fig. 2. The low-frequency part corresponds to a
but, unlike References 24 and 25, in both cases shifted
symmetry-reduced C molecule with the principal T
60 1u upby12cm−1comparedtothe1468cm−1pristineA (2)
modes at 523, 576 and 1193cm, and severalnew modes, g
mode. Such blue shift, albeit smaller, has been observed
especiallyintheregionbetween700-900cm−1. Thelower
in C monolayersadsorbedonplatinum,28 where it was
symmetry confirms polymerization in accordance with 60
explained by covalent bonding to the metal, resulting in
x-ray diffraction. The line around 815 cm−1 is of par-
electronbacktransferfromthefullerenetothemetalsur-
ticular interest. This line appears in the spectrum of
face. Covalent bonding to at least some of the Mg ions
all C -based polymers20,21 where the C molecules are
60 60
cannot be excluded in the present case either, consider-
connected by single bonds (see Fig. 3). We consider the
ing the close values of the Mg ionization potential and
appearanceofthischaracteristicpeakinMg C asproof
5 60
theLUMOlevelofC . Thephenomenondescribedhere
for single bonds between fulleride ions. 60
warrants further investigation since it was not seen in
The high-frequency region around the T (4) mode
1u bulk fulleride salts before.
showsanunusualpattern,dominatedby a broadfeature
around 1480 cm−1. This feature cannot be explained by
ashiftintheT (4)modebychargetransferfromtheMg
1u
C. Microwave conductivity and ESR
atom, neither by polymerization effects, which are both
knowntoreducethevibrationalfrequencies.22,23 Instead,
weassignthispeaktotheA (2)vibrationalmode,which Both the microwave conductivity and ESR measure-
g
is Raman active in undistorted C . Our assignment is mentsshowthatMg C ismetallicaboveroomtemper-
60 5 60
supported by the fact that the peak is also dominant in ature and becomes insulating at low temperature. Fig.
the Raman spectrum. A similar effect was reported for 4 displaysthe temperature dependence ofthe microwave
C monolayers adsorbed on metal24 and semiconductor conductivity of Mg C normalized to 295 K. At high
60 5 60
surfaces.25 In those systems, an intense infrared absorp- temperatures the conductivity decreases with increasing
tion above the highest-frequency T mode is observed, temperature as it is usual in metals where scattering is
1u
together with a Raman counterpart,26 attributed to the due to phonons. The low temperature behavior is dif-
4

4.0 0.5 2.0
1.8
Arb. Units) 233...505 GHz (mT)00..34 111...246 25 GHz (mT)
sceptibility ( 12..50 e width at 9 0.2 001...680 ne width at 2
n su 1.0 Lin0.1 0.4 Li
pi
S 0.5 0.2
0.0 0.0
0.0 0 50 100 150 200 250 300
0 50 100 150 200 250 300
Temperature (K)
Temperature (K)
FIG. 6: Temperature dependence of the ESR line width of
FIG. 5: Temperature dependence of the spin susceptibility Mg5C60 at9GHz(squares)and225GHz(circles). Thetem-
of Mg5C60. 225 GHz ESR measurements (empty circles) are peraturedependenceofthefrequencyindependentcomponent
normalized to the 9 GHz ones (squares) at 300 K. The con- of the line width a(T) is also reported (triangles)
tinuouslineistheBrillouin functionofspin1/2in8.1Tfield
.
normalized at 50 K to the225 GHz measurement.
liesbelowE thegroundstateisinsulating. Electronsare
ferent from usual metals. The conductivity has a broad c
delocalized and the behavior is metallic if the tempera-
maximumanddecreasesbelow200K.Thisbehaviorcor-
ture is higher than E /k .
responds to a smooth transition between metallic and c B
insulating states. An additional insight into the electron relaxation
The ESR spectrum consists of a single Lorentzianline mechanism is given by the frequency and temperature
atalltemperaturesbothat9and225GHzconfirmingthe dependence of the ESR line width. The 9 GHz line is a
phase purity of the sample. Phase inhomogeneities usu- symmetric Lorentzian with a 4.5 G line width at room
ally split the line orat leastinhomogeneouslybroadenit temperature. The line broadens inhomogeneously with
duetodifferencesintheg-factor. IftheMgconcentration increasing frequency. The frequency and the tempera-
were inhomogeneous then the high frequency ESR lines ture dependences of the line width may be separated to
wouldbe distorted,especiallyinthe lowtemperaturein- give ∆H = a(T)+b(T)H. The frequency independent
sulating state where spatial variations are not averaged term a(T) is almost constant below 100 K (see Fig. 6)
by spin diffusion. and increases linearly with temperature as in metals in
The change from the low temperature insulating to which the main relaxation mechanism is the electron-
the high temperature metallic state is observed in the phonon interaction.31 The term b(T) increases with fre-
temperature dependence of the spin susceptibility also quency and with the lowering of temperature. The dis-
(Fig. 5). For T ≥200 K the spin susceptibility is almost tortionof the line andthe broadeningat 225GHz is due
independent of temperature, as expected for the Pauli to a partiallyaveragedg-factor anisotropy. We attribute
susceptibility of a degenerate electron gas. In normal the broadening at T < 200 K (see Fig. 6) to the in-
metals the spin susceptibility χ is related to the density homogeneous interaction between paramagnetic defects
of states at the Fermi level n(E ): χ = 1/4g2µ2n(E ), randomly distributed in the lattice.
F B F
where g is the spectroscopic g-factor and µ is the Bohr We measured a lower limit of the depolymerization
B
magneton. At lower temperatures a Curie contribution temperature by searching for changes in the ESR spec-
characteristic of a small concentration of localized spins tra after annealing at higher temperatures. The sealed
dominatestheESRspectrum(Fig.5). Thechangeofthe Mg C samplewasheatedatarateof10K/mintoeach
5 60
temperature dependence of the static susceptibility from temperature, kept there for 15 minutes, quenched into
Pauli to Curie like is gradual, there is no well defined waterandthen the 9 GHz ESRspectrawere recordedat
phase transition. We suggest that the smooth transition ambienttemperature. We repeatedthis cycletwicein25
to an insulating ground state is due to Anderson local- K steps between 300 and 848 K. We found that treat-
ization. At a certain degree of disorder of the lattice the ment up to 823 K leaves the ESR spectrum unchanged.
electronic wave functions with an energy below the mo- At 848 K the ESR spectrum and the color of the quartz
bilityedge,E ,becomelocalized.29,30 IftheFermienergy sample holder changed. These changes indicate a reac-
c
5
tionofMg(diffusingoutofthesample)withquartz. Itis We attribute the transition from metallic to paramag-
notclearwhetherwithoutthisreactiondepolymerization netic insulating states below 200 K to a disorder driven
would take place. We conclude that the polymer phase Anderson localization. The stability of the polymeric
is stable for at least 30 minutes at 823 K. structure of Mg C up to temperatures above 800 K is
5 60
exceptional,all other polymers of chargedfullerides con-
nectedbysingleordoubleC-Cbondsdecomposeatmuch
IV. CONCLUSION lower temperature.
Mg C with x = 5− 5.5 is a single phase, two di-
x 60
mensional fulleride polymer. X-ray diffraction indicates
V. ACKNOWLEDGMENT
afinite stoichiometryrangewiththesamestructure. Vi-
brational spectroscopy reveals that the Mg-fullerene in-
teraction is more complicated than simple charge trans- We gratefullyacknowledgeG.Oszl´anyifor helpful dis-
fer, with apossible covalentcomponent. Microwavecon- cussionsandL.Forr´oforprovidingtheX-bandESRspec-
ductivity shows that Mg C is metallic for T > 200 K trometer at the EPFL . This work was supported by
5 60
as confirmed by the temperature dependence of the spin theHungarianNationalResearchFund(OTKA)through
susceptibility and this is the reason for the temperature grants No. T 049338, T 046700, TS049881, F61733 and
dependence of the ESR line width in the metallic state. NK60984.
∗ Corresponding author. Electronic address: State Commun. 127, 311 (2003).
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