Vapor-Phase Growth and Structural Characterization of Single

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Vapor-Phase Growth and Structural Characterization of Single Crystals of Magnesium Doped Two-dimensional Fullerene Polymer MgC 2

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Masashi Tanaka, and Shoji Yamanaka Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00186 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Crystal Growth & Design

Vapor-Phase Growth and Structural Characterization of Single Crystals of Magnesium Doped Two-dimensional Fullerene Polymer Mg2C60 Masashi TANAKA1, 2*, Shoji YAMANAKA2 1

Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata, Kitakyushu 804-8550, Japan

2

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan

Abstract Single crystals of magnesium doped fullerene polymer Mg2C60 can be directly grown via a binary vapor-phase mixture of Mg and C60, in sealed glass tubes at elevated temperatures. This is the first single crystal, in metal-doped two-dimensional fullerene polymers, which enables precise X-ray structural refinement. The Mg2C60 crystallizes in a monoclinic space group, I2/m, with lattice parameters of a = 9.324(2) Å, b = 9.041(2) Å, c = 14.817(3) Å, and β = 91.699(10)°. A Mg atom is located at each tetrahedral fullerene ball interstice, where the shortest Mg-C distance is 2.341(2) Å, suggesting that the Mg cation is in van der Waals contact with carbon p orbitals. The precise structure of the two-dimensional fullerene polymer network is characterized by comparison with structural data reported previously on powder samples.

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1. Introduction

The reactivity of fullerene C60 with alkali and alkaline earth metals has attracted continued interest by physicists and chemists. The products referred to as metal fullerides, show unique electronic properties; heavy metal intercalated fullerides such as K3C60,1 Rb3C60,2 and CsxRbyC603 show high transition temperature (Tc) superconductivities.4,5 The fulleride superconductors are cubic monomer phases. In some metal fullerides, polymerization of C60 molecules is induced by charge transfer from metal atoms, coupled with the contraction of the fullerene lattice through Coulomb interactions: C60 molecules are linked by C-C single bonds and/or [2 + 2] cycloaddition.6-8 Alkali metal fullerides with large atomic sizes MC60 (M = K, Rb, and Cs) form one dimensional (1D) polymeric C60 chains.9-11 Smaller alkali metals, such as Li and Na result in the two-dimensional (2D) polymeric phases Li4C60 and Na4C60, respectively.12,13 In Na4C60, each C60 molecule is linked to four other neighbors by single covalent C-C bonds, forming a 2D network.13 In Li4C60, C60 molecules are linked by mixed types of interfullerene bonds, single C-C bridging and [2 + 2] cycloaddition: the two different types of bonding chains run orthogonally in the polymer sheet.14 Divalent metal fulleride Mg2C60 also has a similar 2D polymeric C60 network isostructural with that of Li4C60.15 The 2D polymer fullerides, Li4C60 and Mg2C60, have high ionic conductivity, and are expected to be useful as battery materials in the future.16,17 In early studies of C60 polymer phases, interfullerene bond formation was estimated from the contraction of the C60 crystal lattice, as C60 has high molecular symmetry and sufficient stiffness.18,19 The center-to-center distances between the C60 units, in van der Waals contact within

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the monomer crystals, are reduced by the formation of single bridging bonds and [2 + 2] cycloaddition. Single crystals composed of 2D (Immm and R-3m) and 3D (Immm) C60 polymers, without metal doping, were obtained, for the first time, via topotactic polymerization of C60 monomer single crystals under varied high-pressure and elevated temperature conditions.20-22 The orientation and C-C bond lengths of the [2 + 2] cycloaddition of the C60 molecules in the 2D and 3D polymers have been precisely determined by X-ray refinements on the respective single crystals.20-23 The intramolecular bond lengths and bond angles for the C60 units in the polymers were also precisely determined. C60 molecules are easily polymerized by the doping (intercalation) of alkali and alkaline earth metals without the use of high pressure. Metal fullerides are normally prepared via solid-solid reaction techniques at elevated temperatures using a stoichiometric ground mixture of C60 and the respective metal powders in vacuum-sealed glass tubes. The treatment is usually repeated after the products are reground to confirm compositional uniformity. Therefore, the resulting samples are fine powder aggregates. Pioneering studies have observed in detail the metal fulleride crystal structures of Li4C60 and Mg2C60 using powder samples and the Rietveld analysis of X-ray or neutron diffraction data.12,15 The results are precise, and sufficiently reliable in order to understand the basic polymer structures. However, an ongoing challenge is the synthesis of single crystals of metal fullerides that are large enough for more precise X-ray structural refinement. Single crystals are also useful for physical property characterization in the future. In this study, we have successfully

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obtained single crystals of Mg-doped 2D fullerene polymer Mg2C60, and precise X-ray structural data of these crystals.

2. Experimental Section 2.1. Synthesis of Mg2C60 Polymer Single Crystals C60 powder (MTR Ltd., 99.5+%) was purified by sublimation in a vacuum-sealed glass tube with a temperature gradient of 500–600°C. A mixture of the purified C60 and Mg metal powder (Sigma Aldrich, 200–300 mesh) was vacuum-sealed in a glass tube with an inner diameter of 10 mm, which was then placed in a two-zone horizontal furnace with a temperature gradient of 500–600°C for one day. The loaded sample mixture was in the higher temperature (T2) zone, whereas single crystals were grown on the inner surface of the glass tube in the lower temperature (T1) zone, shown schematically in Figure 1. The crystals are identical to the monoclinic 2D polymer Mg2C60 that is described below.

Figure 1. A schematic illustration of the two-zone horizontal furnace and glass tube used for the synthesis of Mg2C60 single crystals. 4


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The temperature control in the furnace is crucial in order to produce the single crystals. In optimal furnace conditions, with a temperature gradient of 500(T1)–600°C(T2), the color of the glass tube changed to dark brown after the reaction. When T1 < 450°C, the glass did not change to a dark color, and only monomer C60 single crystals were deposited in the lower temperature (T1) zone, suggesting that the Mg vapor had not sufficiently filled the glass tube. On the other hand, when T1 > 550°C, although the color of the glass tube changed to dark brown, C60 monomer crystals were only found in the T1 zone. A study has recently worked on the thermal decomposition of Mg2C60: Mg2C60 polymer decomposes to a C60 monomer when heated above 550°C.17 This study also suggests that Mg2C60 decomposes to monomer crystals even within a higher Mg vapor pressure at T1 = 550°C. When prolonged heating (i.e., for longer than one day) was tried, there was C60 crystal contamination observed. The nominal composition of the loaded mixture did not influence the stoichiometry of the resulting single crystals: we used a molar ratio of Mg:C60 = 6:1 in most reactions.

2.2. Characterization Single crystal X-ray structural analysis was performed on a SMART-APEX II (Bruker) diffractometer with a CCD area detector, using graphite monochromated Mo Kα radiation (λ = 0.71069 Å) from a fine focus X-ray tube. The structure was solved by direct methods using Sir97.24 The structure was reﬁned with the program SHELXL-2018 and the WinGX software package.25,26 The crystals were non-merohedrally twinned with the twin law of [−1 0 0, 0 −1 0, 0.094 0 1].

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TWINABS was used to make absorption corrections to create an hklf5 file for the refinements. 27 The twin fraction parameter (BASF) was refined to be 0.371(2). The chemical composition of the single crystals was measured by an energy-dispersive X-ray (EDX) analyzer (EDAX Genesis XM2) equipped with a scanning electron microscope (SEM, Hitachi, model S3400).

3. Results and Discussion 3.1 Synthesis of Mg2C60 Polymer Single Crystals Single crystals, with sub-millimeter dimensions, were grown onto the inner surface of the glass tube in the lower temperature zone (T1 = 500°C). The SEM images and EDX analysis spectrum of the crystals are shown in Figure 2. The atomic ratio Mg:C is approximately 1:30, suggesting that the crystal composition is similar to the polymer phase Mg2C60. The Mg2C60 polymer is directly grown via the binary vapor-phase mixture of C60 and Mg, by their reaction at elevated temperatures. Single crystals of the 1D polymer KC60 were prepared by a similar co-evaporation method of K metal and C60 in a stoichiometric composition.28 Previous studies have reported that there are rhombohedral magnesium fullerides with a higher Mg concentration, MgxC60 (x = 4–6).29,30 In this study, we obtained only the monoclinic Mg2C60, despite the Mg-C60 mixture that has higher Mg compositions. Recently, Mg-doped epitaxial thin C60 films have been deposited on a mica substrate using molecular beam epitaxy (MBE).31 Although the doping level was as low as Mg/C60 < 0.6 in the film, and Mg2C60 phase formation was not discussed, it is likely that Mg2C60 single crystal growth, by

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binary vapor-phase, may occur via a similar mechanism as the MBE epitaxial deposition of Mg-doped C60 films. The 2D polymer Mg2C60 is stable in air, and can be left exposed to air for longer than one month. A similar stability was reported for the 1D fullerene polymer KC60.32 The significant stability when exposed to air is due to the complete ionization of the Mg and K atoms in the fulleride polymer structures.

Figure 2. SEM image and typical EDX result of a single crystal.

3.2 Mg2C60 Crystal structure The result of the single crystal analysis, and the crystallographic details of Mg2C60 are summarized in Tables S1–S4 in the Supporting Information. The Crystallographic Information File (CIF) is also given in the Supporting Information. As shown in Table S1, we used unique reflections, which are more than ten times larger than the total number of crystallographic parameters, including atomic coordinates and anisotropic atomic displacement parameters, to complete the refinements. R values (R1/wR2) converged to sufficiently small values (Table S1). As shown in Table S2, all carbon atoms 7



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in the C60 polymer have similar atomic displacement parameter values, within a narrow range of Ueq = 0.0055(3)–0.0071(3) Å2, as well as positive values along the three main axes (U11, U22, and U33), which all indicate that the refinement results are reliable. The lattice and the atomic displacement parameters of various 2D fullerides and a 2D fullerene polymer, without metal doping, are compared in Table 1.

Table 1. Lattice and atomic displacement parameters of the 2D metal fullerides and a 2D fullerene polymer (Immm) without metal doping. Mg2C60

Mg2C60

Li4C60

C60 (2D polymer)

Space group I2/m Sample Single crystal Radiation X-ray Mo Kα Lattice parameters

I2/m Powder Neutron

I2/m Powder Synchrotron

Immm Single crystal

a (Å) b (Å) c (Å)

9.3114(2) 9.0370(2) 14.8010(3) 91.62(2)

9.3264(4) 9.0478(4) 15.03294(2) 90.967(3)

9.026(2) 9.083(2) 15.077(3) 90

β (°)

9.324(2) 9.041(2) 14.817(3) 91.699(10)

Interfullerene bond type Mixed: Mixed: single, [2+2] single, [2+2] Atomic Displacement (thermal) parameters (Å2) C Mg Reference

Ueq: 0.0055(3)-0.0071(3)

Uiso: 0.0077(4)

Ueq: 0.0144(2)

Uiso: 0.0170(4)

This study

Ref. (15)

Mixed: single, [2+2]

X-ray Mo Kα

[2+2] cycloaddition Ueq: 0.0354(9)-0.038(1)

Ref. (12)

Ref. (20)

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Figure 3. A schematic illustration of the structure of the Mg-doped fullerene polymer Mg2C60; (a) a view along the direction parallel to the polymer sheets, and (b) a view along the direction perpendicular to the polymer sheet.

The Mg2C60 crystal structure is schematically shown in Figure 3. Pontiroli et al. reported a similar structure on the basis of Rietveld analysis of powder samples using neutron diffraction.15 Their results are compared with ours in Table 1, S2, and S3. Both studies are basically in good agreement. In the 2D Mg2C60 polymer sheet, the C60 units are linked by [2 + 2] cycloaddition along the b axis, which are connected by C-C single bonds along the a axis (Figure 3b). Pontiroli et al. refined the structure, varying the rotation angle of the polymer chains connected by [2 + 2] cycloaddition. They found that the optimal rotation angle is at φ = 78°. In this study, the structure was directly solved without using any structural model. We refined the atomic coordinates and the anisotropic atomic displacement parameters for each atom. The refined structure obtained in this study corresponds to the chain rotation model above, where φ = 77.89(8)°. This angle corresponds to the tilt of the polymeric chains running along the b axis by 12.11(8)° (= 90° – φ), as shown in Figure 3a. The 2D polymer sheets are stacked along the c axis (Figure 3a). In the Immm 2D C60 polymer 9



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without metal doping, which is producible under high-pressure and elevated temperature conditions, the C60 units are linked by [2 + 2] cycloaddition along both directions.20 The a and b lattice parameters reflect the differences in bonding type, listed in Table 1. Pontiroli et al.15 refined the powder data by using isotropic temperature factors (Biso), constraining all C atoms to have the same Biso parameter. The refined parameter Biso = 0.61(3) Å2 for C corresponds to Uiso = 0.0077(4) Å2, and the Uiso for the Mg atom to 0.0170(4) Å2 (Tables 1 and S2). These Uiso values are quite small for such light elements. Note that the atomic displacement parameters, Ueq, obtained in this study are also as small as Ueq = 0.0055(3)–0.0071(3) Å2 for carbon, and 0.0144(2) Å2 for Mg (Table 1 and S2). The Ueq values for the carbon atoms are much smaller than those (Ueq = 0.0354(9)–0.038(1) Å2) found for the carbon atoms in the Immm polymer (Table 1). The reason is not clear, but it is likely that carbon atoms in the Mg2C60 polymer are strongly bound to the polymer structure in the presence of Mg cations between the layers. Mg atoms are intercalated between layers occupying the tetrahedral interstitial sites formed by the C60 packing units. Li4C60 is isostructural with Mg2C60, where two tetrahedral interstices and one octahedral interstice per C60 unit are fully occupied by a Li ion and a Li ion pair (Li2), respectively.12 In Mg2C60, no residual electron intensity was detected in the octahedral site by the difference Fourier analysis. Figure 4 shows the carbon coordination surrounding the Mg atom in Mg2C60. The atomic distances Mg-C smaller than 2.7 Å are shown with dotted lines. Carbon atoms nearest to Mg are C10 × 2 and C16 × 2 with atomic distances of 2.341(2) and 2.349(3) Å, respectively, which are much longer than a covalent magnesium-carbon bond of 2.15 Å.33 The van

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der Waals radius of the C60 carbon atoms is 1.47–1.76 Å,34 depending on the orientation of the unit. The ionic radius of Mg is 0.72 and 0.89 Å for 6 and 8 coordination, respectively.35 The above atomic distances from Mg to the nearest carbon atoms, Mg-C10 and Mg-C16, are close to the sum of van der Waals radius for the carbon atom of the C60 molecule, and the ionic radius of Mg. This suggests that the Mg ion is in van der Waals contact with the carbon p orbitals in the C60 molecule. Figure 4 shows that there are seven such van der Waals contacts with Mg-C distances of less than 2.7 Å. The extended Mg-C10 line passes through the centroid of C60.

Figure 4. Carbon coordination surrounding the Mg atom, which has been placed in the tetrahedral interstice of four C60 units. Mg-C distances that are less than 2.7 Å are shown by dotted lines with their respective values. The intermolecular C-C bonds are depicted in yellow. Anisotropic atomic displacement ellipsoids are shown at the 80% probability level.

Note that Ueq values for C1 and C15 are slightly smaller than Ueq values for the other carbon atoms listed in Table S2. These carbon atoms link the C60 units to neighbors in order to form

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the 2D polymer sheets. They have deformed sp3 hybrid orbitals, and are bound more strongly than the other carbon atoms (sp2) in the structure. In Table S3, selected Mg2C60 bond distances are compared with the calculated values obtained by geometrical optimization based on the local density approximation (LDA) in the framework of the density functional theory. The observed and calculated bond distances in Table S3 are also compared on the ORTEP drawing shown in Figure S2. Note that the theoretically calculated values are in good agreement with the experimentally refined values, within 0.02 Å. The geometrical optimization calculation of 2D-C60 polymers, without metal doping, is well known for reproducing the experimental structure.36,37 The good agreement between the observed and calculated bond distances in Figure S2, and Table S3 indicates that the LDA calculation is a good theoretical platform even for the study of metal-doped fullerene compounds. The electronic structure and the density of states (DOS) for Mg2C60 calculated by LDA are shown in Figure S3. Several bond distances, taken from the powder data by Pontiroli et al.15, are also compared in Table S3. There are slight differences for the interfullerene bond distances between the single crystal and the powder data on C1-C1* created by [2 + 2] cycloaddition, and C15-C15* of the single bonds. In the single crystal analysis, C1-C1* and C15-C15 distances are 1.591(2) and 1.584(3) Å, respectively, whereas in the powder analysis, the distances were 1.52(2) and 1.69(2) Å, respectively.

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4. Conclusion We prepared single crystals of magnesium doped 2D polymer Mg2C60 by binary vapor-phase growth, and performed precise X-ray structural characterization. The results are comparable, and in good agreement, with those reported by Pontiroli et al.15 on powder samples. Our results present more detailed bond distances and a detailed picture surrounding the intercalated Mg atoms. This is the first single crystals of 2D fulleride polymer created, which are relatively easy to prepare. These crystals are useful for further studies by physicists as well as chemists. The compression of Mg2C60 polymer single crystals, under high-pressure and high-temperature conditions, is an interesting subject for further study, similar to a high-pressure study performed on Ba3C60.38

ASSOCIATED CONTENT Supporting Information: CIF file for Mg2C60. Details of bonding distances and angles in Mg2C60 polymer. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author: *M.T. E-mail: [email protected]. Address: Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata, Kitakyushu 804-8550, Japan. Tel. & FAX: (+81)-93-884-3204.

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ACKNOWLEDGMENTS This work has been supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) Program” and JSPS KAKENHI Grant Number JP18K04707. The authors would like to thank Associate Professor Kazuma Nakamura of Kyushu Institute of Technology for helpful discussion about the interpretation of the band calculation results.

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For Table of Contents Use Only Vapor-Phase Growth and Structural Characterization of Single Crystals of Magnesium Doped Two-dimensional Fullerene Polymer Mg2C60 Masashi Tanaka, Shoji Yamanaka

Synopsis Single crystals of magnesium doped 2D C60 polymer Mg2C60 were obtained by direct growth through a binary vapor-phase mixture of Mg and C60. The single crystals enabled us to perform precise X-ray structural refinements.

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Vapor-Phase Growth and Structural Characterization of Single

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