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

Abstract

In the present study, the results of investigation of physical properties of heterofullerides A_3−𝑥M_𝑥C₆₀ (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.

1. Introduction

It is well known that critical temperature of superconducting transition (𝑇_𝑐) of alkali-intercalated fullerides depends on the crystalline lattice parameter [1]. The value of 𝑇_𝑐 increases with the increase in some interval of the lattice parameter a. Thus one can expect that heterofullerides with Cs may possess higher 𝑇_𝑐 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 𝑇_𝑐=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 𝑇_𝑐.

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 120–130^∘C 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 A₄C₆₀+MCl−−−−→₂^THFA₂MC₆₀A+2ACl,₅C₆₀+2MCl−−−−→₂^THFAM₂MC₆₀A+4ACl,₅C₆₀+M^′Cl−−−−→₃^THFA₂M^′C₆₀A+3ACl,₃C₆₀+M^′′Cl−−−−→^THFA₂M^′′C₆₀+ACl, (1)(2)(3)(4) 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 interval 4.2<𝑇<300K.

3. Results and Discussion

3.1. Superconductivity

Fullerides RbCsTlC₆₀ and Rb₂TlC₆₀ have superconducting transition temperature 𝑇_𝑐 higher than others. For KCsTlC₆₀, the value of 𝑇_𝑐 = 21.7 K is the highest for fullerides with K. Fullerides with composition Cs_3−𝑥M_𝑥C₆₀ (M—metal, 𝑥=1,2) are non-superconductors. We found that 𝑇_𝑐 of fullerides with composition K₂MC₆₀ is lower than that of K₃C₆₀ (excluding KCsTlC₆₀, with 𝑇_𝑐 =21.7 K). Moreover, there is a correlation between lattice constant of these fullerides and their 𝑇_𝑐, 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 heterofullerides RbCsTlC₆₀ at 𝑇_𝑐 = 26.4 K, KCsTlC₆₀ at 𝑇_𝑐 = 21.7 K, and a heterofulleride Rb₂TlC₆₀ at 𝑇_𝑐 = 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 𝑇_𝑐 values of our previously investigated heterofullerides, synthesized by the same method. The 𝑇_𝑐 = 24.5 K of Rb₂BeC₆₀ was maximal, that was achieved previously [4, 6]. The fulleride KCsTlC₆₀ possesses the highest 𝑇_𝑐 = 21.7 K among all synthesized in the present study of heterofullerides with potassium. This confirms the suggestion that the increase of the 𝑇_𝑐 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 (𝑇_𝑐), crystal lattice parameter (a), and ionic radius of heterometal (𝑟⁺) for some investigated fullerides.

Figure 1 

Temperature dependencies of magnetic susceptibility of RbCsTlC60, 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 homofullerides Cs_𝑛C₆₀ 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.

(a)

(b)

(c)

Figure 2 

Diffractogram of Cs2AlC60, Cs2FeC60, and Cs2C60, Cs3C60. Vertical lines show peaks of CsCl. There is no reflection from fulleride crystal lattice.

The results of interaction of Cs₃C₆₀ 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 fulleride RbCsTlC60 synthesized by (4) mixed with the product of reaction CsCl.

3.3. Raman Scattering

The results of measurement and investigation of Raman scattering of fullerides Cs₂C₆₀, 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.

(a)

(b)

Figure 4 

Raman scattering spectra of fullerides Cs2C60, 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.

The absence of the majority of oscillatory modes observed in K₃C₆₀ 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 of C₆₀ molecules that mediate electron-electron attraction [7]. Besides, polymerization may cause transition to the dielectric state.

3.4. Electron Paramagnetic Resonance

In EPR spectrum of CsMg₂C₆₀, 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 width ΔHpp and 𝑔-factor of EPR for non-superconducting fullerides of Cs and Mg.

The 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 fullerides A₂MC₆₀ (A=K, Rb, M=Be, Mg)—Figure 5.

Figure 5 

Asymmetric of EPR spectra heterofullerides with Cs pointed out to asymmetric environment of molecule C60 due to possible polymerization.

Temperature dependence of paramagnetic susceptibility (double-integrated EPR signal) of narrow line is described by Curie-Weiss law Figure 6. 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 susceptibility 𝜒 (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 composition Cs_𝑥C₆₀ (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 Figure 7, 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 of Cs2FeC60.

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 of RbCsTlC₆₀ with 𝑇_𝑐 = 26.4 K and KCsTlC₆₀ with 𝑇_𝑐 = 21.7 K. As it is known for alkali metal fullerides, 𝑇_𝑐 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 composition Cs_𝑛C₆₀ (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.

Acknowledgment

The work was supported by RFBR, Grant no. 08-03-00237-a. The measurements of EPR and Raman spectra were carried out on the equipment of the Center of Users' Facilities of Moscow State University.