Phase Of C60 Research Articles

The Structures of C60‐Phases, Formed by Thermobaric Treatment: HREM‐Studies

The present study was undertaken for determination of microstructures of C60 phases, formed at different stages of high pressure and high temperature (HPHT) treatment by TEM methods. A sequence of fcc‐lattices was found after HPHT treatment at moderate temperature and pressure conditions. Increasing the pressure and temperature values further results in the formation of distorted fcc‐phases, explained by the formation of chains of bonded C60‐molecules. Curved carbon structures—ellipsoids and onions with compressed inner layers were observed in C60‐specimens after next stage of thermobaric treatment. Diffraction patterns of the structure, formed after higher parameters, show diffuse arc reflection, separated by a 70° angle. We can conclude, that the materials's hardness increases with increasing temperature and pressure. The hardest material has a structure consisting of onions and 70° disoriented fringes.

Low‐Frequency Phonons in High‐Pressure High‐Temperature C60 Polymers

Inelastic neutron scattering investigations of the orthorhombic (O), tetragonal (T), and rhombohedral (R) pressure‐polymerized phases of C60 as well as the pressure‐dimerized state give evidence for differences in the responses of low‐energy excitations between these phases. These differences are assigned to changes in low‐energy interball modes dynamics as a function of the structure of pressure‐polymerized phases. Far‐infrared transmittance measurements complete the information of the low‐frequency vibrational dynamics of these phases. The temperature dependence of these low‐frequency excitations reveals a strong anharmonicity and possible phase transitions.

Single Crystals Synthesis and Refinement of the Crystal Structure of the Polymerized Tetragonal Phase of C60

The possibilities of single crystals synthesis in conditions of nonisotropic one‐axis contraction were studied. As a result of the work the single crystals of the tetragonal polymerized phase of C60 without orientational domains were obtained. The crystal structure of this phase was resolved using single crystal x‐ray diffraction data. A structural model of this phase proposed early was confirmed and refined to final R = 0.075. The structure is slightly disordered. It consists of a random combination of the P 4 2/mmc layers (84%) and of the Immm layers (16%), along the c axis.

Photoluminescence Study of the Two‐Dimensional Tetragonal and Rhombohedral Polymers of C60 at High Pressure

The pressure behavior of the photoluminescence (PL) spectra of the planar polymeric phases of C60 have been studied at pressure up to 4 GPa. The PL spectra and the pressure‐induced shift of the principal bands differ considerably for the pristine C60, two‐dimensional tetragonal (2D‐T) and rhombohedral (2D‐R) polymers of C60. The changes in the PL spectra may be related with the transformation of the electron energy spectrum of polymers.

Observation of ordered phases of fullerenes in carbon nanotubes.

We have assembled molecular arrays of C60 inside double-walled carbon nanotubes (DWNTs) with internal diameters of 11-26 A and directly observed the existence of different crystalline phases of C60 previously predicted theoretically. The structure of the encapsulated C60 crystal is defined by the internal diameter of the DWNT, as the molecules adjust their packing arrangement in order to maximize van der Waals interactions. We have also shown that fullerenes in C(60)@DWNT interact with the outer layer of DWNTs, as demonstrated by the efficient filling of DWNTs with internal diameters of less than 12 A.

Calorimetric study of crystalline dimer and polymerized phases of fullerene C60

By high precision dynamic calorimetry the temperature dependences of heat capacity C p 0 = f ( T ) for dimer of C 60 , orthorhombic and rhombohedral polymerized phases of fullerene C 60 were determined in the temperature range 300–650 K. Besides, endothermic transformations caused by the depolymerization of the polymerized (dimeric) phases of C 60 were studied and their standard thermodynamic characteristics a temperature interval, enthalpy, entropy and Gibbs function were analyzed. Based on the thermodynamic and oscillation spectroscopy data, it was established that the depolymerization proceeds completely and the products formed are a monomer fcc phase of C 60 . From the data of the present work and the heat capacity values measured by us earlier in a low-temperature interval, the thermodynamic functions of the substances, viz. the heat capacity C p 0 ( T ), enthalpy H 0 ( T )− H 0 (0), entropy S 0 ( T )− S 0 (0) and Gibbs function G 0 ( T )− H 0 (0), were calculated over the range from T →0 to (380–490) K at standard pressure. The results were discussed taking into account literature data. A sequence of a relative thermodynamic stability at 298.15 K and atmosphere pressure of the crystalline polymerized C 60 phases was arranged.

Influence of mechanical stress and temperature on the photoluminescence in the low-temperature phase of C60 fullerite

The results of a study of the low-temperature (5–90 K) photoluminescence of thin films of C60 obtained by vacuum deposition on heated mica substrates are reported. The structure of the films is analyzed by the method of high-energy electron diffraction. The features of the luminescence of structural traps (X traps), which arise on account of mechanical stresses created by bending of C60 films of different structure on mica substrates, are investigated for the first time. The temperature behavior of the photoluminescence bands due to defects of this kind is investigated. The processes of trapping and transport of electronic excitations in the low-temperature phase of C60 are discussed.

Effective photopolymerization of C60 films under simultaneous deposition and UV light irradiation: Spectroscopy and morphology study

An effective method for uniform photopolymerization of C60 films using simultaneous deposition and irradiation with ultraviolet (UV) light is described. The photopolymerization process was monitored as a function of irradiation time using Raman and infrared (IR) spectroscopy. New features appeared in the Raman (near the pentagonal pinch Ag(2) mode) and IR spectra (400–1500 cm−1) after more than 20 h of UV irradiation testifying to the transformation of pristine C60 to polymerized C60 phases. Band shape analysis of the vibrational data revealed: (i) the degree of photopolymerization to be ∼90% after 20 h of irradiation, and (ii) the presence of orthorhombic, tetragonal, and rhombohedral phases in the photopolymerized films. Electron microscopy and diffraction studies revealed the amorphous nature of the photopolymerized films which comprised of crystals with a linear dimension of ∼40–60 nm. No evidence for cracks in the surface of the polymerized film was found. The proposed route for photopolymerization provides an opportunity to prepare extended polymeric C60 films suitable for technological applications.

Orientational correlations in the high-temperature phase of solid C60

The diffuse scattering in the high-temperature phase of solid C60 forms a very complicated pattern. It is analyzed from microscopic interactions within a mean-field theory. Different local orientational configurations of the C60 molecules are found.

Intermolecular potential and the equation of state of solid C60

First-principles total-energy calculations are performed for the low-temperature ordered phase of solid C60 in the wide range of lattice parameter. The intermolecular potential obtained from these calculations is successfully modeled in terms of the interaction between carbon atoms on different C60 molecules and the multipole Coulombic interaction between molecules. This model enables us to calculate structural and thermodynamic properties of both the orientationally ordered and disordered phases in a consistent manner. We find that the equation of state at finite temperature obtained for this potential is not in good agreement with experiments and the intermolecular potential is much shallower than that expected from the experimental heat of sublimation. These discrepancies are interpreted as arising from the limited capability of the density-functional calculations to appropriately incorporate the effect of electron correlations at large separation, which is responsible for the long-range behavior of the van der Waals interaction between molecules. In order to circumvent this difficulty we take an empirical approach to estimate the additional van der Waals interaction, which is not taken into account in the current density-functional calculations.

Thermodynamic properties of crystalline polymeric C60 phases in the temperature region from O → 0 to 340 E

The temperature dependences of the heat capacity Cp° = f(T) were studied in an adiabatic vacuum calorimeter for the orthorhombic, tetragonal, and rhombohedral polymeric C60 phases in the 7—340 K temperature interval with an error of ∼0.2%. Comparative analysis of Cp° of these phases formed by stacking of one-dimensional and two types of two-dimensional polyfullerenes C60, was performed, and their fractal dimensionalities D were determined for temperatures below 50 K. The thermodynamic functions of the crystalline polymeric C60 phases were calculated in the temperature region from O → 0 to 340 K: Cp°(T), H°(T) — H°(0), S°(T) — S°(0), and G°(T) — H°(0). Assuming that S°(0) = 0, the standard entropies of formation ΔfS° of these phases from graphite at T = 298.15 K and standard pressure were calculated. In addition, the entropies of transformation of the initial face-centered cubic phase of fullerite C60 in the crystalline polymeric C60 phases and entropies of their interconversions under the same conditions were estimated. The thermodynamic characteristics of the polymeric C60 phases were reviewed.

Crystal structures and electronic properties of haloform-intercalatedC60

Using density functional methods we calculated structural and electronic properties of bulk chloroform and bromoform intercalated C60, C60 2CHX3 (X=Cl,Br). Both compounds are narrow band insulator materials with a gap between valence and conduction bands larger than 1 eV. The calculated widths of the valence and conduction bands are 0.4-0.6 eV and 0.3-0.4 eV, respectively. The orbitals of the haloform molecules overlap with the $\pi$ orbitals of the fullerene molecules and the p-type orbitals of halogen atoms significantly contribute to the valence and conduction bands of C60 2CHX3. Charging with electrons and holes turns the systems to metals. Contrary to expectation, 10 to 20 % of the charge is on the haloform molecules and is thus not completely localized on the fullerene molecules. Calculations on different crystal structures of C60 2CHCl3 and C60 2CHBr3 revealed that the density of states at the Fermi energy are sensitive to the orientation of the haloform and C60 molecules. At a charging of three holes, which corresponds to the superconducting phase of pure C60 and C60 2CHX3, the calculated density of states (DOS) at the Fermi energy increases in the sequence DOS(C60) < DOS(C60 2CHCl3) < DOS(C60 2CHBr3).

Pressure-induced transformations and optical properties of the two-dimensional tetragonal polymer of C60 at pressures up to 30 GPa

The Raman spectra of the two-dimensional tetragonal (2D(T)) polymeric phase of C60 have been studied in situ at pressures up to 30 GPa and room temperature. The pressure dependence of the phonon modes shows an irreversible transformation of the material near 20 GPa into a new phase, most probably associated with the covalent bonding between the 2D polymeric sheets. The Raman spectrum of the high-pressure phase is intense and well resolved, and the majority of modes are related to the fullerene molecular cage. The sample recovered at ambient conditions is in a metastable phase and transforms violently under laser irradiation: the transformed material contains mainly dimers and monomers of C60 and small inclusions of the diamond-like carbon phase. The photoluminescence spectra of the 2D(T) polymer of C60 were measured at room temperature and pressure up to 4 GPa. The intensity distribution and the pressure-induced shift of the photoluminescence spectrum drastically differ from those of the C60 monomer. The deformation potential and the Gruneisen parameters of the 2D(T) polymeric phase of C60 have been determined and compared with those of the pristine material.

Viewpoint

Most magnetic materials which are used in technologicalapplications are metals, alloys or simple compounds, all basedon inorganic chemistry. The search to find organicmaterials which show magnetism has three principal motivations.First, organic materials are extremely chemically tunablebecause of the flexibility of carbon chemistry; there is anobvious advantage in choosing a route already favoured bybiological systems. Second, organic materials can possessinteresting optical properties, thus allowing the possibility ofdevices which implement new functionality. Third, there is thepleasure of achieving something which was once thoughtimpossible: making ferromagnetism in materials containing only sand p electrons. Heisenberg's theory offerromagnetism [1], which introduced the concept ofexchange, had explained the observation that all ferromagnetsknown at that time had d and f electrons. Conventionalwisdom has it that, for example, carbon (containing only s and pelectrons) does not have a spontaneous magnetic moment in any ofits allotropes.Organic ferromagnetism was first achieved using organic radicalscalled nitronyl nitroxides. Many organic radicals exist whichhave unpaired spins, but few are chemically stable enough toassemble into crystalline structures. Even when that ispossible, aligning these spins ferromagnetically is usuallydifficult to achieve. Therefore, the discovery of long-rangeferromagnetism in one of the crystal phases of para-nitrophenyl nitronyl nitroxide (C13H16N3O4)was particularly exciting, eventhough the transition temperature was a disappointingly low0.65 K [2] (though a similar material was soon foundwhich more than doubled this record [3]). Weakferromagnetism in a dithiadiazolyl radical(C8N3F4S2) has been found below 36 K [4](rising to 65 K under pressure [5]). The orderingtemperature can be raised to well above room temperature bypreparing `hybrid materials' known as molecular ferromagnets inwhich organic groups are combined with transition metalions [6,7]. Here the organic groups are themselvesnot magnetic but are used to mediate the magnetism betweentransition metal ions.A further strategy is to use carbon `buckyballs' to makeorganic materials with novel properties. CertainC60-containing compounds can exhibitsuperconductivity [8], and weak magnetism below16 K was found in TDAE-C60 (TDAE is tetrakis(dimethylamino)ethylene)[9], although thenature of the ordering provoked some controversy; this was foundto be due to the existence of two distinct phases ofTDAE-60 which are associated with different relativeorientations of the C60 molecules [10]. Untilrecently, C60 itself was not thought to be a source ofmagnetism, although superconductivity below 52 K has beeninduced in a C60 film using hole injection in afield-effect transistor geometry~[11] (or below 117 Kin lattice-expanded C60 films [12]). Inpristine solid C60, the buckyballs are arranged in aface-centred cubic lattice and held together by van der Waalsbonding. The application of pressure allows the production of anumber of different crystalline, polymeric and amorphous phasesof C60 [13], one of which is a two-dimensionalpolymerized rhombohedral phase containing layers of covalentlybonded C60 molecules [14]. This phase recently showed a signature of spontaneous magnetization up to about500 K [15], raising the possibility that an allotropeof carbon might have a room temperature spontaneousmagnetization after all.In this issue of Journal of Physics: Condensed Matter,Wood et al [16] describe a structural andmagnetic study of this intriguing two-dimensional polymerizedrhombohedral phase of C60, although prepared underslightly different conditions. Reports of weak magnetization inorganic materials have often proved to be wrong, so the newconfirmation of the earlier work [15] is extremelyencouraging. More importantly, using electron microscopy andx-ray diffraction Wood et al show that the buckyballsare undamaged in the most magnetic phase. With higherpreparation temperatures (or higher preparation pressures) thecages can collapse and an sp2 amorphous phase is produced [14]; this amorphous phase could contain unpaired defects which would explain the magnetic properties.However, by demonstrating that the C60 molecules are notcollapsed in the most magnetic phase, Wood et al haveruled out this explanation of the magnetic properties and theirwork consequently strongly supports a model ascribing themagnetism to radical centres formed by the dangling bonds whichare left following the breaking of the intermolecular bridgingbonds. The mechanism for the magnetic coupling between theseradical centres is not yet elucidated and the magnetization isvery small and far from being uniformly distributed throughoutthe sample. Nevertheless, this paper demonstrates thatperforming careful structural studies of samples prepared atdifferent temperatures and pressures is the right way forward tounderstand this effect. The lesson from TDAE-C60 is thatthe precise orientations of the buckyballs can be of crucialimportance in determining the magnetic ground state,demonstrating that combined structural and magnetic studies aredefinitely needed. The letter of Wood et al reminds usthat organic materials continue to be full of surprises.

Spin−Lattice Relaxation and 13C NMR Line Shape at Multiaxial Reorientation of Molecules in Fullerite C60

Molecular dynamics in the fcc-sc phases of fullerite C60 (13C NMR line shape and spin-lattice relaxation time T1) is described in terms of discrete orientational states of molecules. Molecular reorientations in the low temperature orientationally ordered sc phase are assumed to take place about molecular symmetry axes. Transition into the high-temperature disordered fcc phase is modeled by additional molecular reorientations about C4 symmetry axes of the cubic unit cell. At low temperatures, the NMR line shape functions are demonstrated to depend significantly on the model of molecular reorientations in contrast with T1. In the vicinity of the phase transition at 260 K, T1 changes dramatically for all models, unlike the NMR line shape. The model of relatively fast molecular jumps about a single C3 symmetry axis along with slow reorientations about the other axes is found to adequately reproduce the T1 experiment and qualitatively reflects the temperature changes in NMR line shape.

Pressure-induced transformations in two-dimensional polymeric phases of C60

The structural stability of the tetragonal and rhombohedral two-dimensional (2D) polymeric phases of C60 was studied under pressures up to 27 GPa at room temperature by means of in situ Raman scattering spectroscopy. The results show that the tetragonal 2D phase undergoes an irreversible transformation in the region of 20 GPa while no pressure-induced transitions were observed for the rhombohedral 2D phase. The obtained data are discussed within the framework of recent numerical calculations, which predict the pressure-induced transformation of the 2D polymeric phases of C60 into three-dimensional (3D) polymers in the pressure range 14–20 GPa.

Relaxation contribution to shear moduli of the low-temperature phase of solid C60

The relaxation contribution of the molecular reorientation in an elastic field of an acoustic wave to the effective shear moduli is calculated in the framework of the phenomenological two-level model of orientational states in the low-temperature phase of solid C60. The polarity of the rotation axis of C60 molecules and the possible existence of orientational domains in the structure of the low-temperature phase are taken into consideration. The estimates obtained are compared with the available experimental data.

Modeling mechanical properties of carbon molecular clusters and carbon nanostructural materials

ABSTRACTThe concept of 2D elasticity of a graphene sheet together with the idea of stiffness of a single sp3 bond have been applied to theoretical evaluating elastic properties of diverse carbon states. 2D elastic moduli have been extracted from data on elastic moduli of crystalline graphite. Stiffness of the sp3 bond has been estimated from data on the elastic modulus of diamond. Efficiency of Van-der-Waals interaction has been taken from the elastic modulus C33 of crystalline graphite. Characteristics of single fullerene deformability have been computed by the molecular dynamics method. Theoretical estimations have been performed for single molecular clusters, pristine fullerite, HPHT phases of polymerized C60, etc. The estimations are in good agreement with experimental data on elastic properties and nanoscale structure of carbon states. The approach is effective for establishing interrelation between nanostructure and elastic properties, for prediction and classification of nanostructure in novel carbon materials.

Phase transitions and the structure of the C60 crystal doped with lithium by electrodiffusion

The structure and phase transitions of C60 crystals doped with lithium by injecting metal ions from the superionic crystal-C60 single crystal heterojunction under electrodiffusion conditions are reported. The sample experienced irreversible transitions resulting in the virtually complete disappearance of EPR signals and MW conduction in the temperature range 320–370 K. In this temperature interval, a new C60 phase was formed; the phase contained polymeric chains of C60 molecules along the crystallographic c axis and lithium clusters. The structure of this phase was determined. Annealing at 620 K restored the EPR signal and, according to the X-ray data, the initial cubic structure of pure C60. The X-ray pattern, however, contained additional diffraction peaks, which was evidence of the presence of one more phase with a structure yet unknown.

Electronic structure and properties of rhombohedrally polymerized C60

The valence electronic structure of rhombohedrally polymerized C60, synthesized at 6 GPa and 725 °C, has been studied using x-ray emission spectroscopy. The C Kα spectrum of the polymer was found to differ from that of the C60 fullerite especially in the high-energy region. The observed spectra were compared to densities of states, calculated using a tight-binding approximation for three two-dimensional hexagonal C60 networks, distinguished by the nature of intermolecular bonding. Theoretical spectra of the polymers agree well with experiment and indicate that changes in the valence band of C60 upon polymerization are mainly due to the formation of intermolecular bonds, and only to a negligible degree due to a distortion of the C60 cage. Rotation of C60 within the hexagonal layer changes dramatically the electronic properties of the polymer from a semiconducting behavior for the structure with 66/66 connections to a metallic behavior for a layer containing C60 molecules that are linked through 56/65 bonds. The occurrence of the latter configuration may explain the observed metalliclike in-plane conductivity of the rhombohedral phase of C60.