Fullerite crystal

The interaction character between H-O pairs is repulsive what corresponds to the negative values of Pi, p2, yi, y2, 51 52 energies. For this case, as an example, Figure 21 shows the plots for c° = c°(T) dependence for fullerite crystals of the stoichiometric composition (ci = c2 = 0.5) and for the energies defined at kT = 0.86T0-1 eV by relations... 

Fig. 5 Mode Griineisen parameters for IR- and Raman-active intramolecular modes in C60 and C70. Squares - Raman data for C60, dots - IR data for C70, open circles - Raman data for C70. Reprinted with permission from KP Meletov, AA Maksimov, and II Tartakovskii, Energy spectrum and phase transitions in C-70 fullerite crystals at high pressure , J. Exp. Theor. Phys. (JETP) vol. 84 (1997) 144-50 [40]. Copyright 1997 American Institute of Physics...

Figure 2. The elementary cells of the fullerite crystal lattice of sc phase (a) of B1 structure (NaCl type) and fee phase (b) of Ll0 structure (CuAu type).

Peculiarity of the fullerene molecule formation also reveals itself in a fullerite crystal structure. Cubic crystal lattices of fullerites and hydrofullerites behave like those of different metals and alloys. Fullerene molecules are distributed in the lattice sites while atoms of elements are distributed in the octa- and tetrahedral interstitial sites forming the interstitial solid solutions. Fullerene molecules substitute each other in the sites of lattice and form the substitution solid solutions. Forming exo- and endocompounds, fullerene molecules that are in the lattice sites can change considerably the properties of crystal, whereas its crystalline structure remain unchanged. 

Figure 4.8-27 Cgo fullerene molecule (a) and Cfio fullerite crystal (b). The dashed circles in b represent the octahedral and tetrahedral interstitial sites at (1/2,1/2,1/2) and(l/4,l/4,l/4), respectively.

Supported bimetallic clusters are used as catalysts for the conversion of automobile exhausts to nontoxic gases and the refinement of crude oil in the petroleum industry [1], Great expectations exist for the synthesis of exotic materials, an example being the fullerite crystal, in which the units are Cgo clusters [2], The miniaturization of electronic components may soon reach sizes at which cluster physics becomes relevant. The existence of particularly stable clusters is often advocated in the construction of models of amorphous systems. These example provide evidence for the technological importance of small or medium-size atomic clusters. 

There is interest in assembling solids from clusters because of the possibility of building synthetic solid materials with novel properties. One of the best known examples is the Ceo crystal. Recently, assemblies of mass selected gold nanoparticles passivated by a monolayer of thiol molecules have been prepared but much work remains to be done in developing such assemblies. For this purpose the clusters must retain much of their individual character after assembling, which is what happens to Ceo in the fullerite crystal. This condition is difficult to meet for metallic clusters because the interatomic interactions are more delocalized. Proposals for assembling doped aluminium clusters have been investigated by ab initio computer simulations ... 

Chemists were greatly surprised when soccer-ball-shaped carbon molecules were first identified in 1985, particularly because they might be even more abundant than graphite and diamond The C60 molecule (10) is named buckminsterfullerene after the American architect R. Buckminster Fuller, whose geodesic domes it resembles. Within 2 years, scientists had succeeded in making crystals of buckminsterfullerene the solid samples are called fullerite (Fig. 14.32). The discovery of this molecule and others with similar structures, such as C70, opened up the prospect of a whole new field of chemistry. For instance, the interior of a C60 molecule is big enough to hold an atom of another element, and chemists are now busily preparing a whole new periodic table of these shrink-wrapped atoms. 

FIGURE 14.32 These small crystals are fullerite, in which buckminsterfullerene molecules are packed together in a close-packed lattice. 

Fullerenes are the third natural form of carbon. These have been found to exist in interstellar dust and in geological formations on Earth, but only in 1985 did Smalley, Kroto and co-workers discovered this class of carbon solids and their unusual properties [447, 448]. It has been shown that Ceo, the most common fullerene, could be transformed under high pressure into the other forms of carbon, diamond, and graphite [449] or, at moderately high pressures and temperatures, into new various metastable forms [450 53]. Ceo crystals, fullerites, have/cc structure with weak van der Waals interactions. This structure is stable at ambient temperature up to 20 GPa and at ambient pressure up to 1800 K [454, 455]. 

The aim of present work is theoretical study of hydrogen solubility in ordering bcc and fee crystals of fullerites on the assumption that hydrogen atoms are distributed over interstitial sites of different types (a) octahedral, tetrahedral in bcc lattice (b) octahedral, tetrahedral, trigonal, and bigonal in fee lattice, i.e., the lattice distribution of hydrogen atoms has been considered. 

We assume that the crystal lattice of fullerite in its ordered state is the one of the Ll0 type (Figure 16). In this lattice the sites of the first and the second types valid for and 2 fullerenes, respectively, alternate in layers. In this case O, 0, Q, D interstices, depending on their surrounding by the... 

Summing up the attained numbers (50) for all configurations /, we can find the total number Nh of hydrogen atoms in the crystal or the hydrogen solubility in fullerite by Eqs. (45) and (46). 

Currently, intercalation of Ceo with the rare gases into octahedral and tetrahedral interstitial voids of the fullerite Ceo crystal is broadly studied (see... 

In the present paper, we report on the dynamics of He filling the fullerite C6o fee lattice octahedral and tetrahedral interstitial voids with the respective sizes of 4.12 and 2.26 A [2], both larger than the helium Van der Waals diameter of 2.14 A [3], We also present results of study of influence of He intercalation on of photoluminescence spectra of Ceo single crystal in the low temperature phase. The measurement technique as well as the experimental setup for structural [4-6] and luminescent [5, 6] studies have been reported elsewhere. 

In the next stage of this work we studied effects of intercalation on low-temperature photoluminescence spectra of fullerite Ceo single crystal. Figure 3 shows photoluminescence spectra, normalized to integrated intensity, for pure fullerite and fullerite with helium impurities taken at 5 K, as well as a difference between the two. 

Figure 3. Normalized to their integral intensities photoluminescence spectra of C6o single crystal at 5 K under excitation of light with energy of 2.84 eV a) pure fullerite C60 (b) helium-intercalated fullerite C6oi (c) differed spectrum. The (a) and (b) PL spectra were corrected for instrumental response. The recording spectrometer slit width was 2.6 nm.

The accepted nomenclature is used in the work. An individual fullerene molecule is referred to as "fullerene". A crystal from fullerene molecules is referred as "fullerite". 

Influence of EF on magnitude of photoconductivity of fullerene C6o single crystal in a weak MF can be explained in the following way. Increasing intensity of electric field causes the increase of radius of initial distance r0 between the components of electron-hole pairs and, consequently, the decrease of probability of geminate recombination. As a result, AI rises at small values of EF. At higher values of EF the probability of dissociation of pairs in states with uncorrelated spins increases, that causes nonlinear behavior of electrofield dependences of photoconductivity of fullerite C6o in MF. The distance between components of electron-hole pairs in states with uncorrelated spins was estimated as R>3.4 nm. 

The first report of the existence of fullerenes in 1985 [1], and the subsquent discovery in 1990 of a method to produce them in macroscopic amounts [2], paved the way to a new era of carbon science that involves curved surfaces on the nanoscopic scale. As is well known, the aggregation of fuUerene molecules at moderate temperatures and pressures leads to molecular sohds termed ful-lerites. The (buckminsterfuUerene) and Cyg fullerenes and the corresponding fullerites are the easiest to produce, and for this reason they have been the subject of most experimental works. Certain aspects of the solid-state science of fullerenes (e.g., crystal structures, phase transitions, formation of exo- and... 

Solid fullerene displays interesting properties. After condensation, the Cgo molecules forms a face centered cubic (f.c.c.) structure fullerite. This is the only material which consists of quasi-spherical molecules, all atoms of which are of one kind. X-ray dispersion experiments show that fullerite forms a closely packed f.c.c. crystal in which the distance between the nearest molecules is 10.04 A [2]. The least distance between two molecular surfaces is 2.9 A, and the distance between the nearest atoms in a crystal is 1.42 A. Thus, the experiments specify that the molecular structure of Ceo is preserved in the solid. Strong orientational disorder is observed at room temperature [64] and this disorder decreases as the temperatures decreases. 

Doped fullerites are called fullerides. The doping process proceeds by intercalating electroactive atoms or molecules into the crystal lattice in a very similar way as it is well known for the conducting polymers. Sofar only strong donors like alkali metals or earth alkali metals were found to dope the fullerites if the latter are exposed to the vapor of the metal. Doping induced conductivity has been observed for various metals and various fullerenes but metallic behavior and superconductivity was found sofar only for Ceo- Table 1 compiles a selected number of doped systems together with some of their transport properties. 

Fullerene C70 does not form a complex with the same calixarene in toluene, but it does so in benzene, crystallizing as the 2 1 complex, (C7o)2(p-Bu -calix[8]arene) (structme unknown).The same fullerene also forms a 2 1 complex with / -Bu -calix[6]arene, and its precipitation from toluene solutions can be used to retrieve 87% purity C70 from Ceo-depleted fullerite. 

Kratschmer and Huffman called the soot Fullerite , but they were always aware that to have a new allotrope of carbon it was necessary to isolate the actual crystal form. By subliming the soot and extracting the sublimate with benzene, Fostiropoulos obtained orange-brown crystals, which could be in the form of hexagonal rods, platelets, or starshaped flakes. Lowell Lamb, working for Huffman at Tucson, confirmed the UV spectra. They published their results [76], but the definitive X-ray diffraction of a single crystal had still not been achieved, or alternatively, a single line NMR spectrum would have been sufficient to show that Ceo did, indeed, have a soccer ball structure. 

This procedure also produces less stable carbon clusters, such as C70. Buckminsterfullerene can be produced more conveniently using an electric arc between graphite electrodes in an inert gas. The allotrope is soluble in benzene, from which it can be crystallized to give yellow crystals. This solid form is known as fullerite. 

Molecular Crystals Fullerites. Organic crystals are usually prone to ionization damage and decompose very rapidly under electron irradiation they can thus be studied for only a short time (a few. seconds) and only with a very low electron beam intensity. Transmission electron microscopy has, therefore, seldom been applied to organic crystals. However, the all-carbon molecules Qo, C70. etc., (fullerenes) discovered at the end of the 1980s resist electron radiation fairly well. Early structural studies on the crystalline phases of ftil-lerenes (fullerites) were performed mainly by electron microscopy because only small quantities of sufficiently pure material were available. At room... 

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