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High pressure study of Li-doped fullerides, LixC60 (x = 4,12), by x-ray diffraction and Raman

spectroscopy

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1. Introduction

The search for new carbon structures with a three-dimensional (3D) network similar to silicon clathrate superconductors has attracted the attention of many theoretical and experimental research groups in the last decades. These new low-density cage structures are predicted to have exceptional mechanical properties with bulk modulus and strength greater than dia-mond [1–3]. In particular, the possibility of building carbon clathrate structures from fullerenes has attracted much atten-tion. Fullerene has a cage structure similar to that of silicon clathrate, although fullerenes are molecular compounds formed of sp2 rather than sp3 carbon atoms. In solid C60 (ful-lerites) the molecules are held together by relatively weak van der Waals forces and they can accommodate atoms or ions to

form a variety of new compounds. One way to build harder and stiffer forms of fullerene is to induce polymerization among the C60 molecules to create strong covalent intermo-lecular bonds. This was first achieved with photopolymer-ization [4]. Polymerization can also be induced by applying high pressure and high temperature. Using this method sev-eral forms of sp2‒sp3 superhard C60 phases have been formed [5–9]. Recently, compressing solvated fullerenes, a carbon material, with incompressibility comparable to diamond has been synthesized [10].

Insertion of metal atoms into the fullerene lattice leads to electron transfer and sometimes the formation of fullerene polymers. Intercalation of alkali-metal atoms in solid C60 can result in metallic behaviour [11, 12] and the M3C60 (with M alkali metal) compounds were observed to be superconducting

Journal of Physics: Condensed Matter

High pressure study of Li-doped fullerides, LixC60 (x = 4,12), by x-ray diffraction and Raman spectroscopy

V Pischedda1, M Yao2, R Debord1, G Gabarino3 and A San-Miguel1

1 Institut Lumière Matière, UMR5306 Université Lyon 1-CNRS, Université de Lyon 69622 Villeurbanne cedex, France,2 State key lab of superhard materials, Jilin University, Changchun, 130021, People’s Republic of China3 European Synchrotron Radiation Facility, Grenoble 38043, France

E-mail: vittoria.pischedda@univ-lyon1.fr

Received 15 May 2014, revised 30 June 2014Accepted for publication 10 July 2014Published 19 August 2014

AbstractIn this article we study the alkali metal-intercalated 2D polymeric Li4C60 and the monomeric Li12C60 under pressure up to 40 GPa at room temperature, using x-ray diffraction and Raman spectroscopy. Li4C60 undergoes several transitions in the studied pressure range. At pressures lower than 8 GPa, we observed changes in both diffraction patterns and Raman scattering spectra, probably due to the displacement of Li atoms. At 8 GPa another structural and electronic transition occurs. We observe an enhancement of background and a broadening of diffraction peaks. Raman modes weaken and broaden considerably. An important structural transition occurs at around 16 GPa, in which new Raman bands exhibit features similar to those of a reported 3D C60 polymeric structure. The XRD data shows a collapse in volume with the simultaneous formation of amorphous material. The cell parameters deviate from their early pressure evolution and become less compressible. The high pressure study of highly doped monomeric Li12C60 shows that its structural integrity is retained up to 13 GPa, with increasing pressure-induced structural distortion and disorder. Above 13 GPa, Li12C60 transforms to a highly disordered state.

Keywords: alkali metal-intercalated fullerides, phase transformation, high pressure

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with a critical temperature as high as 33 K for RbCs2C60 [13] and 40 K for Cs3C60 [14].

These preliminary results suggest that when combining intercalation and extreme conditions of pressure and tempera-ture, it should be possible to synthesize carbon-cage-like com-pounds with exceptional properties such as high hardness/stiffness, low density and superconductivity [15].

The intercalation of fullerene with large-radius alkali metals such as Rb or Cs with stoichiometric X6C60 has not proved to be favourable to the high-pressure synthesis of 3D intercalated polymers [16, 17]. Importantly, these experiments have shown that alkali intercalation allows the fullerene mol-ecules to be stabilized up to much higher pressures, compared to the pristine form of C60, for which 22 GPa is the limit of sta-bility at ambient temperature [18]. The monomeric structure of Cs6C60 is stable up to 45 GPa [16], while Rb6C60 shows a reversible structural transition to a two-dimensional (2D) pol-ymeric structure above 35 GPa and 600 K [17]. This difference is probably due to the smaller ionic radius of Rb atoms com-pared to Cs, which allows C60s to get close enough to polym-erize under high compression.

Recently, by compressing 2D-Na4C60 polymers, we syn-thesized the first 3D polymeric fulleride, with the forma-tion of new random covalent bonds between the fullerene molecules [19]. An isostructural collapse in volume occurs by cold compression at ~ 15 GPa, with the new phase pre-serving most of the C60 molecules. An irreversible transition to a rather disordered phase was observed by compressing 2D polymeric R-phase of C60 above 15 GPa [20], while 2D polymeric T-phase of C60 transforms into a metastable 3D polymeric phase near 20 GPa by cold compression [21], or at 15 GPa and 550 °C [22].

The presence of intercalated Na ions seems to enhance the stability of the C60 molecules, thus promoting the forma-tion of 3D polymeric structures at lower pressure and room temperature compared to pure C60. The intercalation of the smaller and more reactive Li may help trigger a conversion to 3D polymeric structures at even lower pressures than for Na-doped or un doped C60. Since the observation of super-conductivity in LixCsC60 (3  <  x  <  4) [13], the importance of Li+–C interactions in sensitively controlling the structural, conductive and electronic properties of lithium fullerides has been established [12].

Ab initio calculations on the energetic and structural properties of hypothetical carbon-based cage-like materials suggest the possibility of synthesizing Li-doped carbon clathrate under extreme conditions of pressure and tem-perature [23]. This potential 3D polymeric Li intercalated fulleride, constituting an alternative to carbon clathrates of type-I or type-II structure, may exhibit excellent mechanical properties and high Tc due to a very high phonon-electron interaction. Even in ambient conditions, Li-doped fuller-enes behave differently depending on the amount of inter-calated lithium. Highly doped LixC60 (x > 7) share similar properties, with Li atoms forming clusters in the octahe-dral site of the original fcc lattice. At lower doping levels (x < 7), the volume occupied by the intercalants decreases, while the efficiency of charge transfer increases. Both these

factors reduce the fullerene distance, giving rise to fullerene polymerization.

In this paper we investigate the high-pressure behaviour of low-doped Li4C60 and highly doped Li12C60 using Raman and synchrotron x-ray diffraction (XRD). Li4C60 is the only 2D polymeric fullerite containing both [2 + 2] and single C‒C bridging motifs between fullerenes propagating at right angles within the polymeric plane [24]. In Li12C60, lithium atoms cluster at the largest interstices of the pseudo-tetragonal fullerene structure. The cell volume is reduced with respect to pristine C60. This may be due to the electrostatic attrac-tion between C60

n− anions and the alkali-metal cations [25]. Subjecting these two LixC60 compounds to high-pressure treatment, we will analyze the influence of the intermolecular environment in stabilizing new polymerized structures.

2. Experimental section

The samples LixC60 (x = 4,12) were synthesized in our lab-oratory following the procedure described elsewhere [26]. Well-mixed stoichiometric amounts of presublimed C60 (purity 99.98%, MTR Ltd) and alkali metal Li (Aldrich, 99.95%) were loaded into copper or stainless-steel crucibles and annealed at elevated temperatures (350 °C) for about one month in an oxygen-free glove box. The water and oxygen levels in the glove box were always kept below 0.1 ppm and 0.5 ppm, respectively. The samples were reground once every week and the progress of the reaction was monitored by XRD and Raman spectroscopy. The final products were confirmed to be single-phase Li4C60 or Li12C60.

High-pressure room-temperature Raman spectra were recorded using a DAC and a Horiba (Jobin Yvon) HR-800 LabRAM spectrometer with double-notch filtering and an air-cooled charge-coupled device detector at ENS, Lyon. The exciting laser beam (514.5 nm line of an Ar+ laser) was focused down to a 2 μm spot on the sample, and the back-scattered light was collected through the same objective. Pressure was measured in situ by the R1 fluorescence shift of a ruby sphere placed into the gasket hole. To avoid reac-tion or intercalation with the sample, no pressure-transmit-ting medium was used. We did not collect Raman scattering data in the frequency shift region around 1332 cm−1 so as to avoid the very strong peak associated with the diamond of the high-pressure cell.

Angle dispersive XRD experiments at high pressure were performed at the insertion device ID27 beamline at ESRF, Grenoble. Monochromatic beams with wavelength λ = 0.3738 Å were selected using a Si (1 1 1) monochro-mator and focused on the sample using multilayer mirrors in the Kirkpatrick-Baez geometry. The focal spot size was 1   ×  1 µm2 (FWHM). The sample was loaded in a 200 µm diameter cavity in a stainless-steel gasket pre-indented to 80 µm thickness inside an oxygen-free glove box. Pressure was increased from ambient pressure to 40 GPa at intervals of about 2 GPa. At each pressure step we waited 5–10 min to allow pressure to stabilize before collecting the data. The 2D diffraction rings were recorded on a fast large-area scanning

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MAR345 image plate and converted into one-dimensional diffraction patterns using the FIT2D software package. The sample-to-detector distance and the image plate tilt angles were calibrated using a silicon standard located at the sample position. Data were analyzed using the Rietveld method for ambient pressure data and the LeBail pattern decomposition technique for high-pressure data. (No Rietveld refinement was possible at high pressure).

Ex situ high-pressure, high-temperature experiments on Li4C60 were performed using a ‘Paris-Edinburgh’ (PE) device at ILM, Lyon, with 5 mm sintered diamond anvils [27]. Sample loading was done under inert atmosphere conditions using a water-cooled ‘clamp’, which allowed us to pre-pressurize the PE anvils with the gasket containing the sample inside the glove box [27]. Characterization of quenched samples that were previously sealed inside a glass capillary inside the glove box was performed in ambient conditions on a Bruker D8 Advance powder diffractometer (Kα1,2 Cu wavelengths) at the Centre de Diffraction Henri Longchambon (Université Lyon 1).

3. Results

3.1. In situ XRD synchrotron compression study of Li4C60 up to 40 GPa

The layered polymeric Li4C60 adopts a body-centred mono-clinic structure (space group I2/m). The C60 unit bonds have [2 + 2] and single C‒C bridging motifs in the b and a direc-tions, respectively. Two Li+ ions are located near the pseudo tetragonal sites and the other two are symmetrically displaced along the c axis with respect to the centre of the pseudoctahe-dral site [28].

Figure 1 shows selected XRD patterns obtained from pressurizing Li4C60 up to 40 GPa. The low-pressure XRD patterns can be well fitted by the body-centred monoclinic structure and a small amount of pristine fcc C60. After the sample loading, due to the imperfect sealing properties of the DAC and the high reactivity of Li, a certain amount of Li4C60 likely reacted with oxygen or water and transformed in molecular C60.

Rietveld refinement in the I2/m space group was performed using starting-cell parameters from [28]. The ambient-pressure Rietveld refinement gives the following results: a0 = 9.331(2) Å, b0 = 9.054(3) Å, c0 = 14.988(2) Å, β = 90.95(4)°, which are in good agreement with previous reports [24, 28, 31]. However, the quality of the data did not allow for a Rietveld refinement at higher pressures and only cell parameters could be extracted up to ~ 25 GPa. Beyond this pressure the LeBail fitting was no longer possible either.As the pressure increases, the diffraction patterns gradually change. Bragg peaks shift to higher 2θ angles due to the contraction of the lattice. Minor changes can be observed between 1.5 and 3.3 GPa and again at 8 GPa. Between 8 and 13 GPa we observe a transition with enhancement of back-ground and broadening of diffraction peaks. These changes, preserving the monoclinic unit cell symmetry, suggest minor structural transformations in the material. We will see later

that these observations are concomitant with local disorder signs from Raman spectra. Above 13 GPa peaks become increasingly broader and after 32 GPa almost no signs of crystallinity can be detected.

The pressure dependence of the lattice parameters (a, b, c) up to 25 GPa are shown in figure 2 and the cell volume evalu-ation is shown in figure 3.We notice that the lattice of Li4C60 up to 16 GPa is much more compressible in the c direction, whereas the a and b lattice parameter are quite rigid in this pressure range. This strong anisotropy in the compressibility can be easily rationalized in terms of compression along or perpendicular to, covalent intermolecular bonds. The b axis, where polymerization is characterized by double covalent bonding between fullerene molecules, is almost pressure inde-pendent up to 25 GPa. The a axis, characterized by single C‒C bonds, is more sensitive to the applied pressure and undergoes an abrupt change of compressibility above 16 GPa. The most compressible axis is c, which is almost perpendicular to the polymeric layer (β = 90.95° in ambient conditions) and domi-nated by van der Waals forces.The centre-to-centre C60‒C60 distances decrease as pressure increases. As has been observed, the electrostatic attraction between C60

n− anions and the alkali-metal cations shrinks the structure, reducing the interstitial voids and the inter fullerene distances [24, 28]. The material becomes less compressible. The centre-to-centre C60‒C60 distances in the (100) plane, dominated by the van der Waals interactions, decrease faster as pressure increases. Due to the anisotropy in the 2D-layer of Li4C60, the shortening of in-plane centre-to-centre C60‒C60 distances could lead to the deformation of C60 molecules. (See in particular the variation of the b parameter, which seems to slightly increase above 10 GPa and again above 16 GPa). Pressure-induced modification of the shape of the C60 molecules was observed previously [7, 9, 29]. Considering the rather high reactivity and mobility of Li+ atoms, we cannot exclude that this is accompanied to some degree by the deintercalation and/or displacements of Li+ atoms.

Fitting the experimental volume versus pressure points up to 16 GPa with the Birch-Murnaghan equation of state, we obtained an experimental value of the bulk modulus at room temperature B0 = 78(± 8), as well as its pressure derivative B0’ = 7(± 4). The average bulk modulus calcu-lated from linear fit is 81(± 2) GPa for the ambient-pressure phase up to 8 GPa.

A phase transition can be observed between 16 and 18 GPa. This phase transition is accompanied by a volume drop of about 14% (figure 3) and the amorphization of the material (indicated by the increase of background in the x-ray patterns). The average bulk modulus calculated from linear fit is 184 (± 26) GPa for the high-pressure phase up to 25 GPa (figure 3). The compression data obtained above 16 GPa strongly indi-cates the occurrence of polymerization between layers. As this transition is accompanied by a large increase in back-ground intensity, it is probably associated with the breaking of some intramolecular bonds, concurrent with the formation of new random intermolecular covalent bonds. (See below.) The formation of a highly incompressible 3D polymeric phase

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with mixed sp2‒sp3 character also occurs in the high-pressure polymerization of C60 and Na4C60 [18–21].

3.2. In situ Raman study of Li4C60 up to 40 Gpa

Raman spectroscopy is a powerful technique to study inter-calated fullerides, providing information on both the struc-ture and the charge state of the C60 molecules [30, 31]. A free isolated C60 molecule has Ih symmetry and ten (two Ag + eight Hg) observable frequencies. The Raman spectrum of 2D-Li4C60 has richer vibration modes due to the concomitant effects of C60 polymerization lowering the symmetry of the structure and the introduction of the dopant ions. This leads to the splitting of degenerate modes and the appearance of new

ones, when compared to the free isolated C60 molecule. More details on the Raman modes of pristine polymerized Li4C60 can be found in [31, 32].

Figure 4 shows the Raman spectra of Li4C60 at different pressures up to 40 GPa. Raman modes gradually broaden and merge as pressure increases. At 3.3, 4.8 and 6.9 GPa we observe peak changes mostly in the low-frequency region of the spectra (between 200 and 600 cm−1). These changes are likely due to inhomogeneity of the sample and/or displace-ment of Li+ atoms with a resulting variation of Li+‒C inter-actions. At 8.5 GPa the background increases and all Raman modes weaken and broaden considerably – in particular the Raman peaks near 950 cm−1, which are characteristic of inter-molecular polymer bonds. Rapidly, Raman modes become too

Figure 1. Selected raw XRD patterns of Li4C60 under pressure up to 40 GPa.

2 3 4 5 6 7 8 9 10

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. uni

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2θ angle (°)

(110

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20)(0

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Figure 2. Variation of Li4C60 cell parameters with pressure up to 25 GPa. A significant change in compressibility is observed above 16 GPa for the a and c direction.

0 5 10 15 20 25

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weak to be detected except for a broad band at around 600–800 cm−1. This indicates an important structural and electronic transition occurring in the material.

In a recent high-pressure electrical resistance measure-ments and Raman spectroscopy study on Li4C60, Yao et al [32] suggest that a metallization transition occurs above 9 GPa. The pressure-induced metallization of Li4C60 is con-sistent with the intensity lowering of the Raman mode around 950 cm−1. The very weak Raman modes observed above 8 GPa can be related to a smaller Raman scattering cross-section due to the increased electron-phonon coupling. The introduction of some disorder in the structure could explain the increase of background in the Raman and x-ray patterns.The Raman spectra change again at around 20 GPa, reflecting a structural transition. We observe the increase of the inten-sity and broadening of the bands around 500, 680 and 1590 cm−1, and the appearance of a band around 1830 cm−1.

These spectra are similar to those of 3D polymeric fullerites reported in the literature [20, 33] – i.e. they have in common the broad asymmetric band at ~1600 cm−1 and weaker bands in the 200–800 cm−1 region. The broad asymmetric band at ~1600 cm−1 is related to the presence of both sp2 and sp3 bonds in the fullerite structure and can be attributed to either a 3D polymer or an amorphous carbon phase [18–21, 33–35]. The 1830 cm−1 band also occurs at similar frequencies for pressure-induced 3D polymeric C60 [36] and the 3D poly-meric Na4C60 obtained by pressurizing 2D Na4C60 polymer above 15 GPa [19]. This mode has been attributed to the eth-ylenic-like stretching vibration of the interfullerene bridging C‒C bond when a 3D polymerization occurs [37]. This mode disappears above 33 GPa, suggesting the destruction of the C‒C bond in the 3D polymer. This further indicates that the Li intercalated 3D polymer is not stable at such high pres-sure. Above 33 GPa, the shorter molecular distances enhance

Figure 3. Variation of the unit cell volume as a function of pressure at room temperature for Li4C60. Red line shows the Birch-Murnaghan EOS fit; blue line corresponds to the linear fit for the high-pressure phase. A drop in volume and a significant change in compressibility can be observed at pressures above 16 GPa.

900

950

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1150

1200

1250

1300

0 5 10 15 20 25Pressure (GPa)

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Figure 4. Raman spectra of Li4C60 at different pressures up to 40 GPa in the lower- (a) and higher-frequency regions (b).

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the interactions between the dopants and C60s, weakening the C‒C bonds and destroying the cage structure.

The 2D polymeric Na4C60 shows a different behaviour under pressure. A structural and electronic transition with higher electron hopping has been observed above 3 GPa [19]. A second transition occurs at around 15 GPa with the appear-ance of new Raman bands similar to those observed for the Li4C60 above 20 GPa. Despite the strong pressure-induced electron-phonon coupling observed in Na4C60, no supercon-ductivity was measured down to 2 K [38]. It would be inter-esting to investigate whether the higher pressure-induced electron-phonon coupling of Li4C60 is accompanied by a high Tc-superconducting transition.

3.3. Stability of the high-pressure Li4C60 phases and the effect of temperature

After releasing pressure from 12 GPa to ambient pressure, Raman data shows that the Li4C60 phase is reversible (see figure 5), although after the sample is released from 39 GPa to ambient pressure and exposed to air, Raman data shows high background with a very weak signal of C60 (figure 6). XRD data in ambient conditions, after sample compression to 40 GPa, shows that the Li4C60 structure remains amor-phous. This data indicates that only a small proportion of C60 molecules remain intact after exposure to pressure as high as 40 GPa. In figure 6 we compare the Raman spectrum of Li4C60 with that of Na4C60 recovered from similar high-pressure compression [19]. A larger amount of C60 molecules were recovered for Na4C60 than for Li4C60, which suggests higher stability of the pressure-induced Na intercalated 3D polymeric structure.In a separate experiment we investigated whether temperature could contribute to decreasing the pressure of phase transfor-mation and consequently, metastabilizing the Li4C60 high-pressure structure. We treated Li4C60 at 15 GPa and 900 K using a Paris-Edinburgh press with sintered-diamond anvils [27]. Sample loading was done under inert atmosphere inside the glove box. The quenched sample was characterized inside a glass capillary using a conventional x-ray diffractometer. A sample treated at 2 GPa and 400 K is also shown for compar-ison [32] (Figure 7). The diffraction patterns show the revers-ibility of the high-pressure phases when released from 2 GPa and 400 K. For the sample released from 15 GPa and 900 K, we observe diffraction peaks broadening and merging in a very broad signal at 2θ angles above 20°. This could be related to important residual stress. This effect apart, the overall signal is recovered, which indicates a reversible transformation from 15 GPa and 900 K.

At 15 GPa and 900 K, pure 2D polymeric C60 single crystal is already irreversibly transformed into 3D-C60 polymer [7]. Our results indicate that either Li intercalation induces

Figure 5. Reversibility of Li4C60 high-pressure phase when released from 12 GPa. Raman spectrum of Li4C60 after decompression from 12 GPa to 0.8 GPa (top red line), to be compared with the ambient-pressure Raman spectrum of Li4C60 (bottom black line).

Inte

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nits

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500 1000 1400 1600

~0.8 GPa

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1441 cm-1

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60

~0 GPa

Release from 12 GPa

Figure 6. Top line is the Raman spectrum of Li4C60 after decompression from 39 GPa to ambient pressure, to be compared with the ambient-pressure Raman spectrum (bottom line) of Na4C60 after release from 41 GPa [19]. Background was subtracted in both spectra.

Figure 7. XRD of the Li4C60 in ambient conditions (I), after cycling the sample to 2 GPa and 400 K (II) ([32]), and to 15 GPa and 900 K (III).

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reversible 3D phase transformation or even at a high temper-ature such as 900 K, higher pressure is needed to transform Li4C60 into a 3D polymeric structure.

3.4. In situ XRD synchrotron compression study of Li12C60

In order to investigate the effect of Li ion intercalation on phase stability, we prepared saturation-doped Li12C60 and performed an in situ XRD synchrotron compression study up to 31.5 GPa. We refined Li12C60 cell parameters using initial values from [25]. In this case, Li12C60 adopted an Immm body-centred pseudo-tetragonal crystal structure with Li forming metallic clusters in the octahedral voids.

The pressure evolution of the XRD patterns from ambient pressure up to 31.5 GPa is shown in figure 8. Under compres-sion, we observe a shift of Bragg peaks towards higher 2θ angles and increasingly, peak broadening with no sign of phase transformation up to the highest measured pressure. Above 13 GPa Li12C60 becomes gradually amorphous, and above 25 GPa no diffraction line can be detected.

The face-centred cubic C60 phase remains stable under hydrostatic compression up to 20 GPa or transforms to a crys-tallographic structure of lower symmetry at around 16 GPa under non-hydrostatic compression [39]. Even though Li12C60 has a smaller lattice constant compared to the bulk C60, no evi-dence for a symmetry-lowering structural transition is found in the pressure range studied here (figure 8–9). In figure 9 we observe that an important structural distortion occurs above 10 GPa. As the centre-to-centre C60‒C60 distances decrease with increasing pressure, the electrostatic attraction between C60

n− anions and the alkali-metal cations become stronger. These strong interactions probably preclude orientationally ordering transitions and subsequent C‒C bond formation between fullerene units.

Intercalation usually enhances the stability of molecular fullerenes, as has been observed for Rb6C60 and Cs6C60, where

the C60 molecules are preserved at pressures higher than 40 GPa [16, 17]. In Li12C60, the presence of excess Li in the interfullerene spacing leads to an appreciable steric crowding that does not allow the C60 units to move close enough to form C‒C bonds. The disorder is due to either the increase of dis-locations in the C60 molecules or the destruction of C60 cata-lyzed by Li ions.

To compare the bulk modulus of Li12C60 with that of Li4C60, we fitted the experimental volume versus pressure points up to 8 GPa with the Birch-Murnaghan equation of state, fixing the pressure derivative B0’ = 7. We obtained for Li12C60 an experimental value of the bulk modulus at room temperature, B0 = 36.9(± 7). As expected, the 2D polymeric Li4C60 (B0 = 78(± 8)) is stiffer than monomeric Li12C60.

4. Discussion

In figure 10 we plot the relative volume evolution versus pres-sure at ambient temperature of Li4C60 and Li12C60, together with Na4C60 and tetragonal 2D polymer of C60 from the lit-erature [21]. From the V/V0 curves, we can clearly see that a volume collapse occurs when compressing 2D polymerized Li4C60 and Na4C60 above 15 GPa. As pointed out in our pre-vious work [19], the small size of the dopants, together with the pressure-induced shortening of the centre-to-centre C60 distances, are important factors in obtaining 3D polymeriza-tion of the intercalated fullerides by cold compression at rela-tively low pressure (16 GPa for Li4C60 compared to at least 20 GPa for 2D polymeric C60).No volume drop is observed for Li12C60 up to 31.5 GPa. In this case, the presence of additional Li does not catalyze but, on the contrary, precludes polymerization. Polymerization depends on both the favourable centre-to-centre C60 distance and the negative doping of the system. It has been postulated that the addition of electrons to the system should decrease the

Figure 8. Selected XRD patterns of Li12C60 under pressure up to 31 GPa. High background arises mainly from Compton scattering in the high-pressure cell diamond windows.

2 4 6 8 10

(002

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energy barrier for the formation of covalent bonds between fullerenes [40]. This is true for Li4C60, where polymerization occurs at lower pressure than pristine C60. Changing the elec-tron doping, however, affects the height of the energy barrier but not the fullerene distance at which polymerization occurs. Our results show that when the dopants fully occupy the voids of C60 lattice, as for Li12C60, the proximity of C60 molecules and thus polymerization under pressure, is prevented.

It is interesting to notice that both 2D polymers, Li4C60 and Na4C60, with host-guest interaction, show lower compressi-bility compared to the 2D polymeric C60 below ~15 GPa. Above 15 GPa, very low compressible phases are formed in the two intercalated materials. The slightly higher compressibility and

larger drop in the cell volume of Li4C60 compared to Na4C60 is probably related to the structural and intercalation differ-ences between the two fullerides. Na4C60 forms only single C-C bonds and Li4C60 forms single and double bonds within the polymer plane. Li is smaller (Li+ 90 pm, Na+ 116 pm ionic radii) and has higher reactivity than Na. Thus, a different inter-action between dopants and C atoms is expected, which may affect the stability of C‒C bonds and influence the mechanical properties of the resulting 3D polymer.

The new Li4C60 high-pressure phase exhibits a high degree of disorder, as demonstrated by an increase of XRD and Raman spectroscopy background, a decrease of intensity, and a broad-ening of the peaks. Probably the formation of new covalent bonds

Figure 10. V/V0 evolution versus pressure at ambient temperature of Li4C60 (2D) and Li12C60 (monomeric), compared with Na4C60 (2D) [19] and tetragonal 2D-C60 [21]. Above 15 GPa we observe a drop in volume for Li4C60 and Na4C60.

0 5 10 15 20 25

0.75

0.80

0.85

0.90

0.95

1.00

Li4C60 Na4C60 T-C60Li12C60

V/V 0

Pressure(GPa)

Figure 9. Cell parameter evolution versus pressure up to 21 GPa for Li12C60. Above 10 GPa we observe a distortion of the structure.

0 5 10 15 20

9

10

14

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para

met

er (

Å)

a b c

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in Li4C60 at high pressure happens in a random way, accompa-nied by the destruction of some C60 molecules. The structure can be close to that of amorphous carbon, which is characterized by the presence of ~15% sp3 bonding [40]. The Li4C60 amorphous phase is metastable, and only a small percentage of C60 mole-cules are recovered when the pressure is released from 39 GPa to ambient pressure (figure 6). By heating at high temperature and compressing at 15 GPa, the Li4C60 polymerized phase cannot be quenched. The heating could trigger the displacement of the ions inside the structure and higher pressures are needed to metasta-bilize the pressure-induced 3D polymeric phase.

5. Conclusions

In summary, the alkali-metal intercalated 2D polymer Li4C60 and the highly doped monomeric Li12C60 have been studied by XRD and Raman spectroscopy measurements up to 41 GPa and 31 GPa, respectively. Li4C60 undergoes several phase transitions in the pressure range studied. At pressures below 8 GPa, changes in the pressure behaviour of Raman modes and the intensity of x-ray peaks are attributed to some degree of displacements of Li+ atoms inside the monoclinic structure. At ~8 GPa, we observe an important structural and electronic transition. All Raman modes weakened and broadened con-siderably, leading to a picture of higher electron hopping.

A further transition occurs above ~16 GPa and can be con-sidered as an isostructural volume-collapse transition. XRD data shows a rapid increase of background and a decrease in the cell parameters, mainly along the a and c axes, with the formation of a lower compressible structure. The anisotropic compression of the Li4C60 lamellar structure leads to an inter-plane distance corresponding to the formation of C60 cova-lent bonding between planes. All Raman peaks disappear and become very broad and diffuse, exhibiting Raman features similar to those of 3D polymeric structures. All these effects suggest the formation of a 3D polymeric phase with the random creation of new polymeric bonds between fullerene molecules of the initial Li4C60 monoclinic structure.

Summarizing the entire sequence of Li4C60 phase tran-sitions, with increasing pressure we observe: electronic displacement of Li+ atoms → higher electron hopping and dis-tortion of the structure → formation of random 3D polymeric bonding → amorphous material with progressive destruction of C60 cages. This last transformation is mostly irreversible and only a small percentage of C60 molecules are preserved after releasing the pressure from 39 GPa.

On the other hand, we observe that Li fulleride with high stoichiometry (e.g. Li12C60) is not favourable to the formation of a polymerized phase under pressure up to 31 GPa. XRD data shows a progressive, pressure-induced structural disorder with the formation of a highly disordered phase at pressure above 13 GPa.

Our results show that the small size of the dopants, together with the pressure-induced shortening of the centre-to-centre C60 distances, can favour 3D polymerization of intercalated fullerites at relatively low pressure by cold compression. However, when the Li atoms saturate the voids of the C60 lat-tice, it inhibits the polymerization of C60s under pressure.

Acknowledgment

This work was partially supported by the National Natural Science Foundation of China (Grant No. 11104105).

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