In Situ Synchrotron X-ray Diffraction and Raman Spectroscopy

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C: Plasmonics, Optical Materials, and Hard Matter

In-situ Synchrotron X-ray Diffraction and Raman Spectroscopy Studies of Gd@C -S under High Pressure 82

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Huanli Yao, Hu Cheng, Xuejiao J. Gao, Cheng Li, Rongli Cui, Huan Huang, Xihong Guo, Xiaodong Li, Lele Zhang, Bing Liu, Binggang Xu, Jinquan Dong, Xingfa Gao, Yanchun Li, and Baoyun Sun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02180 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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The Journal of Physical Chemistry

In-situ Synchrotron X-ray Diffraction and Raman Spectroscopy Studies of Gd@C82-S8 under High Pressure Huanli Yao,1,4,‡ Hu Cheng,2,5,‡ Xuejiao J. Gao,3 Cheng Li,1,4 Rongli Cui,1,4 Huan Huang,1,4 Xihong Guo,1,4 Xiaodong Li,2 Lele Zhang,1 Bing Liu,1,4 Binggang Xu,1 Jinquan Dong,1 Xingfa Gao,3 Yanchun Li,2,* Baoyun Sun1,4,* 1

CAS Key Lab for Biomedical Eﬀects of Nanomaterials & Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

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Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China 3

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China 4

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University of Chinese Academy of Sciences, Beijing 100049, China

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China

ACS Paragon Plus Environment

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ABSTRACT

The structural transformation and mechanical properties of fullerenes under high pressure has attracted significant interest. However, the research data on the large cage endohedral metallofullerenes are still rare. Here, we first report high pressure studies on a new cocrystal Gd@C82-S8 by in-situ X-ray diffraction (XRD) and Raman spectroscopy. The high-pressure XRD data and Raman findings provide consistent evidence of no structural phase transition in Gd@C82S8 before a pressure-induced irreversible amorphization at about 23 GPa and an anisotropic deformation of the C82 cage upon compression. The bulk modules is as high as 44 GPa and the embedded Gd atom has great effect on the pressure behavior of Gd@C82, especially on the restraint of the compression of the adjacent bonds. The assignment of Raman-active vibrational modes and their corresponding eigenvectors of Gd@C82 are presented for the first time by theoretical calculation. Our findings provide new insight into the structural transformation of metallofullerenes under pressure and even contribute to prepare desired carbon materials with new structures and properties.


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1.

INTRODUCTION It is well known that high pressure technology is a useful tool to find or construct new materials1-

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. Usually, under atmospheric pressure, fullerene molecules are bonded by weak van der Waals

interactions, which are very sensitive to high pressure and can be modified to form new phases by pressure4. Over the last few years, high pressure studies on the structural stability and evolution of fullerenes have attracted interest and indeed a number of interesting phenomena have been reported5-10. For instance, the hexagonally packed C70 single crystals can form zigzag polymeric C705. The nonconductive 2D polymeric C60 can transform into new electron conductive 3D C60 polymer via [3 + 3] cycloaddition7. The C60/m-xylene can form a novel carbon phase composed of ordered amorphous carbon clusters (OACC) structure under high pressure, which is hard enough to indent diamond anvils8. This interesting finding not only breaks our inherent understanding of the categorization of various phases of solid-state materials but also initiates the rapid development of high-pressure study of other solvated fullerenes11-15. Compared with the hollow fullerenes, endohedral metallofullerenes (EMFs), formed by encapsulation of metal atoms, ions or clusters inside fullerene cages, have some superior performance due to the electron transfer from the encaged species to the fullerene cage16-18 and the properties of the entrapped species such as optical or magnetic properties of rare earth element 19. Potential applications in chemistry20, catalysis21, electronics22, biomedicines23-25 and many other fields26-27 have been proposed. Carrying out high pressure experiments on EMFs is very important in characterization and understanding of this unique material, as well as creating new structures. Up to now, Buga et al. studied La@C82 under pressure 9.5 GPa and elevated temperature (520 K and 720 K) by powder X-ray diffraction and transmission electron microscopy and found that the


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La@C82 may form polymeric crystalline structures28. Cui et al. also investigated the pressure behavior of Sm@C88 and solvated Sm@C90 by infrared spectroscopy studies29-30. However, the research of the structural deformations and the electronic properties of this extraordinary material under pressure are quite limited. An in-depth understanding of the effects of high pressure on EMFs is urgently desirable. In theory, the interaction between embedded atoms and fullerene cage may also affect the deformation of the carbon cage. High pressure as a useful tool is expected to tune this interaction and may bring new ideas on the structural stability and the deformation process of metallofullerenes at the atomic level, making the forecast of new carbon phases possible31. In the practical application, the carbon cage itself has many polymerization ways and exhibits high hardness after polymerization. Combing with the special properties of the embedded atoms, the high pressure study on the EMFs is promising for developing new materials with different structures and properties, even a special ultrahard material. In previous work, we have successfully synthesized Gd@C2v(9)-C82·2.5S8·0.5CS2 (simplified to Gd@C82-S8) single crystals through a facile solution growth method32. In this work, we present the first study of the structural stability and the cage deformation of the material under high pressure by using in situ synchrotron angle-dispersive X-ray diffraction (AD-XRD) and Raman spectroscopy. The Raman spectrum of Gd@C82 and the assignment of the corresponding Raman vibrational modes are simulated. To our knowledge, there is no investigation about the high pressure properties of metallofullerene with single crystals at present. Gd@C82-S8 thus provides an ideal model to study the behavior of metallofullerene molecules under pressure and a good building block for designing new carbon materials with special properties such as by use of magnetic gadolinium.


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2.

EXPERIMENTAL SECTION

2.1

Materials

Sufficient amount of highly purified Gd@C82 (99.5%) was obtained by the industrial-scale production line of metallofullerenes in Institute of High Energy Physics, Chinese Academy of Sciences (CAS)33-34. The crystals of Gd@C82-S8 were grown by slow evaporation of a mixture containing o-dichlorobenzene solution of Gd@C2v(9)-C82 and CS2 solution of sulfur at room temperature. X-ray diffraction measurements were performed at varied temperature (i.e., 90, 170, and 298 K), showing that the structure of the crystal was solved in the monoclinic space group C2/c (No.15): a = 36.533(13) Å, b = 17.668(7) Å, c =19.782(8) Å and β = 114.760(10) °, V = 11594.7(8), and Z = 4332. 2.2

Characterizaton

The crystalline structure and morphology were also characterized by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan) and transmission electron microscopy (TEM, FEI Tecnai G2 T20, American) with an acceleration voltage of 200 kV (see Supplementary Information Part 1, Figure S1 and Figure S2). 2.3

High pressure XRD and Raman measurements

High-pressure experiments were performed at room temperature in diamond anvil cells (DACs) with flat anvils culet diameter of 300 µm in all runs of experiments. The Gd@C82-S8 were loaded into 140 μm diameter hole in a preindented T301 stainless steel gasket with 40 μm thickness. Ruby fluorescence technique was used for calibrating pressure35. Liquid argon or a 4:1 methanol–ethanol mixture was used as the pressure medium, respectively, in XRD experiments to provide the quasi-


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hydrostatic environment. Another pressure medium, silicone oil, served as the pressure transmitting medium for the Raman experiment. In situ high-pressure synchrotron AD-XRD experiments up to about 40.6 GPa were performed at 4W2 beamline of the Beijing Synchrotron Radiation Facility (BSRF) with a beam wavelength of 0.6199 Å. The powder diffraction patterns were detected with a Pilatus image plate detector and the two-dimensional patterns were radially integrated using FIT2D software36. In situ high-pressure Raman measurements were perfomed with a commercial confocal laser Micro-Raman spectrometer (LabRAM, HR Evolution, Japan) using 600 gr/mm grating blazed at 500 nm. A laser beam at 633 nm was used for excitation and a 20× long focal length microscope objective was used to focus the laser beam on the sample. The laser output power presented at the sample was maintained at about 1 mW to avoid sample heating. All high pressure measurements in this article were carried out at room temperature. 3.

RESULTS AND DISCUSSION Figure 1a shows the in situ AD-XRD patterns of Gd@C82-S8 at different pressures. The

background was carefully subtracted. The Le Bail fitting of the diffraction pattern at 3.3 GPa has been conducted using the single crystal structure parameters32 as initial data (as illustrated in Figure 1b). The well-fitted result confirms that the sample at 3.3 GPa belongs to a monoclinic C2/c space group with unit cell parameters a = 34.8069 (19) Å, b = 17.0886 (13) Å, c = 19.0867 (10) Å and β= 114.569 (4)°, which is in good agreement with our simulated powder diffraction pattern of Gd@C82-S8 (Figure S3 and Table S1) and indicates that no phase transition appears under this pressure.


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Figure 1. (a) In situ AD-XRD patterns of Gd@C82-S8 at different pressures and (b) Le Bail full-profile fitting result of the AD-XRD pattern of Gd@C82-S8 at 3.3 GPa. As pressure increases, almost all of the diffraction peaks become broader and weaker and gradually shift to higher angles, indicating the compression of the lattice. During compression up to 23.6 GPa, no dramatic change is observed and no new diffraction lines appeared in the patterns, revealing that the monoclinic structure phase of the molecules still remains. When the pressure surpasses 23.6 GPa, all peaks disappear and only one broad peak is detectable, which suggests that the monoclinic crystalline structure of Gd@C82-S8 begin to lose the cage feature and transform into an amorphous structure at such a high pressure. Very similar results are also obtained using the methanol/ethanol mixture as pressure medium (Figure S4). It is remarkable that the phase transition pressure of Gd@C82-S8 is higher than those observed in some other fullerenes. For example, Liu et al. showed the occurrence of amorphization of C60 nanotubes at 20.41 GPa37. Wasa et al. reported that C70 became amorphous above 12 GPa38. Cui et al. observed the amorphization of Sm@C88 at 18 GPa31. The reported results show that Gd@C82-S8 is stable enough against higher compression. The high pressure phase transition of Gd@C82-S8 may favor its practical application as the magnetic materials with high hardness.


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Figure 2. Pressure dependence of (a) the selected d-spacings and (b) the lattice parameters for Gd@C82-S8. On the basis of the in situ high pressure collected XRD data, we plot the evolution of several selected d-spacings as functions of pressure in Figure 2a. It can be seen here that all the d values continue to decrease with pressure and no obvious anomaly change is observed for all the peaks, which manifest there is no structural type transition before the sample starts to transform into the amorphous at pressure 23.6 GPa, in agreement with our above XRD results. The pressure dependence of the lattice parameters (a, b, c) for Gd@C82-S8 is shown in Figure 2b. The lattice constants exhibit declining trend as increasing pressure. Moreover, the lattice of Gd@C82-S8 is much more compressible in the a direction than in the b and c directions, suggesting that the axial compressibilities of Gd@C82-S8 are highly anisotropic with increasing pressure. The pressure dependence of the relative volume relationship for the monoclinic phase of Gd@C82-S8 up to 19.1 GPa is presented in Figure 3a. The data were fitted using the third order Birch-Murnaghan equation of state: 𝑉

7⁄3

𝑃(𝑉) = 1.5𝐵0 [( 𝑉0 )

𝑉

5⁄3

− ( 𝑉0 )

3

𝑉

2⁄3

] × {1 + 4 (𝐵0′ − 4) [( 𝑉0 )

− 1]}

(1)


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Where 𝑉0 is the unit cell volume at ambient pressure, 𝐵0 is the isothermal bulk modulus, and 𝐵0′ is pressure derivative of 𝐵0. The bulk modulus for Gd@C82-S8 was determined to be 44.0 GPa with the pressure derivative 𝐵0′ fixed to 4. It is remarkable that the bulk modulus of Gd@C82-S8 is larger than that of C84 with similar cage size (𝐵0: ~ 20 GPa)39. This suggests that the lattice in Gd@C82-S8 is less compressible, which may be due to the effect of embedded atom or the cocrystalline S8. Figure 3a also shows that the derivative of the unit cell volume with pressure is continuous below 19.1 GPa, indicating no structural type transition before the transformation to the amorphous occurring. This result is consistent with the above XRD results on Gd@C82-S8.

Figure 3. (a) Pressure dependence of the relative unit cell volume of Gd@C82-S8, the solid line is the fitting result according to the Birch–Murnaghan equation and (b) the XRD patterns of the sample collected in the releasing pressure process. In order to verify whether or not the pressure induced phase transition in Gd@C82-S8 is reversible, we also studied the XRD patterns during pressure release in Figure 3b. When pressure is released to ambient pressure, the original peaks of the pristine sample are not recovered, indicating that the pressure induced phase transition to an amorphous carbon phase in Gd@C82-S8 is irreversible up to 40.6 GPa.


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We further employed Raman spectroscopy to study the structure and bonding states of Gd@C82S8 single crystal. As shown in Figure 4a, the Raman spectrum of Gd@C82-S8 single crystals shows an almost identical spectral pattern as those of pristine Gd@C82 reported earlier40-41 and the absence of additional peaks in the Gd@C82-S8 spectrum suggests that the interaction between Gd@C82 and S8 rings is weak van der Waals interaction (Figure S5). The comparison of our experimental data with those reported in the previous studies shows that the Raman spectrum of the Gd@C82-S8 has smaller background and intense peaks. Especially, the high frequent modes from 1100 to 1700 cm-1 present well-resolved intense peaks. These differences are probably a result of better single crystal structure and higher purity of our Gd@C82-S8.

Figure 4. (a) Experimental Raman spectrum of Gd@C82-S8 crystal and (b) calculated Raman spectrum of Gd@C82 at ambient conditions. For better analysis of the vibrational modes of high pressure Raman spectra of Gd@C 82-S8, we calculated the Raman spectrum of one Gd@C82 molecule using Gaussian 09 package42 at ambient conditions to understand the origin of these vibrational modes and provided it in Figure 4b. We can see that both spectra show similar features except for small shift of some vibration modes and slight differences in the strength of some peaks. This phenomenon may be due to the following


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reasons: (i) The simulated spectrum of Gd@C82 was taken from one Gd@C82 molecule in which the entrapped metal is at the fixed position (i.e., the most thermodynamically stable position). However, the Gd@C82 molecule in reality has been proved to be highly delocalized by X-ray diffraction measurements32; (ii)The experiment samples are single crystals, so the interaction between Gd@C82 and adjacent molecules is likely to cause the deviation of the spectrum31; (iii) The weak van der Waals interaction between S8 and Gd@C82 may also has some effect on the bond vibrations of the Gd@C82 molecule13. Based on the simulation of Gd@C82, the Raman spectrum can be roughly divided into four regions: (1) Spectral bands between 1045 and 1700 cm-1 are mainly from the tangential stretching vibrations of the C82 cage. (2) The bands from 802 to 1045 cm-1 are a gap like region that separates radial and tangential vibrational modes. (3) The bands in the range from 216 to 802 cm -1 are attributed to the radial vibrations of C82 cage. (4) The bands below 200 cm-1 are assigned to vibrations between the included metal and the C82 cage. The above assignments of the Raman vibrational modes are similar to the Raman spectral analysis by previous researchers for other endohedral fullerenes40, 43. To further investigate the nature of Gd@C82-S8 under high pressure and confirm the pressure induced amorphous phase transition of Gd@C82-S8 in AD-XRD results, in situ high pressure Raman scattering measurements were performed at ambient temperature, as shown in Figure 5. We can find that the Raman spectrum of Gd@C82-S8 at 0.3 GPa contains almost the same Raman active modes as Gd@C82-S8 at ambient pressure except for the Raman mode at 1345 cm-1 which is completely covered up by the strong peak of the diamond. With increasing pressure, all Raman modes shift, become broaden and weaken. Above 22.9 GPa, all modes could not be observed


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clearly, which has been identified as an evidence for the collapse and amorphization of Gd@C82 molecules. This pressure is consistent with the amorphization phase transition in our AD-XRD.

Figure 5. In situ Raman spectra of Gd@C82-S8 under different pressure and released from 31 GPa. Moreover, the Raman spectrum of Gd@C82-S8 decompression from 31.3 GPa is also shown in Figure 5. All the characteristic Raman peaks belonging to Gd@C82-S8 disappeared and only a broad band centered at 1554 cm-1 can be seen. This band is attributed to the amorphous carbon containing sp3 bonds, suggesting Gd@C82-S8 undergo irreversible collapse above 31 GPa. This result is also in good agreement with that obtained from the in situ AD-XRD studies. We further analyze the pressure dependence of several selected Raman modes of Gd@C82-S8 in Figure 6. The Gd@C82-S8 vibration peaks disappear gradually below 22.9 GPa and all the peaks exhibit linear pressure dependences with no abnormal changes occurring, indicating that no phase transitions take place in the range of pressure studied. In addition, all Raman peaks shift to higher frequency with increasing pressure. The pressure induced blue shift of the vibrational modes is a result of the contraction of bond length with increasing pressure. Detailed studies of the pressure dependence dω/dp were carried out on these peaks.


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Figure 6. The pressure dependence of several selected Raman modes of Gd@C82-S8. The shifts of four radial vibrations of C82 cage at 351, 617, 729 and 790 cm−1 in Figure 6a gave the different pressure coefficients of these vibrational modes. The closer the relevant regions of the carbon cage are to the Gd atom, the smaller the pressure coefficients of the corresponding vibrational modes are. For instance, the magnitude of the pressure coefficient of the mode at 729 cm-1 (1.30 cm-1GPa-1), which is the radial vibrations from the part of C82 cage far away from the Gd atom, is larger than that of the mode at 790 cm-1（0.11 cm-1GPa-1）from the part close to the Gd atom. This may be due to the nonuniform charge distribution on the carbon cage, which results in a stronger interaction between the Gd atom and the neighboring C atoms. Such interaction may stretch the object bonds, thus weaken the contraction of bond length caused by pressure, and ultimately slow down the corresponding radial vibrations with increasing pressure. When comparing the pressure coefficients of five intense tangential stretching vibrations of the C82 cage at 1134, 1181, 1233, 1581 and 1617 cm-1 ( Figure 6b), the same rule is found. For example, the pressure coefficient of the vibrational mode at 1617 cm−1 (at the region far away from the Gd atom) is 4.53 cm−1GPa-1, which is larger than the pressure coefficient of 4.04 cm-1GPa-1 of the vibrational mode at 1581 cm−1 (at the region close to the Gd atom), indicating that the interaction


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between Gd and the near area of the carbon cage can restrain the compression of the adjacent bonds. The corresponding eigenvectors of the Raman-active vibrational modes at 729, 790, 1581 and 1617 cm−1 of Gd@C82-S8 are shown in Figure 7, and the animation in video format of these vibrational modes is available in the supplementary information.

Figure 7. The eigenvectors of Raman active vibrational modes at (a) 729, (b) 790, (c) 1581 and (d) 1617 cm−1. Normally, the bands below 200 cm-1 are attributed to the metal-cage vibrations. In our experiment, Gd@C82-S8 show a metal-cage stretching vibration at about 153 cm-1 at ambient pressure, which are plotted versus the pressure (Figure 8a). The peak exhibits obvious blue shift with increasing pressure. Upon increasing the pressure to 22.9 GPa, a shift of approximately +21(0.5) cm-1 is observed in this peak. The vibrational frequency of this metal-carbon cage vibration could be approximated given by using a simple harmonic oscillator model, where the metal atom moves against the rigid cage41:


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ν (𝑐𝑚−1 ) = 1302.1(𝑓⁄𝜇)

1⁄2

(2)

f is the force constant (N·cm−1) between the metal atom and the carbon cage and μ is the reduced mass of the Gd@C82 in atomic mass units. The force constants of the Gd-C82 vibrational mode at different pressure can be derived using the above equation. Figure 8b shows the plotted curve for the force constant as a function of pressure. It is evident from the figure that the force constant (f) versus the pressure in the range from ambient pressure to 22.9 GPa is increasing. At ambient pressure, the f value for Gd-C82 is about 1.87 N·cm−1, while at 22.9 GPa, it increases to 2.43 N·cm−1, indicating that Gd atom is still at the +3 charge state40, 44. Therefore, the blue shifts of both Gd−C and C-C Raman vibrations are caused not by the change of the oxidation state of the Gd ion but by the contraction of the C82 cage at high pressure which then can modify the bond length and their strength of interaction45.

Figure 8. Pressure dependence of (a) Raman shift at 153 cm-1 and (b) the force constants for Gd-C82 interaction. Overall, the deformation of Gd@C82-S8 is anisotropic and the deformation from the part of the carbon cage close to the embedded metal atom is less than the part far away from the metal atom under high pressure. The observed blue shifts of the Raman vibrations are due to external pressure effect rather than the change in the oxidation state of the metal atom.


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CONCLUSIONS In summary, we have employed in situ synchrotron X-ray diffraction patterns and Raman spectra

to study the structural stability and phase evolution of Gd@C82-S8 under high pressure up to 40.6 or 31.3 GPa, respectively. Moreover, the Raman spectrum of Gd@C82 and the eigenvectors of the corresponding vibrational modes were simulated and assigned at ambient conditions for the first time by Gaussian 09. Gd@C82-S8 does not show any structural phase transition before the pressure induced amorphization at about 23 GPa in both experiments and the amorphization is irreversible when the pressure releases to ambient pressure. Lattice of Gd@C82-S8 is much more compressible in the a direction than in the b and c directions and the deformation from the part of the carbon cage close to the embedded metal atom is less than the part far away from the metal atom under high pressure, suggesting that the deformation of Gd@C82-S8 is anisotropic. By studying the pressure coefficients of Raman modes in different parts on the carbon cage and the force constant between the metal atom and the carbon cage, we find that the trapped Gd atom plays an important role in supporting the carbon cage against the deformation though it is still at the +3 charge state. It is remarkable that both the phase transition pressure (up to 23 GPa) and the bulk modulus (44.0 GPa) of Gd@C82-S8 are higher than observed in some other fullerenes. These high pressure studies on EMF (Gd@C82) provide available information for future research on the structural evolution and physical properties of EMFs, especially may favor its practical application as the magnetic materials with high hardness. ASSOCIATED CONTENT Supporting Information. SEM and TEM analysis, simulated powder diffraction pattern, high pressure XRD using methanol-ethanol mixture as pressure transmitting medium of Gd@C82-S8; Raman spectra of


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Gd@C82-S8 and S8; The animation in video format of the Raman-active vibrational modes at 729, 790, 1581 and 1617 cm−1 of Gd@C82-S8. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Baoyun Sun: Tel: +86-10-88233595. Email: [email protected]. * Yanchun Li: Tel: +86-10-88235981. Email: liyc@ ihep.ac.cn. Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We would like to acknowledge Dr. Yilin Lu and his colleagues at the Horiba (China) Trading Co., Ltd in Beijing Branch, Ke Zhu, Yang Yang and Xin Li from Institute of Physics, CAS and Min Lv at Perking University for helping with the Raman work. This work was supported financially by the National Basic Research Program of China (2016YFA0203200), National Natural Science Foundation of China (U1632113, 21402202, 11505191, and 11705211). REFERENCES (1) McMillan, P. F. New Materials from High-Pressure Experiments. Nat. Mater. 2002, 1, 19-25.


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