Recent progress on high-pressure and high-temperature studies of fullerenes and related materials

Polymerizationof fullerenes is an interesting topic that has been studied for almost threedecades. A rich polymeric phase diagramof C60 has been drawn under a variety of pressure P and temperature T conditions. Knowledge of the targeted preparation and structural control of fullerene polymers has become increasingly important because of their utility in producing novel fullerenebasedarchitectureswithunusual properties andpotential applications. This paper focuseson thepolymeric phases of fullerenes and their derivatives under high P and/or high T. First, the polymerization behavior and the various polymeric phases of C60 and C70 under such conditions are briefly reviewed. A summary of the polymerization process of intercalated fullerenes is then presented, and a synthetic strategy for novel structural and functional fullerene polymers is proposed. Finally, on the basis of the results of recent research, a proposal is made for further studies of endohedral fullerenes at high P. © 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5086310


I. INTRODUCTION
Pressure is a fundamental thermodynamic variable that can dramatically alter the atomic and electronic structures of materials, resulting in concomitant changes to their properties. 1he C=C bonds in fullerenes allow them to form covalent intermolecular bonds when treated at high pressure and high temperature (HPHT).The preparation path based on the reaction diagram and the treatment time greatly influences the completeness of transformation and the degree of polymerization.In addition, the hydrostatic nature of the compression environment plays an important role in bonding orientation and polymerization ordering.[4][5][6][7][8][9][10][11] The controllable polymerization of fullerenes to create new materials by using molecular confinement or co-intercalation by template molecules in the reactant has been extensively studied under a wide range of conditions of pressure P and temperature T. Solvated fullerenes, in which solvent molecules separate C 60 and C 70 molecules, are a series of crystalline materials with high stability, tunable metrics, and functionality.The intercalated solvent molecules play an important role in the construction of solvate lattices with different structures and further influence the phase transition at high P. [12][13][14][15][16][17] In addition, several metastable doped fullerene polymers have been synthesized under HPHT, such as C 60 /Ni(OEP), 18,19 C 60 /CNT, 20 and Na 4 C 60 . 21The unique initial lattice structures and the different degrees of spatial confinement provided by the dopants lead to different polymeric phases accompanied by distinctive physical and electrical properties.
In 2001, Buga et al. 22 reported the P-and T-induced polymerization of the mono-atom endohedral fullerene La@C 82 for the first time.Later, Liu and co-workers studied the structural transformations of Sm@C 88 (Ref.23) and solvated Sm@C 90 (Ref.24).The HOMO-LUMO gap decreased with increasing P until the carbon cage deformed, but no phase transition occurred.Pei et al. 25 observed for the first time that Lu 3 N@C 80 underwent a reversible n-to p-type inversion under high pressure, with the p-type semiconductor being stable up to 25 GPa.The role of the intercage metal atom is to restrain the compression of the adjacent bonds and protect the endohedral fullerene against collapse, as well as decreasing and postponing the change in band gap.To date, however, the structure and properties of endohedral fullerenes under high P remain undefined.
This paper focuses on the properties and phases of fullerenes and their derivatives under HPHT.Four topics will be discussed: (i) how fullerenes are covalently incorporated into a polymer phase under HPHT; (ii) the correlation between the structural evolution and the novel properties of the polymeric phase; (iii) the effect of pressure on the polymerization behavior of doped or intercalated fullerenes; (iv) the polymeric fullerenes and their derivatives as new and versatile building blocks to prepare a wide variety of new fullerene-based materials governed by weak intermolecular interactions, showing unprecedented architectures and a wide range of potential applications.

II. HPHT RESEARCH ON C 60
To date, many different strategies have been exploited to prepare polymeric fullerenes.Chemical reactions can link C 60 cages either covalently or supramolecularly, and the polymeric phases exhibit outstanding structural, electrochemical, and photophysical properties. 26Photopolymerization, 27 pressureand/or temperature-induced polymerization, [2][3][4] charge-transfer polymerization mediated by metals, 28 electron-beam-induced polymerization, 29 and plasma-induced polymerization 30,31 can be used to prepare novel polymeric fullerenes.In this section, we will present a general "panoramic" view of all-carbon polymeric fullerenes under high P and/or high T. Since the discovery of polymeric fullerene synthesized at high P in 1994, 32 there has been intense interest in the characterization of the polymeric phase products and in clarification of the mechanisms underlying the process of polymerization, because of the potentially useful physical, biological, and chemical properties of these products. 33,34In particular, there have been extensive investigations of the influence of P and T treatment on polymerization, 35 on infrared spectroscopy (IR) and Raman vibrational properties, 36 and on photoluminescence 37 and magnetism. 38-induced polymerization under various T and P has been studied extensively in recent years. 2 When moderate hydrostatic or quasi-hydrostatic P at high T is applied to pristine C 60 , irreversible polymerization occurs over a limited range of T and P. As shown in the P vs. T phase diagram (Fig. 1), C 60 polymers have orthorhombic, tetragonal, rhombohedral, or cross-linked phases.Recently, single crystals of these polymeric phases have been successfully prepared (the orthorhombic phase in Ref. 39, the tetragonal phase in Refs.40-43, and the rhombohedral phase in Ref. 44).Under ultrahigh P and T, 3D polymers have been discovered, but, as yet, without a uniform structure.
A. Bonding in HPHT polymeric C 60 C 60 , an electron-deficient polyene, tends to react with other molecules via radical, nucleophilic, or cycloaddition mechanisms.Fullerene can be functionalized via [2 1 n] cycloaddition (n = 1, 2, 3, 4) reactions. 45In P-induced polymerization, a [2 1 2] cycloaddition reaction between two [6,6] double bonds of near-neighbor C 60 cages is the favored reaction pathway (Fig. 2).A new cyclobutane ring is formed by covalent linkage in the polymeric product.On the other hand, the formation of C 60 intermolecular bonds is a thermally activated process, and the polymerization reaction is very slow at room temperature.Davydov et al. 46 have shown that the activation energy for the formation of the C 60 dimer is approximately 135 kJ mol 21 .However, at ambient P and T between 300 °C and 400 °C, the polymeric product is metastable and reverts to pristine C 60 . 32Hence, P is necessary for the formation of a stable polymeric product because it reduces the intermolecular distance sufficiently for chemical bond formation to take place.
In the 3D C 60 polymer, [3 1 3] cycloaddition bonds form between 2D layers to construct a 3D framework, and it is a fragment of the di-(1.5)(6,10)methanol-cyclodecanemolecule. 47,48All the equivalent 3 and 6 atoms of each C 60 molecule are bonded in pairs with the corresponding atoms of the neighboring (x,y) layer molecules.The intermolecular bond lengths are 0.151 nm and 0.161 nm for the [3 1 3] and [2 1 2] cycles, respectively. 47he distance between the center-of-mass positions of the two nearest-neighbor C 60 in face-centered cubic (fcc) bulk C 60 is 10.1 Å. 49 Generally, it is 9.1 Å in C 60 [2 1 2] cycloaddition polymers and about 9.4 Å when a single C-C bond links two cages. 50,51For example, the intermolecular distance along the C 60 orthorhombic chains is 9.14 Å, as characterized by singlecrystal X-ray diffraction experiments. 39In confined C 60 , the nearest-neighbor cage distance approaches a value of 8. 45  Å near 10 GPa.Alkali-metal-doped fullerene polymerizes under high T or HPHT.However, the framework is constructed using single carbon bonds. 53In the case of endohedral fullerenes with C 80 , [2 1 2] cycloaddition reactions occur at both the [6,6] and [5,6]  bonds of the cage.The [6,6]-regioisomer is a kinetic product, while the [5,6]-regioisomer is a thermodynamic product. 54 Spectroscopy of monomeric, dimeric, and polymeric phases Polymerization changes both the microscopic and macroscopic properties of a material.Insolubility of the polymeric materials in those solvents that do dissolve C 60 is direct proof of the polymerization.Polymerization is also confirmed by shifts in the vibrational modes and activation of Raman and infrared modes due to the reduced molecular symmetry as well as to the greater degree of freedom, and these also indicate the degree of polymerization (Fig. 3).According to the symmetry of the displacement patterns, there are 46 distinct intramolecular vibrations for C 60 .Group theory indicates ten first-order Raman-active modes, including two breathing A g modes related to the symmetric oscillations of the entire molecule and pentagons together with eight H g modes.[55][56][57][58][59][60] In addition, four intramolecular T 1u modes are infrared-active at first order.55,61,62 The Raman (Fig. 3) and infrared (IR) spectra of the pressure-dimerized state, as well as those of the orthorhombic, tetragonal, and rhombohedral pressure-polymerized phases of C 60 , have been investigated.55 The number of covalent bonds per C 60 molecule increases with increasing P and T. Accordingly, the formation of intermolecular bonds shifts the electron density away from the remaining double bonds, which become weaker.In particular, the A g (2) mode is one of the most sensitive to polymer structure and charge transfer.As shown in Fig. 3, it downshifts from 1470 cm 21 in the monomer to 1465 cm 21 in the dimer, which corresponds to two sp 3 -like coordinated carbon atoms per fullerene molecule.63 Upon further polymerization, the softening is 12 cm 21 in the linear polymeric chains, 20 cm 21 in the planar tetragonal phase, and 60 cm 21 in the 2D rhombohedral polymers.55 Apart from this, the presence of sp 3 intermolecular bonds leads to the appearance of new bonds in the range 900-1000 cm 21 .
Thermally induced dissociation of the polymeric bonds in C 60 polymers can be monitored as a function of time at high T.For example, the band shape and the intensity of the A g (2) mode change with increasing time. 64Novel polymeric C 60 phases can also be recognized by Raman spectroscopy measurements.For instance, a branched-chain C 60 polymer shows a characteristic mode at 1454 cm 21 , a more pronounced splitting of H g (1), and a new Raman-activated mode around A g (1). 63he symmetry of C 60 is reduced from I h to D 2h in the orthorhombic and tetrahedral polymers and to D 3d in the rhombohedral phase. 65Accordingly, the T 1u (2) mode is primarily polarized in the stretched directions and dramatically downshifts more than 50 cm 21 in the IR spectra of the orthorhombic and rhombohedral polymers.The large distortions of the C 60 sphere due to the numbers of cycloaddition connections demand a distorted polymer C 60 ball with relaxed atomic positions.

C. Dimerization of C 60 at HPHT
Before discussing the formation and structures of polymeric C 60 , we briefly review the structural features of C 60 at ambient P. C 60 molecules exhibit a rotationally disordered fcc structure above 260 K, free rotation at high T, and an orientationally disordered phase at 90 K.Under compression, the crystal undergoes an orientational transition at around 0.4 GPa and ambient T, which transforms into a simple cubic (sc) structure. 2,13,66uring further shortening of the intermolecular distance, [6,6]  double bonds open and four-membered rings link nearest neighbor cages.A solid-state mechanochemical method shows that the C4 ring connecting the cages is square rather than rectangular.As shown in Fig. 4, C 60 dimers are disordered in position and orientation within an average cubic lattice derived from the monomer. 67C 60 dimerization begins either within the fcc phase, in which the cage rotates freely, or within the sc structure with nearest neighbors in the P-DB (C=C double bond facing pentagon) or H-DB (C=C double bond facing hexagon) configuration. 68This mechanism can be observed even at room temperature at about 1.0 GPa. 69However, this thermally activated process is very slow at ambient T. As a result, direct synthesis of pure C 60 dimers at HPHT is not yet possible.Typical conditions for synthesizing the C 60 dimer are 1.5 GPa, 423 K, and 1000 s, with a yield of about 80%. 55When the C 60 molecules are linked, the icosahedral symmetry is drastically reduced.As a consequence, the degenerate energy levels near the Fermi level are removed, and thus the optical band gap narrows. 70 1D polymerization of C 60 at HPHT At higher T (<700 K) and P (;0.7-9GPa), long linear chains with two four-membered rings per C 60 cage form.Parallel straight chains along the original AE110ae directions in the ordered orthorhombic structure are observed in powder and singlecrystal diffraction results, although disordered branched chains possibly exist as well (Fig. 5).At the same time, a large and rapid change in volume at P ; 1 GPa has been observed.71 The XRD pattern confirms that the nearest-neighbor distance (about 9.1-9.2Å) in the orthorhombic phase polymer decreases by almost 10% compared with 10.02 Å in pristine C 60 , owing to the formation of covalent bonds.39,55 It is interesting to note that two different orthorhombic phase C 60 polymers can be formed, corresponding to "high" (above ;2-3 GPa) and "low" P regions.These structures are recognized by the different orientations of the chains around their axes.The chain links are parallel to the plane in the orthorhombic (O) structure, 33 while they are alternately tilted by angles 1m and 2m from chain to chain in another orthorhombic (O9) phase.39,72 As a result, the O and O9 phases demonstrate a pseudo-tetragonal Immm space group and orthorhombic Pmnn space group, respectively.It is hard to estimate the angle m, but lattice energy minimization predicts an optimal value of 29°.68 E. 2D polymerization of C 60 at HPHT At higher T (>700 K) and P (1.5-9 GPa), four or six fourmembered rings per C 60 molecule participate in the construction of the 2D C 60 polymer.The formation of the polymer layers by cross-coupling of chains appears in two possible unit cells (tetragonal and trigonal).After that, the layers stack in a close-packed arrangement to form the corresponding tetragonal or rhombohedral structures.When [2 1 2] cycloaddition occurs in the (001) plane of the original fcc phase, a 2D tetragonal phase is formed [shown in Fig. 3(e)]. 2 For example, a single crystal of the "tetragonal" polymer was obtained when C 60 was treated at 2.5 GPa and 500 °C.43 A single-crystal study identified the tetragonal phase to be a pseudo-tetragonal  packing of translationally identical adjacent 2D layers with an orthorhombic space group Immm.43 In this case, the chain orientations are almost parallel.In another case, the chain links are alternately parallel and perpendicular to the stacking axis and exhibit a P4 2 /mmc space group.Interestingly, W ågberg et al. 74 reported that the structural transition from an orthorhombic to a tetragonal structure occurs via an intermediate dimer state as an orthorhombic phase to a dimer and then from this dimer to a tetragonal phase.In addition, the second process is slower than the first.
When treated at higher P (500-800 °C at 5 GPa), C 60 polymerized via [2 1 2] cycloaddition in the (111) plane of the original fcc phase into a rhombohedral structure with a hexagonal lattice. 2,32It has been reported that during decompression from 6 GPa to 2.2 GPa at 873 K, the rhombohedral polymer could transform to a tetragonal phase. 40A trigonal symmetry instead of a hexagonal one was confirmed to be the most stable mode by an XRD experiment. 75The hexagonal layers stack in a close-packed arrangement of the ABCABC type with space group R3-m. 33,76All the former four-membered rings rotate 60°, forming another structure with ACBACB stacking type.Single-crystal XRD studies confirm this structure, 42 and a density functional theory (DFT) calculation demonstrates slightly more stability. 77 3D polymerization of C 60 at HPHT Under much higher P (>13 GPa) at elevated T, 3D polymers are obtained.For example, one superhard phase of a 3D polymer quenched from 13 GPa and 820 K features [3 1 3] cycloaddition bonding between the [2 1 2] cycloaddition-bonded 2D tetragonal layers.47,48 Single-crystal XRD results demonstrate that the 3D C 60 polymer is in a body-centered orthorhombic space group Immm structure (Fig. 6).78 The resulting sp 3 -sp 2 hybridized system is expected to show metallic conductivity.In contrast, the 2D rhombohedral phase transforms into a disordered phase at around 15 GPa, 79 and the 2D tetragonal phase transforms into a 3D metastable polymer over 20 GPa at room temperature.80 In situ Raman spectra strongly support the above conclusion by revealing a transformation from a 2D to a 3D phase.[81][82][83][84] Also, the in situ X-ray measurements reveal a first-order irreversible transition of the tetragonal polymeric phase at around 24 GPa.82 Although several studies have demonstrated the existence of 3D C 60 polymers, there is still no consensus on the preparation conditions, owing to experimental difficulties and the structural complexity of the products.Generally, under 8-9 GPa, a cubic structure is observed up to about 600-700 K. Disorder increases, and covalent linking extends in all directions with rising T. 5,68 Over 9.5 GPa, a body-centered structure presents.78 When T rises above 900-1000 K, the C 60 cages start to collapse, and this, combined with the covalent bonding distortion of the cage, makes the structure become amorphous or even graphitic at higher T. 4 Superhard materials, even cubic diamond crystallites, have also been found.,76 It should be noted that the polymeric phase formation and purity depend strongly on T, P, and treatment time.Typical conditions for the formation of C 60 polymers are as follows: for the 1D orthorhombic phase, 1-1.2 GPa, 550-585 K for 5 h; 39,55 for the 2D tetragonal phase, 2-2.5 GPa, 700-873 K for 0.5-4 h; 42,43,55 and for the 2D rhombohedral phase, 5-6 GPa, 773-873 K for 0.5-1 h.44,55 Another problem is establishing the stability of the polymeric phases.A quenched intermediate transition or experimental artifacts all possibly exist, but only a few in situ studies have been carried out.
It is important to highlight that heating-then-pressing from rapidly rotating monomers leads to disordered states, because chains form in all directions.Compression benefits the formation of well-ordered 1D or 2D polymers.Nevertheless, pressing-then-heating favors the optimal [2 1 2] cycloaddition route to dimerization, but orientational tuning at high T promotes linear or planar bonding. 68However, different paths still give distinct products.For instance, the heating-thenpressing path tends to form rhombohedral polymers. 82hen we consider the polymeric phase transition shown in Fig. 1, the orthorhombic-to-tetragonal transformation involves proper rotations of the chains followed by interchain cross-linking in the {100} c planes.Furthermore, the transformation from an orthorhombic to a rhombohedral phase demands that the chains rotate to a fixed angle, while the tetragonal-to-rhombohedral transition requires bond rearrangement (Fig. 5). 73

G. Properties of polymeric C 60 phases
The physical properties of the low-dimensional phases have already been well studied. 2,4The essential difference between the C 60 monomer and the polymers is the covalent intermolecular linkage in the latter. 13C nuclear magnetic resonance (NMR) studies of orthorhombic, 86,87 tetragonal, 88 and rhombohedral 86,89 C 60 clearly demonstrate that these structural differences are based on the distinguishable frequencies of sp 2 and sp 3 hybridized carbon.The relative numbers of sp 2 and sp 3 hybridized atoms can be used to identify the intraand intermolecular bonds in the polymeric phase.In particular, the sp 2 peak can be decomposed into lines corresponding to the number of unique carbon atom environments (Fig. 7). 88,89he reduced symmetry of icosahedral C 60 and the formation of intermolecular bonds in the polymeric phase lead to splitting of degenerate molecular orbitals and a narrowing of the HOMO-LUMO gap.The electronic structures of 1D and 2D C 60 polymers have been studied theoretically.The 1D linear polymer is a semiconductor with a finite band gap of 1.148 eV and almost pure s-type intermolecular bonding. 90The 2D tetragonal and rhombohedral C 60 polymers are also semiconductors.With different intermolecular covalent bonds, the tetragonal polymeric and rhombohedral polymeric phases exhibit band gap values of 1.2 eV and 1.0 eV, respectively. 34 60 is reportedly stable up to 950 °C under low-pressure inert gas conditions.91 High-temperature synchrotron radiation XRD results show that C 60 is stable below temperatures of 900 °C at 80 MPa.92 However, all the polymers are less stable than the monomer with free C 60 molecules.For example, dimers are stable to about 420 K, 93,94 the orthorhombic phase breaks down near 500 K, and the tetragonal and rhombohedral 2D polymers are stable to 550 K. 94,95 Thus, the stability sequence is dimer < 1D polymer < 2D polymer < monomer.34 It should be noted that the thermal "stability limit" is dependent on time scale. Th activation energy E a reflects the stability of the polymers; it is 1.75 eV and 1.9 eV for the dimers and orthorhombic C 60 polymeric phases, respectively.34,94 The degree of polymerization, the polymeric structure, and the ratio of sp 2 to sp 3 C dominate the conductivity of polymerized C 60 .In contrast to C 60 , an n-type semiconductor, the 1D plasmapolymerized C 60 polymer acts as a p-type semiconductor in field effect transistor (FET) experiments owing to variable-range hopping (VRH). 96With the increasing degree of polymerization, the band gap reduces and the electrical conductivity increases.95 The conductivity of 2D C 60 polymers is anisotropic.In the covalent bond linking layer, it shows a metallic-like behavior.In the weak van der Waals interacting direction of the interlayers, it exhibits semiconductor-like behavior.97,98 This phenomenon is ascribed to the weak localization of the carriers, and the metallic behavior could be attributed to a self-doping effect.Structural calculations show that lattice defects may induce metallic behavior. 99 Electricl resistivity measurements show that amorphous polymeric C 60 phases are semimetallic, with VRH and semiconducting behavior.100 Recent first-principles DFT calculations have demonstrated that the structures of 3D C 60 polymers in which each molecule is in one of the two standard orientations are analogous to ordered binary-alloy-type structures.101 Furthermore, all the investigated structures in this case are predicted to be metallic, which is supported by an experimental result where a polymeric 56/56 2 1 2 cycloaddition bond is analogous to the Ising antiferromagnetic interaction in a 3D C 60 polymer.102 Thermal conductivity theoretically increases with increasing P. In addition, it increases with the formation of covalent bonds between near neighboring C 60 's because the resulting appearance of new phonon modes contributes to heat transport. Poymeric phase disorder will compensate for the thermal conductivity effect.Under compression, the rotational motion is reduced to uniaxial rotation around the molecular axis.Subsequently, the C 70 molecules tend to arrange in a line, and a rhombohedral structure is obtained (Fig. 8).106,107 Regarding symmetry, the C 70 monomer with an fcc lattice cannot form a long-range ordered polymeric structure.In contrast, C 70 crystallized in the hcp structure can polymerize into an ordered zig-zag chain structure with an orthorhombic lattice.108 At T between 400 and 500 K and P around 1 GPa, a dimerrich phase is observed, while the dimer ratio reduces at higher temperatures.Notably, C 70 exhibits lower reactivity than C 60 , 105 so the dimerization of C 70 is much slower under high P.
Several polymeric phases have been observed in C 70 at higher P and T. Incompatibility between the lattice symmetry and the geometry of the reactive bonds means that an ordered polymeric phase of C 70 is hard to obtain at P in the range of 0.82 GPa at T between 350 and 600 K.However, Blank et al. 109,110 and Marques et al. 111,112 reported that C 70 with an fcc lattice could transform to an ordered structure under sufficiently high P, which was thought to occur only in hcp C 70 .It should be noted that the low reactivity and topochemical constraints on the polymerization lead to the slower formation of C 70 oligomers and polymers .Above 700 K, a "disordered cross-linked layered" structure and a 3D polymerized tetragonal structure have been reported. 109

IV. HPHT RESEARCH ON INTERCALATED FULLERENE A. Solvent effect on parent structure at ambient conditions
The interaction between fullerenes and solvents leads to the formation of solvated fullerene. 113C 60 and C 70 can form solvated crystals with almost all common solvents. 12aporating fullerene solutions is the general preparation method to obtain solvated fullerenes.6][117][118][119][120][121][122] Several conclusions have been drawn based on this research: (a) Solvates are typical van der Waals complexes with negative excess volumes.Bonding between fullerenes and a solvent will enhance the stability of the solvate. 120) in solvated fullerenes.An interesting case is a static cubane, which occupies the octahedral voids of the fcc structures and forms C 60 p C 8 H 8 and maintains the fcc structure (Fig. 9). 49Accordingly, solvents with symmetrical geometry and a small size tend to occupy the octahedral voids of fullerene lattices and maintain their initial structure, while solvents with nonsymmetric geometry or polar molecules are ideal structure controllers. 12,120,123,124,126Besides the solvent species, the stoichiometric ratio of solvent to C 60 will also have an impact on the structure of the solvated crystals. 116rthermore, the solvent molecule also affects the physical properties.Świetlik et al. 123 and Graja and Świetlik 127 reported that the rotation of C 60 is hindered by the intercalated solvent molecule, which is reflected in the vibrational spectrum.The influence on the rotation of C 60 depends dramatically on the intermolecular interaction and structural arrangement.On the other hand, the symmetry decrease due to interactions between fullerene and solvent molecules enhances the photoluminescent intensity.For instance, the intensity rises about two orders of magnitude in C 60 p m-xylene compared with pristine C 60 crystal, 12,14 and it is about 30 times higher in C 70 p mesitylene than in pristine C 70 powder. 128The solvent effect on the photoluminescent behavior correlates with the interaction of the cages.7 GPa, 0.7 GPa, and 0.5 GPa, respectively. 132,133In the case of iodinedoped C 60 , which crystallizes in a base-centered orthorhombic structure, 134 P enhances the charge transfer between the C 60 and the iodine atoms.As a result, the electrical band gap is 0.3 eV at 17 GPa.
It must also be mentioned that any given solvent molecule generally plays more than one role in the structure control and property tuning of solvated fullerene.For instance, ferrocene acts as a spacer as well as a charge reservoir to tune the polymerization process and promotes a layered structure of the intercalated ferrocene in the C 70 (Fe(C 5 H 5 ) 2 ) 2 crystal. 135The m-xylene in a C 60 solvate plays an important role as a spacer and a bridge by forming covalent bonds with neighbors.It preserves the stability of the highly deformed or amorphous C 60 molecules to form a long-range ordered structure under high P. 83 Hence, the solvent effects on the structure and properties of solvated molecules are various and diverse under high P.
The P-induced polymeric products can be divided into two categories: copolymerized products of C 60 and solvent molecules, or C 60 self-condensed materials.C 60 p C 8 H 8 and C 60 S 16 are in the former category.An anomaly at 0.5 GPa in the P dependences of the molecular vibrational frequencies indicates an orientational ordering transition at room temperature, while an anomaly at 1.3 GPa is attributed to a fullerene-cubane interaction in C 60 p C 8 H 8 . 136In C 60 S 16 , the peaks from the S 8 rings disappear, and new peaks are observed above 7.2 GPa, suggesting that the sulfur rings break and covalent C-S bonds form between the C 60 and solvent molecules. 137In the second category, the C 60 solvates grown from different solvent molecules exhibit various polymeric products under high P.For instance, C 60 p TDAE [where TDAE is tetrakis(dimethylamino) ethylene] (Refs.138 and 139) and C 60 (Fe(C 5 H 5 ) 2 ) 2 (Refs.140 and 141) both exhibit a one-dimensional polymerized phase above ;1 GPa and 5 GPa, respectively.C 60 {Pt(dbdtc) 2 } starts to form intercage covalent bonds at 0.5 GPa and a novel polymer, which is coordinated with nearest-neighbor C 60 with 10 and 6 sp 3 -like carbon atoms per molecule above 2.5 GPa. 132The solvent molecule in C 60 p m-xylene confines the formation of ordered amorphous carbon clusters (OACCs) under high P, as shown in Fig. 10. 16This study on a small piece of C 60 solvate indicates the diversity of the structure and properties of these solvates under high P, and additionally predicts potential applications in a wide range of areas.
Recently, a new way to understand the pressure-induced structure formation and transformation in fullerene solvates under high P has been proposed by studies in which their building blocks have been tailored and their boundary interactions have been tuned. 83,84,142,143Solvent molecules with hexagonal carbon rings, such as m-xylene, 1,2,4-trimethylbenzene (TMB), and m-dichlorobenzene (m-Cl), tend to stabilize highly compressed or collapsed fullerene clusters in fullerene solvates and form a long-range ordered structure under compression.In contrast, ferrocene, with its pentagonal carbon ring, generally causes a transformation into an amorphous component together with a carbon cage and leads to the formation of a disordered fullerene solvate structure under high P. Cubane, C 8 H 8 , differs from these aromatic solvents.It reacts with C 60 in solvates under high P, but not all of the C 8 H 8 molecules react with the carbon cage. 142The intercalated C 8 H 8 and the reacted C 8 H 8 both protect the C 60 cages from early collapse under compression.Furthermore, unlike C 60 p mxylene, in which the C 60 molecules overcome the confining effect of the solvent molecules and take part in polymerization between the cages, 16,84 the absence of intermolecular bonds

C. HPHT studies on CNT-confined fullerenes
][146][147][148][149][150] C 60 's packed inside single-walled CNTs are considered "peapods" 145 and provide an opportunity to study original quasi-1D fullerene phases.Using high-resolution transmission electron microscopy, Smith and co-workers 145,146 discovered selfassembling C 60 fullerene chains inside CNTs.Molecular dynamics studies suggest that (9,9) and (10,10) nanotubes with diameter larger than 6.27 Å give the best fit for C 60 . 147,148The intercage distance in a CNT was reduced to 8.7 Å under compression to 25 GPa, and a one-dimensional polymer chain was obtained. 20,52Larger CNTs have more space for the dimensional polymerization of C 60 , as proven by the observed zigzag structure in C 60 @CNT (15,15).
In contrast, the anisotropy of C 70 is reflected by two distinct orientations with regard to the nanotube axis.The C 70 long axis parallel to the nanotube long axis leads to a lying-down orientation, while perpendicular orientations form when the long axis is rotated 90°to the nanotube axis.The orientation of C 70 depends on the diameter of the nanotube.C 60 peapods fully polymerize at 1.5 GPa and 300 °C, 149 while C 70 peapods do not give a polymeric product even at higher P and T (1.5 GPa and 545 °C, or 2.5 GPa and 300 °C). 104These results show that CNTs impose much stricter constraints on the reactivity of the ovoid C 70 than on that of the high-symmetry C 60 .
Theoretical simulations have predicted several structures of fullerene peapods under high P. 151 In 2017, a novel superhard sp 3 carbon allotrope was obtained by cold compression of C 70 peapods. 150This so-called V carbon is thermodynamically stable over a wide P range up to 100 GPa and can be recovered to ambient conditions once it is formed, providing a new strategy for synthesizing new superhard materials.

D. HPHT studies on metal-doped fullerenes
Polymerized alkali-metal-doped fullerenes have attracted much attention because of their hardness and stiffness, metallic behavior, 152,153 and especially superconductivity. 1541D and 2D polymeric alkali-metal-doped fullerenes can be obtained under HPHT.For example, AC 60 (A = K, Rb, Cs) polymerized to a 1D orthorhombic phase under high T. 155 At 5 GPa and 573 K, the 2D rhombohedral polymers (LiC 60 ) HP and (NaC 60 ) HP were synthesized starting from C 60 and AN 3 (A = Li, Na).Notably, large-radius alkali metals (Rb or Cs) and stoichiometric X 6 C 60 stabilized the doped fullerene at much higher P compared with pristine C 60 .Cs 6 C 60 retains a monomeric structure until 45 GPa, 156 while Rb 6 C 60 exhibits a reversible structural transition to a 2D polymeric phase at 35 GPa and 600 K. 157 A 3D polymeric fulleride can be synthesized by compressing 2D Na 4 C 60 . 21On the other hand, fullerene will polymerize in a lower P range when doped with smaller alkali metals such as Na or Li. 153,154In addition, an intercalated alkali metal with a small radius could diffuse in the fullerene lattice such as in the case of the superionic conductivity in the Li 4 C 60 polymer, 158,159 Na-ion diffusion in Na 2 C 60 results in the formation of polymeric phases of C 60 . 160

V. HPHT RESEARCH ON ENDOHEDRAL FULLERENES
As pointed out at the start of this paper, pressure is a fundamental thermodynamic variable that can dramatically alter the atomic and electronic structure of materials, resulting in concomitant changes in their properties.Determining the evolution of the geometric and electronic structure of endohedral fullerenes under high P is particularly important because this information is necessary for understanding the electronic conduction mechanism, for discoveries of novel electronic properties, and for fabricating novel fulleride-based materials.On the other hand, the relatively low yield restricts the characterization of endohedral metallofullerenes by standard measurement techniques, while our limited understanding of cluster structure and properties is another challenge.However, in situ measurements at high P in a diamond anvil cell offer a complete workflow to circumvent these problems effectively.
Cui et al. 23,24 have conducted high-P structural transformation studies on the mono-atom encapsulated fullerenes Sm@C 88 and solvated Sm@C 90 .The high-P in situ IR spectra exhibit both red and blue shifts, indicating an anisotropic deformation of the carbon cage under compression.On the basis of these results, together with those of simulations, Cui et al.
propose that the cage deforms in the vertical direction, changing from an ellipsoidal to an approximately spherical shape.With further compression, a peanut-like structure appears, before the cage finally collapses under higher P. At the same time, there is a significant P-induced reduction in the band gap because of the deformation of the carbon cage and the enhanced intermolecular interaction (Fig. 11).In this compression process, the Sm atom plays a role in restraining the compression of adjacent bonds.The structural evolution of solvated Sm@C 90 under high P is almost the same. 24The solvent molecule plays a role protecting against cage collapse and postpones P-induced changes, contributing to the trapping of the metal atom in an OACC.
Polymeric fullerenes such as polymerized C 60 and C 70 exhibit unique properties such as high elastic constants and hardness. 161An HPHT-induced polymerization study of the endohedral fullerene La@C 82 revealed different polymeric phases such as dimers and 2D and 3D polymers. 22X-ray and electron diffraction results showed increasing density and disorder when the sample was treated at 9.5 GPa and 520 K.A fraction of dimer and oligomers mixed with mainly monomeric La@C 82 retained the hcp structure of the pristine sample.The hcp lattice constant decreased dramatically, and small amounts of body-centered tetragonal phase appeared when T was raised to 720 K.

VI. CONCLUSION
HPHT studies on fullerene and its derivatives have been reviewed.The essential differences among the polymeric phases concern the orientational phenomena that they exhibit and the fullerene cage bonding modes.In view of these, various 1D, 2D, and 3D C 60 polymeric structures prepared under HPHT have been studied, utilizing FTIR and Raman spectroscopy and XRD.C 70 , a nonspherical carbon cage with lower symmetry than C 60 , shares the general features of the latter with regard to polymerization, but its polymeric structures exhibit significantly fewer phases.A deeper understanding of the polymerization processes and phase diagrams of the fullerenes will help promote the synthesis of new polymeric forms with interesting and useful structural, electrical, magnetic, and photoluminescence properties.
Based on a molecular engineering strategy, solvent molecules, CNTs, and metal atoms could be used to control the arrangement of carbon cages in the lattice.Under HPHT, the intercalated units dominate the evolution of the fullerene configuration, constrain polymerization behavior, and determine the structure and properties of the polymeric product.Further efforts are required so that the phase diagrams of these intercalated/doped fullerenes can be drawn.In addition, studies of intercalated/doped fullerenes at high P and very high T (>1000 °C) present exciting opportunities to discover unique carbon phases and even diamond-like polymeric structures.
Endohedral atoms/clusters greatly enrich the properties of fullerenes, and the resulting fullerene derivatives have properties ranging from semiconducting to metallic.In particular, the tunable electronic spin state inside the fullerene cage makes endohedral fullerenes candidates for use in quantum computing.The application of high P is a potentially useful method to control electronic spin and phonon behavior.However, to date, there have been few high-P studies of fullerenes that have considered this topic.Study of endohedral fullerenes at high P is a very recent field and much remains unexplored.What happens to the interplay of carbon cages and the endohedral species under HPHT conditions?How is charge transferred between the carbon cages and the encaged species when the density increases under compression?What unique properties due to the evolution of molecular and electronic structure will endohedral fullerenes exhibit under high P and indeed high T? The need to answer these questions is a strong motivation for further research in this field.

FIG. 1 .
FIG. 1. Map of the P-T plane showing the various phases of C 60 created under different conditions.Symbols denote some of the P-T coordinates where experiments have been carried out and samples have been treated for at least 1 min.(-) denotes the diamond/graphite equilibrium line, while (---) divides the semimetal and semiconductor states at T > 1100 K and the "soft" and "hard" states at T < 1100 K. Reprinted with permission from V. D. Blank et al., Carbon 36, 319-343 (1998).Copyright 1998 Elsevier.

FIG. 4 .
FIG. 4. A random distribution of C 60 dimers (and isolated monomers) in the (001) plane of a model crystal, which accounts reasonably well for the experimentally observed diffuse scattering intensity distribution.Only in-plane and out-of-plane C 60 's that form dimers with an in-plane C 60 are shown.Reprinted with permission from R. Moret et al., Eur.Phys.J. B 37, 25-37 (2004).Copyright 2004 Springer Nature.

FIG. 5 .
FIG. 5. Schematic views of the polymer chains running along AE110ae cubic directions of the parent monomer lattice, for the different polymer structures.The chains are normal to the figure.The orientation of the four-membered rings is represented by the shaded bars that form an angle m with the AE001ae cubic directions.The broken lines represent the 2D tetragonal and rhombohedral polymer layers that can be obtained, at least schematically, by connecting the C 60 molecules of the chains by supplementary four-membered rings.Reprinted with permission from R. Moret et al., AIP Conf.Proc.544, 81-84 (2000).Copyright 2000 AIP Publishing LLC.

FIG. 6 .
FIG. 6. Crystal structure of the 3D C 60 polymer (b) in comparison with the starting 2D polymer (a).Carbon atoms marked with p in (c) are from the neighboring C 60 units.Reprinted with permission from S. Yamanaka et al., Phys.Rev. Lett.96, 076602 (2006).Copyright 2006 American Physical Society.

III. HPHT RESEARCH ON C 70 C
70 molecules undergo quasi-free rotation in a closepacked fcc lattice above room temperature.However, only the five bonds radiating out from each polar pentagon have significant reactivity, which leads to topological constraints on the formation of C 70 polymers.The anisotropic C 70 molecule exhibits at least three phases: free rotation at high T, uniaxial rotation at intermediate T, and frozen rotation or ratcheting at low T.3,104

FIG. 7 .
FIG. 7. 13 C magic angle spinning (MAS) NMR spectra of the rhombohedral 2D phase with a spinning rate of 12 kHz and repetition times of 200 and 900 s.The inset shows the structural mode of this phase.Reprinted with permission from F. Rachdi et al., J. Phys.Chem.Solids 58, 1645-1647 (1997).Copyright 1997 Elsevier.

B
. Solvent effect under HPHT Under high P, the incorporated solvent molecule diversified the phase transformation and polymerization of C 60 .There are four kinds of P-induced solvent effect: (a) Through a spacer, such as C 8 H 8 in the C 60 p C 8 H 8 crystal, which effectively shields the P in the interball voids, raising the phase transition P compared with that of C 60 . 130(b) Through a structure confiner, for instance, 1,1,2-trichloroethane (TCAN), which confines the [2 1 2] cycloaddition bonds of the C 60 's that form in the upper and lower layers alternately when C 60 p 1TCAN is treated under high P and high T.As a result, a novel quasi-3D C 60 polymer is obtained. 17Another example is C 60 p m-xylene. 16,131The long-range hcp structure is maintained by the m-xylene confiner even when the C 60 cages collapse at P > 32 GPa.(c) Through a bridge, such as in C 70 p m-xylene, where the m-xylene forms a superhard sp 3 -bonded carbon by bonding with neighboring fullerenes. 83(d) Through a charge reservoir.For example, the formation of a donor-acceptor complex can be monitored by Raman spectroscopy when C 60 {Ni(nPr 2 dtc) 2 }, C 60 {Cu(nPr 2 dtc) 2 }, and C 60 {Pt(dbdtc) 2 } are compressed at P > 0.

FIG. 9 .
FIG. 9. Structures of fullerene-cubane heteromolecular crystals.(a) Space-filling view of the rotor-stator phase of C 60 p C 8 H 8 .Rotating fullerenes occupy the fcc lattice sites, and static cubanes are in between the octahedral voids.(b) The position of cubane in an octahedral void of the fcc cell.The concave outer faces show the perfect match between the cubane and the surrounding fullerenes.(c) The restricted motion of C 70 in the tetragonal rotor-stator phase of C 70 p C 8 H 8 .In the molecular bearing of six cubanes, C 70 rotates about its long axis (C 5 ), which processes about the c axis of the body-centered tetragonal cell.Reprinted with permission from S. Pekker et al., Nat.Mater.4, 764-767 (2005).Copyright 2005 Springer Nature.

FIG. 10 .
FIG. 10.Simulated structures of C 60 p m-xylene under different compression and decompression conditions.Far left, the pristine structure at 0.4 GPa.The deformation of the C 60 cages is elastic below about 30 GPa, and the deformed cages return to their initial shape as they are decompressed back to ambient P. Above 30 GPa, many carbon-carbon bonds start to break, and the cages can no longer return to C 60 upon decompression.OACCs retain long-range periodicity and can be preserved under ambient P conditions.Reprinted with permission from L. Wang et al., Science 337, 825-828 (2012).Copyright 2012 The American Association for the Advancement of Science.
14,15,120,123 (c) Incorporated solvent molecules influence the crystal structure.There are at least five types of packing symmetry (hexagonal in C 60 p 2BrCCl 3 , 116 cubic in C 60 p cyclohexane, 118 monoclinic in C 60 p C 6 H 5 CH 3 , 123,124 triclinic in C 60 p 2ferrocene, 122 and orthorhombic in C 60 p 1,1,2-trichloroethane