Neutron diffraction study of the interaction of iron with amorphous fullerite

The amorphous fullerite C₆₀ has been prepared by mechanical activation (grinding in a ball mill), and its interaction with iron during sintering of powders with 0–95 at % Fe has been studied. After sintering in the range 800–1200°C under a pressure of 70 MPa, the samples have nonequilibrium structures different from the structures of both annealed and quenched steels. In this case, the carbon phase, i.e., amorphous fullerite, undergoes a polyamorphous transition to amorphous graphite. It has also been shown that the interaction of amorphous fullerite with iron is weaker compared to crystalline fullerite or crystalline graphite.     

Mechanism of the formation of a diamond nanocomposite during transformations of C60 fullerite at high pressure

The results of an investigation of the transformation of C₆₀ fullerite to diamond under pressure through intermediate three-dimensionally polymerized and amorphous phases are reported. It is found that treatment of fullerite C₆₀ at pressures 12–14 GPa and temperatures ∼1400°C produces a nanocrystalline graphite-diamond composite with a concentration of the diamond component exceeding 50%. At lower temperatures (700–1200°C) nanocomposites consisting of diamondlike (sp ³) and graphitic (sp ²) amorphous phases are formed. The nanocomposites obtained have extremely high mechanical characteristics: hardness comparable to that of best diamond single crystals and fracture resistance two times greater than that of diamond. Mechanisms leading to the transformation of C₆₀ fullerite into diamond-based nanocomposites and the reasons for the high mechanical characteristics of these nanocomposites are discussed. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 822–827 (10 June 1999)     

Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C₆₀ at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С₆₀ molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.     

Mechanism of three-dimensional polymerization of fullerite C60 at high pressures

A series of polycrystalline phases corresponding to different stages of three-dimensional polymerization and destruction of C₆₀ molecules has been synthesized by heating fullerite C₆₀ under a pressure P=12.5 GPa. The structure of the phases can be identified as fcc, and the lattice period decreases with increasing heating temperature. A model of three-dimensional polymerization in which the lattice parameter is a continuous function of the fraction of covalently bonded molecules is proposed. The model makes it possible to estimate the number of atoms in the sp ³ state. The hardness of the polymerized fcc phases is studied on the basis of percolation of rigidity. It is shown experimentally that the period a≈13.8 Å is the threshold for the formation of a three-dimensionally rigid C₆₀ polymer. It is found that the thermal stability of the strongly and weakly polymerized phases is qualitatively different. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 755–759 (10 December 1996)     

Electroconductivity and pressure–temperature states of step shocked C60 fullerite

A study of electrophysical and thermodynamic properties of C₆₀ single crystals under step shock loading has been carried out. The increase and the following reduction in specific electroconductivity of C₆₀ fullerite single crystals at step shock compression up to pressure 30 GPa have been measured. The equations of state for face centred cubic (fcc) C₆₀ fullerite as well as for two-dimensional polymer C₆₀ and for three-dimensional polymer C₆₀ (3D-C₆₀) were constructed. The pressure–temperature states of C₆₀ fullerite were calculated at step shock compression up to pressure 30 GPa and temperature 550 K. The X-ray diffraction studies of shock-recovered samples reveal a mixture of fcc C₆₀ and a X-ray amorphous component of fullerite C₆₀. The start of the formation of the X-ray amorphous component occurs at a pressure P _m≈ 19.8 GPa and a temperature T _m≈ 520 K. At pressures exceeding P _m and temperatures exceeding T _m, the shock compressed fullerite consist of a two-phase mixture of fcc C₆₀ fullerite and an X-ray amorphous component presumably consisting of the nucleators of polymer 3D-C₆₀ fullerite. The decrease in electroconductivity of fullerite can be explained by the percolation effect caused by the change of pressure, size and number of polymeric phase nuclei.     

Superparamagnetism of magnetite particles in C<Subscript>60</Subscript> fullerite powder

A new magnetic material, C₆₀ fullerite powder doped by magnetite (Fe₃O₄) nanoparticles, is obtained by heating a mixture of fullerite and iron(III) acetylacetonate. It is shown that the material offers superparamagnetic properties. Surface bonding between the nanoparticles and the fullerite is established.     

Mechanical properties,lattice thermal conductivity,infrared and Raman spectrum of the fullerite C24

Lightweight carbon materials with excellent thermal and mechanical properties have important applications in aerospace industry. In this study, the stability, mechanical properties, lattice thermal conductivity, electronic structure, infrared and Raman spectrum of sp³ hybridized low-density fullerite C₂₄ were investigated according to density functional theory (DFT) calculations. It was found that the fullerite C₂₄ was both thermodynamic and dynamic stable. Quasi-harmonic approximation and Grüneisen parameter calculations clarified why the fullerite C₂₄ had a positive thermal expansion coefficient at low temperature. The fullerite C₂₄ also exhibited excellent mechanical properties. Interestingly, the Vickers hardness of carbon allotropes was found to almost be linear proportional to the density of a carbon material. HSE06 electronic structure calculations showed that it was a semiconductor with direct bandgap of 2.56 eV. Anharmonic lattice dynamic calculations showed that its thermal conductivity was higher than semiconductor silicon. Besides, Raman and infrared active modes as well as the corresponding spectra were presented.     

Pressure- and temperature-induced transformations in crystalline polymers of C60

The great advantage of the C₆₀ molecule is its potential for polymerization, due to which the molecule can be the building block of new all carbon materials. In addition, it contains, both sp ² and sp ³ hybridized carbon atoms, which allows synthesizing new carbon materials with desired physicochemical properties using both types of carbon bonding. The one- and two-dimensional polymeric phases of C₆₀ are prototype materials of this sort. Their properties, especially polymerization under pressure and room temperature via covalent bonding between molecules belonging to adjacent polymeric chains or polymeric layers, can be used for further development of new materials. The present review focuses on the study of the pressure-induced polymerization and thermodynamic stability of these materials and their recovered new phases by in-situ high-pressure Raman and X-ray diffraction studies. The phonon spectra show that the fullerene molecular cage in the high-pressure phases is preserved, while these polymers decompose under heat treatment into the initial fullerene C₆₀ monomer.     

Electronic structure and elastic moduli of the simple cubic fullerite C24—A new allotropic carbon modification

The energy band structure, equation of state, density of states, and elastic moduli of a new allotropic carbon modification, namely, fullerite C₂₄ with a simple cubic lattice (known previously as cubic graphite), are calculated by the full-potential linearized augmented-plane-wave (FLAPW) method with geometry optimization for the first time. The dependence of the total energy on the lattice constant exhibits a minimum for a ₀ = 0.60546 nm. In this case, the lengths of the C-C bonds between fullerene molecules, the lengths of the 6,6-bonds shared by hexagons, and the lengths of the 4,6-bonds shared by a square and a hexagon are equal to 0.1614, 0.1503, and 0.1637 nm, respectively. An analysis of the energy band structure and the density of states demonstrates that the simple cubic fullerite C₂₄ is a direct-band-gap insulator or a semiconductor with a band gap of 1.6 eV. The calculated bulk modulus B ₀ = 196 GPa and the elastic moduli C ₁₁ = 338 GPa, C ₁₂ = 139 GPa, and C ₄₄ = 30 GPa indicate that the fullerite under investigation is a mechanically stable material. The inference is made that the simple cubic fullerite C₂₄ is a new diamond-like molecular zeolite with a unique combination of properties, such as the porosity and nonpolarizability, on the one hand, and the mechanical strength, chemical inertness, and high thermal conductivity, on the other hand. The simple cubic fullerite C₂₄ can be considered a promising low-dielectric-constant (low-k) material (?₀ < 5.7) for use in fabricating interconnections and substrates intended for integrated circuits and nanoelectronics.     

Pressure-induced dimerization kinetics of fullerene C60

Dimerization kinetics was studied for fullerene C₆₀ by IR spectroscopy at a pressure of 1.5 GPa in the temperature range 373–473 K. The kinetic curves for the formation of a dimer (C₆₀)₂ were obtained using its analytical IR band at 796 cm^?1. Under the assumption that pressure-induced C₆₀ dimerization is a second-order irreversible reaction, the reaction rate constants were determined at different temperatures. The corresponding activation energy and preexponential factor were found to be 134±6 kJ/mol and (1.74± 0.24)×10¹⁴ s^?1, respectively. The specific features of the solid-phase C₆₀ dimerization in simple cubic and face-centered cubic fullerite phases are discussed.     

Imaging C60 fullerite at 4.5 K by scanning tunneling microscopy

Intramolecular contrast of C₆₀ molecules has been observed by scanning tunneling microscopy at 4.5 K on a C₆₀ fullerite thin film sample. This result provides strong evidence for the freezing of the rotational motion of C₆₀ molecules at low temperature as recently proposed by refinement analysis of neutron diffraction data. Different intramolecular patterns are observed. Interpretation of these patterns is suggested by assigning them to carbon ring structures of the C₆₀ molecules in various orientations.     

The influence of ultraweak ionizing irradiation on the magnetoplastic effect in single crystals of a C<Subscript>60</Subscript> fullerite

The suppression of plasticity sensitivity of a C₆₀ fullerite to the action of a magnetic field is revealed. It is found that the C₆₀ fullerite undergoes temporary softening due to irradiation with ultralow (<0.1 cGy) doses of β and γ radiation.     

Spectroscopic characterization of pressure modified C60

60 are reported. The material associated with the mixture of rhombohedral and tetragonal phases was synthesized under pressure of when treatment temperature was only a little lower than the high-temperature limit of C₆₀ stability. The substance exhibits very rich infrared and Raman spectra as well as luminescence spectra of an unusual shape. They show that vibrational transitions in the prepared carbon architecture substantially deviate from icosahedral symmetry of buckyballs and exhibit a similarity with lattice phonons in graphite. It may originate from mixing of C₆₀ modes and vibrations of a layer structure on deformed and weakly bound fullerene molecules. The luminescence spectra reveal three distinct electronic states located below . One of them responsible for the emission peak at is very characteristic for the pressure modified fullerene. The data should be useful for the accurate determination of structural changes in C₆₀ induced by pressure. Received: 6 September 1996/Accepted: 10 October 1996     

Conducting properties of planar irradiated and pristine silicon–fullerite–metal structures

Au/C₆₀/p-Si sandwich structures can be easily obtained by evaporation of a thin fullerite (C₆₀) film on a silicon substrate and a thin Au film on top of the C₆₀ film. In this case a C₆₀/p-Si p–n heterojunction appears. Both the dark and photoconductivities of the planar pristine and irradiated Au/C₆₀/p-Si structures were measured as a function of the irradiation fluence. Furthermore, the pressure dependence of these structures was determined. A strong dependence on the irradiation damage was found. PACS 81.05.Tp; 72.80.-r; 72.80.Rj; 72.40.+w; 61.82.Rx     

Mechanical, structural, and spectroscopic properties of C70 fullerite phases produced under high pressure and shear

X-ray diffraction patterns, Raman spectra, and the hardness of C₇₀ fullerite subjected to a high pressure with shear are investigated. It is shown that these conditions favor the phase transformation of molecular fullerite into the hard amorphous phase. The hardness of a specimen removed from a diamond anvil cell loaded up to 26 GPa under shear deformation applied is found to be equal to 30 GPa.     

Correlations between the physical properties of the carbon phases obtained at a high pressure from C<Subscript>60</Subscript> fullerite

Correlations between the density, elastic properties, and hardness of the carbon phases prepared from C₆₀ at a high pressure are studied. By varying the high pressure and temperature properly, one can obtain from C₆₀ a broad class of ordered polymerized and disordered phases, for which the fraction of the sp² and sp³ states, the characteristic dimensionality of the structure, the degree of covalent bonding, etc. can be varied successfully. 3D-bonded carbon structures are shown to exhibit a clearly pronounced correlation between the hardness or bulk modulus and the density, with these correlations also apparently applying to the carbon phases in a general case. At the same time, the mechanical characteristics of structures with a lower dimension covalent bonding are worse than those of 3D-bonded phases with similar values of the density.     

Pressure-induced polycondensation of C60 fullerene

Modifications of carbon which are formed from C₆₀ fullerite at pressures up to 10.0 GPa and temperatures up to 1900 K are studied by x-ray diffraction, Raman spectroscopy, and atomic force microscopy methods. The pressures p and temperatures T at which atomic, molecular, and polymolecular structures form under conditions of quasihydrostatic compression are determined. It is shown that, together with polymerization, another type of chemical interaction of the molecules, called polycondensation, which leads to the formation of polymolecular structures with a shortest intermolecular distance of 0.65 nm, is possible in the system. Three-dimensional polycondensation of C₆₀ fullerene is explained by the special properties of the new carbon states. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 10, 778–783 (25 May 1996) The spelling of the authors names are presented here in English as requested by the Russian Editorial office.     

Dielectric and optical properties of C60 material studied by ellipsometry and quantitative IR and UV/VIS spectroscopy

Optical properties in the spectral range from 0.06 to 5.5 eV of fullerite films on different substrates, C₆₀ powder, and dissolved fullerene material are investigated by spectroscopic ellipsometry and optical transmission and reflection measurements. Depolarization effects are taken into account during determination of the dielectric function of fullerite films by ellipsometry. The optical gap for C₆₀ films is found to be 1.63 eV. Three optical absorption bands are observed at 2.69, 3.53, and 4.49 eV.The dielectric function in the infrared shows the four characteristic infrared vibrational modes. The interference pattern seen in UV/VIS reflection measurements are used for high-precision thickness determination of the films. The Clausius-Mossotti formula is successfully applied to reproduce the experimental optical data measured in C₆₀/dichlormethane solutions. Deviations between theory and experiments provide interesting information about the intermolecular interaction of the C₆₀ molecules. A tentative interpretation of the measured absorption bands is presented.     

Properties of the microhardness of single-crystal C60 fullerite in sclerometric tests

The mechanical properties of single-crystal fcc C₆₀ fullerite are investigated by sclerometry and precision contact profilometry. Quantitative estimates are obtained for the microhardness anisotropy on the (100) and (111) planes. Polarity of the mechanical properties is observed in the (111) plane. The mechanisms considered for the orientational deformation of C₆₀ single crystals by a moving indentor confirm existing data showing that plastic deformation in solid C₆₀ occurs along the [011] (111) systems. Fiz. Tverd. Tela (St. Petersburg) 41, 1119–1123 (June 1999)