# Superconductivity, Electron Paramagnetic Resonance, and Raman Scattering Studies of Heterofullerides with Cs and Mg

Authors: V. A. Kulbachinskii; B. M. Bulychev; V. G. Kytin; A. V. Krechetov; E. A. Konstantinova; R. A. Lunin
Journal: Advances in Condensed Matter Physics
 (2008)

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## Abstract

In the present study, the results of investigation of physical properties of heterofulleridesA3−xMxC60 (A=K, Rb, Cs, M=Be, Mg, Ca, Al, Fe, Tl, x=1,2); as well as RbCsTlC60, KCsTlC60, and KMg2C60 are described. All of the fullerides were synthesized by the exchange reactions of alkaline fullerides with anhydrous metal halides. Superconductivity was found in RbCsTlC60 and KCsTlC60.


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## Body

## 1. Introduction

It is well known that critical temperature of superconducting transition (Tc) of alkali-intercalated fullerides depends on the crystalline  lattice parameter [1]. The value of Tc increases with the increase in some interval of the lattice parameter a. Thus one can expect that
heterofullerides with Cs may possess higher Tc values because of larger a values, compared to fullerides
with K and Rb. Fulleride Cs3C60,  synthesized by standard
gaseous method, has primitive cubic lattice and is not a superconductor. Under
high pressure, Cs3C60 is a superconductor with Tc=40 K in FCC lattice as well as K3C60 and Rb3C60. The efforts to synthesize fullerides with
composition from Cs2C60 to Cs6C60 in a solvent (toluol) give amorphous or polymerized non-superconducting
substances [2]. It is worth to note that for KCs2C60 and RbCs2C60 fullerides, both methods produce
superconductors with the same Tc.We showed that exchange reaction of homofullerides of K and Rb with
anhydrous group 8–10 metal halides
rear earth metal halides in tetrahydrofuran substitute a part of alkali metal
to Ln or Fe with surviving of superconductivity [3, 4].
The open question is as follows: are these substances superconductors
themselves or they are partly defected fullerides K3C60 and Rb3C60? To check the effectiveness of exchange
reaction, we tried to synthesize by the same method the heterofullerides with
Cs. The Cs fullerides
under normal conditions are non-superconductors, that is why the observation of
superconductivity when some atoms of Cs replaced by metal of will be an evidence
of superconducting properties of heterofullerides with other not alkali metals.


## 2. Experimental

Toluene and tetrahydrofuran (THF) were cleared
and absolutized by standard techniques. Anhydrous metal chlorides were also obtained
by standard techniques; chlorides of beryllium, aluminum, and iron were
synthesized by chlorination of metal by dry chlorine; chlorides of magnesium,
calcium, barium, and strontium were received by thermal drying of hydrated salts
with ammonium chloride in vacuum; thallium chloride was commercially available.
Potassium, rubidium, and caesium fullerides were synthesized by reaction of
fullerite with the stehiometric quantity of metal in the environment of toluene
at 120–130C° by a method
described in [5].Synthesis
of the initial of alkali metal fullerides, partial removal of toluene, blending of fulleride suspension with suspension of
heterometal halogenide in THF, drying of products of reaction, and their
packaging in ampoules for definition of temperature of superconducting
transition were carried out in vacuum in the full glass facilities equipment.
In more detail, the technique is described in [4, 5]. The exchange reactions
are writing as
(1)A4C60+MCl2⟶THFA2MC60+2ACl,(2)A5C60+2MCl2⟶THFAM2MC60+4ACl,(3)A5C60+M′Cl3⟶THFA2M′C60+3ACl,(4)A3C60+M′′Cl⟶THFA2M′′C60+ACl,
where A—K, Rb, Cs; M—Be, Mg, Ca; M′—Al, Fe; M′′—Tl.The measurements
of RFA were carried out with Guinier G670 HUBER. The sample of fulleride was
located in a thin-walled glass capillary which was brazed in vacuum.C13 NMR spectra are measured by Bruker AC-200 under the one-pulse program with
duration of excitation impulse 4 microseconds. Intervals
between impulses was 2 seconds. For measurements of Raman spectra Perkin-Elmer
Raman spectrometer  with He-Ne, the laser with wavelengths of 632.7 nanometers
was used. Measurements of EPR spectra were carried out on EPR spectrometer Bruker
(model Elexsys 500) in the X-range with flow cryostat in temperature interval
of 105–300 K. The values
of g-factors were calculated based on reference line of Mn2+ions in
MgO (g=1.9812).Temperatures of
superconducting transitions of fullerides were defined by standard
low-frequency induction method by measuring the temperature dependence of a
magnetic susceptibility in temperature interval4.2<T<300K.


## 3. Results and Discussion

### 3.1. Superconductivity

FulleridesRbCsTlC60 and Rb2TlC60 have superconducting
transition temperature Tc higher than others. For KCsTlC60, the
value of Tc = 21.7 K is the highest for fullerides with K. Fullerides with composition Cs3−xMxC60 (M—metal, x=1,2) are non-superconductors. We found that Tc of fullerides with composition K2MC60 is lower than that of K3C60 (excluding KCsTlC60, with Tc =21.7 K). Moreover,
there is a correlation between lattice constant of these fullerides and their Tc, namely,
lattice constant of K2MC60 fullerides (excluding KCsTlC60) is smaller than lattice constant
of K3C60 due
to smaller radii of ion substituting ion K+.Superconductivity of triple heterofulleridesRbCsTlC60 at Tc = 26.4 K, KCsTlC60 at Tc = 21.7 K, and a heterofulleride Rb2TlC60 at Tc = 27.2 K was discovered by
investigation of temperature dependence of magnetic susceptibility. Temperature
dependencies of magnetic susceptibility of these compounds are plotted in Figure 1. It should be mentioned that RbCsTlC60 and Rb2TlC60 transfer to superconducting state at temperatures higher than Tc values of our previously
investigated heterofullerides, synthesized by the same method. The Tc = 24.5 K of Rb2BeC60 was maximal,
that was achieved previously [4, 6]. The fulleride KCsTlC60 possesses the highest Tc = 21.7 K among all synthesized in the present study of heterofullerides with potassium. This confirms
the suggestion that the increase of the Tc value is due to the increase of the lattice constant caused by substitution of
K and Rb by Cs. KCsTlC60 has maximal value of a(a=1.442nm compared to K3C60a=1.431nm) among all superconducting
heterofullerides with potassium. The same is for RbCsTlC60 (a=1.467nm,Rb2BeC60a=1.445nm). Such appreciable increase
of the parameter a of FCC lattice is quite naturally and
also is one more proof of intercalation of atoms of bigger sizes in fulleride
structure. There were no superconducting transitions in heterofullerides with
more than one Cs atom per fullerene C60. In Table 1, parameters of
two superconducting fullerides with Cs are shown together with K3C60 and Rb2TlC60 for comparison.Table 1
Composition, superconducting
transition temperature (Tc), crystal
lattice parameter (a), and ionic
radius of heterometal (r+) for some investigated fullerides.
CompositionTc (K)a (Å)r+ (Å)K3C6018.514.3110(5)1.33Rb2TlC6027.214.448(6)(+3e) 0.95(+1e) 1.47KCsTlC6021.714.424(14)(+3e) 0.95(+1e) 1.47RbCsTlC6026.414.673(17)(+3e) 0.95(+1e) 1.47Figure 1
Temperature
dependencies of magnetic susceptibility ofRbCsTlC60, KCsTlC60,
and Rb2TlC60. Superconductivity was found in new fullerides with Cs.


### 3.2. X-Ray Data

According to RFA, data majority of heterofullerides with Cs are
amorphous. All attempts to synthesize caesium heterofullerides by interaction of
homofulleridesCsnC60 with chlorides of beryllium,
magnesium, calcium, barium, aluminum, and iron on (1) and (3) lead to non-superconducting
X-ray amorphous samples (Figure 2). One can
see only wide peak (halo) at 2θ≈20.2° and peaks identified as crystalline CsCl in X-ray spectrum. As in C13 NMR spectra of these samples, there is no resonance at 170–200 ppm, which is
common for all homo- and heterofullerides with FCC lattice with freely rotating
anion C60−n. We assume that caesium fullerides are not
simply amorphous, but polymeric substances.Diffractogram ofCs2AlC60,
Cs2FeC60, and Cs2C60, Cs3C60.
Vertical lines show peaks of CsCl. There is no reflection from fulleride
crystal lattice.
(a)(b)(c)The results of
interaction ofCs3C60 with TlCl, according to (4), do not
give any new information. But in reaction of thallium chloride with alkali
metal heterofullerides, according to (4), obtained heterofullerides are crystalline
substances (Figure 3) and superconductors.Figure 3
Diffractogram of fullerideRbCsTlC60 synthesized by (4)
mixed with the product of reaction CsCl.


### 3.3. Raman Scattering

The results of
measurement and investigation of Raman scattering of fulleridesCs2C60,
Cs3C60, Cs2MC60 (M=Ca, Ba) showed
that these fullerides are most likely not crystalline. This is testified by the
absence of nearly all characteristic fullerene C60 phonon modes (Figure 4). The lines in Raman spectra are strongly shifted and extended compared to the lines of
pure fullerite. Thus one cannot definitely identify presented lines and
estimate the charge state of C60 molecule in these compounds.Raman scattering spectra of fulleridesCs2C60, Cs3C60,
and Cs2MC60 (M=Ca, Ba) obtained at (a) room temperature
and (b) Raman spectra in the range near line Ag(2) of K3C60 for the same fullerides.
(a)(b)The
absence of the majority of oscillatory modes observed inK3C60 spectrum specifies polymerization (more rigid fixing of molecules C60).On
the basis of X-ray and Raman spectra, one can conclude that fullerides and
heterofullerides of caesium are polymers. This fact can explain the absence of
superconductivity in these compounds because polymerization suppresses the
phonon modes ofC60 molecules that mediate electron-electron attraction
[7]. Besides, polymerization may cause transition to the dielectric state.


### 3.4. Electron Paramagnetic Resonance

In
EPR spectrum ofCsMg2C60, there are two lines, the second
one has higher g-factor and is
much wider than the main line. Parameters of this line are shown in Table 2.
Intensity of this line decreases as temperature decreases and it is not visible for T<120K. The
most probably explanation of the drastically decrease of the intensity is due
to transition of paramagnetic centers to the different spin states. The nature of
these centers at the moment is unknown.Table 2
Average line widthΔHpp and g-factor of EPR for non-superconducting fullerides of Cs and Mg.
CompositionΔHpp and g-factorT (K)297200120Cs2MgC60g2.0010± 0.0003ΔHpp (Oe)4.85.47.3CsMg2C60g2.0010±0.0003ΔHpp (Oe)3.34.47.2CsMg2C60 (wide line)g2.23352.2785—ΔHpp (Oe)248221—K2MgC60g2.0010±0.0003ΔHpp (Oe)3.72.72.1KMg2C60g2.0009±0.0003ΔHpp (Oe)52.41.8The analysis of electron paramagnetic resonance
(EPR) spectra and their temperature dependence revealed the presence of at
least two different types of paramagnetic centers, that is, two lines of
different widths, in superconducting fulleridesA2MC60 (A=K, Rb, M=Be, Mg)—Figure 5.Figure 5
Asymmetric of
EPR spectra heterofullerides with Cs pointed out to asymmetric environment of
moleculeC60 due to possible polymerization.Temperature
dependence of paramagnetic susceptibility (double-integrated EPR signal) of
narrow line is described by Curie-Weiss law Figure6. Therefore, the
appropriate paramagnetic centers obey Bolzman statistics and probably are due
to electron-charged defects like C60–O or C60–O–C60 [8, 9]. Temperature dependence of parameters
of broad line (second type of centers) is weak. This points out to the Fermi
statistics of the centers of the second type, presumably they are conduction
electrons [10, 11].Figure 6
Temperature
dependence of paramagnetic susceptibilityχ (double-integrated EPR signal) non-superconducting
fullerides with Cs. Curie-Weiss law (fitting by solid lines) is a consequence
of the absence of conduction electrons (no contribution to the paramagnetic
susceptibility).At the same time, EPR spectra of
non-superconducting fullerides with the compositionCsxC60 (x=2,3), Cs2AlC60 can be described by anisotropic signal
of only one type of paramagnetic signals with Bolzman statistics, that is,
defects with odd electron charge. This follows from temperature dependence of
lineshape and paramagnetic susceptibility, calculated by double integration of
EPR spectra. In the investigated temperature range (105 K–300 K),
paramagnetic susceptibility can be well fitted by Curie-Weiss law. The
extraction of the signal from conducting electrons with temperature-independent
paramagnetic susceptibility for ERP spectra of these fullerides is not
possible, indicating that the density of electrons is low. This fact is
confirmed by the absence of superconducting transition in this fullerides.In
Figure7, we plotted ERR spectra for Cs2FeC60 fulleride.
The width of the line increases while the amplitude decreases when temperature
decreases, notification of ferromagnetic resonance. Some parameters of EPR
signals for non-superconducting
fullerides are listed in Table 2.Figure 7
EPR spectra at different temperatures ofCs2FeC60.


## 3.1. Superconductivity

FulleridesRbCsTlC60 and Rb2TlC60 have superconducting
transition temperature Tc higher than others. For KCsTlC60, the
value of Tc = 21.7 K is the highest for fullerides with K. Fullerides with composition Cs3−xMxC60 (M—metal, x=1,2) are non-superconductors. We found that Tc of fullerides with composition K2MC60 is lower than that of K3C60 (excluding KCsTlC60, with Tc =21.7 K). Moreover,
there is a correlation between lattice constant of these fullerides and their Tc, namely,
lattice constant of K2MC60 fullerides (excluding KCsTlC60) is smaller than lattice constant
of K3C60 due
to smaller radii of ion substituting ion K+.Superconductivity of triple heterofulleridesRbCsTlC60 at Tc = 26.4 K, KCsTlC60 at Tc = 21.7 K, and a heterofulleride Rb2TlC60 at Tc = 27.2 K was discovered by
investigation of temperature dependence of magnetic susceptibility. Temperature
dependencies of magnetic susceptibility of these compounds are plotted in Figure 1. It should be mentioned that RbCsTlC60 and Rb2TlC60 transfer to superconducting state at temperatures higher than Tc values of our previously
investigated heterofullerides, synthesized by the same method. The Tc = 24.5 K of Rb2BeC60 was maximal,
that was achieved previously [4, 6]. The fulleride KCsTlC60 possesses the highest Tc = 21.7 K among all synthesized in the present study of heterofullerides with potassium. This confirms
the suggestion that the increase of the Tc value is due to the increase of the lattice constant caused by substitution of
K and Rb by Cs. KCsTlC60 has maximal value of a(a=1.442nm compared to K3C60a=1.431nm) among all superconducting
heterofullerides with potassium. The same is for RbCsTlC60 (a=1.467nm,Rb2BeC60a=1.445nm). Such appreciable increase
of the parameter a of FCC lattice is quite naturally and
also is one more proof of intercalation of atoms of bigger sizes in fulleride
structure. There were no superconducting transitions in heterofullerides with
more than one Cs atom per fullerene C60. In Table 1, parameters of
two superconducting fullerides with Cs are shown together with K3C60 and Rb2TlC60 for comparison.Table 1
Composition, superconducting
transition temperature (Tc), crystal
lattice parameter (a), and ionic
radius of heterometal (r+) for some investigated fullerides.
CompositionTc (K)a (Å)r+ (Å)K3C6018.514.3110(5)1.33Rb2TlC6027.214.448(6)(+3e) 0.95(+1e) 1.47KCsTlC6021.714.424(14)(+3e) 0.95(+1e) 1.47RbCsTlC6026.414.673(17)(+3e) 0.95(+1e) 1.47Figure 1
Temperature
dependencies of magnetic susceptibility ofRbCsTlC60, KCsTlC60,
and Rb2TlC60. Superconductivity was found in new fullerides with Cs.


## 3.2. X-Ray Data

According to RFA, data majority of heterofullerides with Cs are
amorphous. All attempts to synthesize caesium heterofullerides by interaction of
homofulleridesCsnC60 with chlorides of beryllium,
magnesium, calcium, barium, aluminum, and iron on (1) and (3) lead to non-superconducting
X-ray amorphous samples (Figure 2). One can
see only wide peak (halo) at 2θ≈20.2° and peaks identified as crystalline CsCl in X-ray spectrum. As in C13 NMR spectra of these samples, there is no resonance at 170–200 ppm, which is
common for all homo- and heterofullerides with FCC lattice with freely rotating
anion C60−n. We assume that caesium fullerides are not
simply amorphous, but polymeric substances.Diffractogram ofCs2AlC60,
Cs2FeC60, and Cs2C60, Cs3C60.
Vertical lines show peaks of CsCl. There is no reflection from fulleride
crystal lattice.
(a)(b)(c)The results of
interaction ofCs3C60 with TlCl, according to (4), do not
give any new information. But in reaction of thallium chloride with alkali
metal heterofullerides, according to (4), obtained heterofullerides are crystalline
substances (Figure 3) and superconductors.Figure 3
Diffractogram of fullerideRbCsTlC60 synthesized by (4)
mixed with the product of reaction CsCl.


## 3.3. Raman Scattering

The results of
measurement and investigation of Raman scattering of fulleridesCs2C60,
Cs3C60, Cs2MC60 (M=Ca, Ba) showed
that these fullerides are most likely not crystalline. This is testified by the
absence of nearly all characteristic fullerene C60 phonon modes (Figure 4). The lines in Raman spectra are strongly shifted and extended compared to the lines of
pure fullerite. Thus one cannot definitely identify presented lines and
estimate the charge state of C60 molecule in these compounds.Raman scattering spectra of fulleridesCs2C60, Cs3C60,
and Cs2MC60 (M=Ca, Ba) obtained at (a) room temperature
and (b) Raman spectra in the range near line Ag(2) of K3C60 for the same fullerides.
(a)(b)The
absence of the majority of oscillatory modes observed inK3C60 spectrum specifies polymerization (more rigid fixing of molecules C60).On
the basis of X-ray and Raman spectra, one can conclude that fullerides and
heterofullerides of caesium are polymers. This fact can explain the absence of
superconductivity in these compounds because polymerization suppresses the
phonon modes ofC60 molecules that mediate electron-electron attraction
[7]. Besides, polymerization may cause transition to the dielectric state.


## 3.4. Electron Paramagnetic Resonance

In
EPR spectrum ofCsMg2C60, there are two lines, the second
one has higher g-factor and is
much wider than the main line. Parameters of this line are shown in Table 2.
Intensity of this line decreases as temperature decreases and it is not visible for T<120K. The
most probably explanation of the drastically decrease of the intensity is due
to transition of paramagnetic centers to the different spin states. The nature of
these centers at the moment is unknown.Table 2
Average line widthΔHpp and g-factor of EPR for non-superconducting fullerides of Cs and Mg.
CompositionΔHpp and g-factorT (K)297200120Cs2MgC60g2.0010± 0.0003ΔHpp (Oe)4.85.47.3CsMg2C60g2.0010±0.0003ΔHpp (Oe)3.34.47.2CsMg2C60 (wide line)g2.23352.2785—ΔHpp (Oe)248221—K2MgC60g2.0010±0.0003ΔHpp (Oe)3.72.72.1KMg2C60g2.0009±0.0003ΔHpp (Oe)52.41.8The analysis of electron paramagnetic resonance
(EPR) spectra and their temperature dependence revealed the presence of at
least two different types of paramagnetic centers, that is, two lines of
different widths, in superconducting fulleridesA2MC60 (A=K, Rb, M=Be, Mg)—Figure 5.Figure 5
Asymmetric of
EPR spectra heterofullerides with Cs pointed out to asymmetric environment of
moleculeC60 due to possible polymerization.Temperature
dependence of paramagnetic susceptibility (double-integrated EPR signal) of
narrow line is described by Curie-Weiss law Figure6. Therefore, the
appropriate paramagnetic centers obey Bolzman statistics and probably are due
to electron-charged defects like C60–O or C60–O–C60 [8, 9]. Temperature dependence of parameters
of broad line (second type of centers) is weak. This points out to the Fermi
statistics of the centers of the second type, presumably they are conduction
electrons [10, 11].Figure 6
Temperature
dependence of paramagnetic susceptibilityχ (double-integrated EPR signal) non-superconducting
fullerides with Cs. Curie-Weiss law (fitting by solid lines) is a consequence
of the absence of conduction electrons (no contribution to the paramagnetic
susceptibility).At the same time, EPR spectra of
non-superconducting fullerides with the compositionCsxC60 (x=2,3), Cs2AlC60 can be described by anisotropic signal
of only one type of paramagnetic signals with Bolzman statistics, that is,
defects with odd electron charge. This follows from temperature dependence of
lineshape and paramagnetic susceptibility, calculated by double integration of
EPR spectra. In the investigated temperature range (105 K–300 K),
paramagnetic susceptibility can be well fitted by Curie-Weiss law. The
extraction of the signal from conducting electrons with temperature-independent
paramagnetic susceptibility for ERP spectra of these fullerides is not
possible, indicating that the density of electrons is low. This fact is
confirmed by the absence of superconducting transition in this fullerides.In
Figure7, we plotted ERR spectra for Cs2FeC60 fulleride.
The width of the line increases while the amplitude decreases when temperature
decreases, notification of ferromagnetic resonance. Some parameters of EPR
signals for non-superconducting
fullerides are listed in Table 2.Figure 7
EPR spectra at different temperatures ofCs2FeC60.


## 4. Conclusions

Exchange reactions of caesium fullerides with anhydrous metal chlorides lead to
polymeric and non-superconducting
phases. The exception is
ternary fullerides with supposed composition ofRbCsTlC60 with Tc = 26.4 K and KCsTlC60 with Tc = 21.7 K. As it is
known for alkali metal fullerides, Tc depends linear on the lattice parameter a and the same linear law was found
for synthesized superconducting heterofullerides.The EPR spectra
of non-superconducting fullerides with the compositionCsnC60 (n=2,3), Cs2AlC60 can be described by anisotropic
signal, that originate from one type of paramagnetic centers with Bolzman
statistics. Most likely, these centers are odd-charged defects C60–O or C60–O–C60. The results of measurement and investigation
of Raman scattering of fullerides Cs2C60, Cs3C60,
Cs2MC60 (M=Ca, Ba) showed that these fullerides are
polymers. This fact simultaneously with absence of conduction electrons can
explain the non-superconducting character of these compounds because
polymerization suppresses the phonon modes of C60 molecules that
mediate electron-electron attraction.



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*Source: 941372-2008-12-21.xml*