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)     

Elastic properties of ultrahard fullerites

The elastic properties of C₆₀ fullerite samples synthesized under pressure P=13.0 GPa at high temperatures were investigated using acoustic microscopy. The velocities of longitudinal (c _L=17–26 km/s) and transverse (c _T=7.2–9.6 km/s) elastic waves in the samples were measured. It was established that the longitudinal sound velocity of ultrahard fullerites is higher than that of any other known solid. The bulk modulus of these ultrahard samples is higher than that of diamond and reaches a value greater than 1 TPa. The high bulk modulus, the relatively large shear moduli, and the substantial Poisson ratio indicate that the structure of the ultrahard fullerites is fundamentally different from that of diamond. Zh. éksp. Teor. Fiz. 114, 1365–1374 (October 1998)     

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.     

Interaction of C60 molecules with a (100) Mo surface

The adsorption, initial stages of film growth, and transformation of an adlayer of C₆₀ molecules on a (100) Mo surface upon heating are studied under ultrahigh-vacuum conditions. It is shown that the C₆₀ molecules remain intact in the adsorbed state up to 760 K. Layer-by-layer growth of a fullerite film is observed at room temperature, while tower-shaped crystallites grow up from a loosely packed monolayer with an approximate concentration of C₆₀ molecules equal to (1.3±0.2)×10¹⁴ molecules · cm⁻² at 500–600 K. In the latter case the percentage of the surface occupied by them depends on the temperature and the impinging flux density of fullerene molecules, but after a certain stationary value has been achieved, it scarcely depends on the exposure time. Zh. Tekh. Fiz. 69, 117–122 (November 1999)     

On the effective Debye temperatures of the C60 fullerite

The effective Debye temperatures Θ_eff determined for solids by different physical methods have been analyzed and compared. Attention has been focused on the original parameter of the Debye theory of heat capacity, i.e., the translational calorimetric Debye temperature Θ _c ^t (0), and the X-ray Debye temperature Θ _x in the framework of the Debye-Waller theory for the C₆₀ fullerite. It has been established that the true Debye law T ³ is satisfied for the C₆₀ fullerite over a very narrow range of temperatures: 0.4 K ≤ T ≤ 1.8 K. For this reason, the experimental Debye temperatures Θ _c ^t (0) obtained for the C₆₀ fullerite by different authors in the range T > 4.2 K are characterized by a large scatter (by a factor of ∼5). It has been revealed that the value Θ _c ^t (0) = 77.12 K calculated in this paper with the use of the six-term Betts formula from the harmonic elastic constants $ \tilde C_{ijkl} $ \tilde C_{ijkl} of the C₆₀ single crystal in the limit T = 0 K is closest to the true Debye temperature. It has been demonstrated using the method of equivalent moments that the real spectral frequency distribution of translational lattice vibrations g(ω) for the C₆₀ fullerite deviates from a parabolic distribution. The effective Debye temperatures Θ_eff involved in applied problems of thermodynamics of crystals and elastic scattering of different radiations from lattice vibrations have been determined. The quantitative measure of anharmonicity of translational and librational lattice vibrations of the C₆₀ fullerite has been determined. This has made it possible to empirically evaluate the lattice thermal conductivity κ of the C₆₀ fullerite at T ≈ 300 K: κ(300) = 0.80 W (m/K), which is in good agreement with the experimental thermal conductivity κ_exp = 0.78 W (m/K) at T ≈ 250 K.     

Ultraviolet Raman analysis of the formation of diamond from C60

The ultraviolet (257 nm) Raman spectrum of C₆₀ compressed to 30 GPa in a Mao–Bell diamond anvil cell with no pressure transmitting medium at ambient temperature indicates the formation of diamond after release of pressure. Previously, more extreme non-hydrostatic compression was reported to be required to form diamond from C₆₀. These results provide confirmation of the transformation of C₆₀ to diamond upon non-hydrostatic compression at room temperature and illustrate the utility of UV Raman spectroscopy for the analysis of carbon phases containing both sp² and sp³ bonding.     

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.     

Anomalies in the microwave conductivity of a polycrystalline C60 membrane

A study is reported of an anomaly in the microwave conductivity of a polycrystalline C₆₀ membrane at T _c=260 K (the transition width is 30 K). Raman scattering measurements indicate that the sample is the C₆₀ fullerite without any signs of graphitization, amorphous phase, or the presence of C₇₀, and that the detected microwave conductivity jump can be unambiguously identified as due to the C₆₀ phase. Fiz. Tverd. Tela (St. Petersburg) 40, 577–579 (March 1998)     

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C₆₀ crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C₆₀, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λ _p, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λ _p∼d=1.4 nm at temperatures T>200 K and reaches values λ_p∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 651–656 (25 April 1997)     

C60 fullerite with a “stretched” fcc lattice

It is shown that deuteration of C₆₀ fullerite followed by thermal decomposition of the resulting deuteride C₆₀D₂₄ leads to the formation of an fcc lattice with a ₀=14.52 Å in the final product, which according to the IR spectra consists mainly of C₆₀ fullerene molecules. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 239–242 (10 August 1998)     

Pressure-induced dimerization of C60 fullerene

The dimerization of C₆₀ fullerene under conditions of quasi-hydrostatic compression at temperatures above 293 K is investigated by IR spectroscopy, Raman scattering (RS) spectroscopy, and x-ray diffraction. The measured dimer (C₆₀)₂ content in the products of the polymerization of fullerite as a function of the pressure, temperature, and treatment time shows that dimerization occurs even at room temperature in the entire pressure range above ∼1.0 GPa. However, at least at temperatures above 400 K dimerization does not result in the formation of a dimer phase as a stable modification of the system, since the dimer is an intermediate product of the transformation. It is shown that increasing the holding time at 423 K decreases the content of the dimer fraction in the samples and results in the formation of linear (at 1.5 GPa) and two-dimensional (at 6.0 GPa) polymers, which are structure-forming elements of the orthorhombic and rhombohedral polymerized phases. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 881–886 (25 December 1998)     

Effect of nickel on the structure and bond type in amorphous diamond-like carbon films

Experimental data are presented from studies of the structure and bond type of carbon atoms in amorphous carbon-nickel films deposited from pulsed vacuum-arc discharge plasma sources. X-ray photoelectron spectroscopy was used. The characteristics of the plasmon loss spectra depend significantly on the deposition parameters. Carbon exists in a mixed sp²+sp³ hybridized state in the carbon–nickel films. The ratio of sp³/sp² carbon bonds increases when the nickel content is reduced (from 5.5 to 1.0 atomic %) and the deposition angle is increased. The structure closest to that of diamond was with a substrate bias voltage of –80 to –100 V and a deposition angle of 90°.     

Ac-conductivity and dielectric relaxations above glass transition temperature for parylene-C thin films

45% semi-crystalline parylene-C (–H₂C–C₆H₃Cl–CH₂–) _n thin films (5.8 μm) polymers have been investigated by broadband dielectric spectroscopy for temperatures above the glass transition (T _g =90°C). Good insulating properties of parylene-C were obtained until operating temperatures as high as 200°C. Thus, low-frequency conductivities from 10⁻¹⁵ to 10⁻¹² S/cm were obtained for temperatures varying from 90 to 185°C, respectively. This conductivity is at the origin of a significant increase in the dielectric constant at low frequency and at high temperature. As a consequence, Maxwell–Wagner–Sillars (MWS) polarization at the amorphous/crystalline interfaces is put in evidence with activation energy of 1.5 eV. Coupled TGA (Thermogravimetric analysis) and DTA (differential thermal analysis) revealed that the material is stable up to 400°C. This is particularly interesting to integrate this material for new applications as organic field effect transistors (OFETs). Electric conductivity measured at temperatures up to 200°C obeys to the well-known Jonscher law. The plateau observed in the low frequency part of this conductivity is temperature-dependent and follows Arrhenius behavior with activation energy of 0.97 eV (deep traps).     

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.     

Kinetics of the sorption of <Superscript>3</Superscript>He by C<Subscript>60</Subscript> fullerite. The quantum diffusion of <Superscript>3</Superscript>He and <Superscript>4</Superscript>He in fullerite

The kinetics of the sorption and subsequent desorption of gaseous ³He in a C₆₀ fullerite powder has been studied in the temperature range of 2–292 K. The temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite have been plotted using the measured characteristic times of filling of octahedral and tetrahedral interstices, as well as previous data. These temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite are qualitatively similar. A decrease in the temperature from 292 to 79 K is accompanied by a decrease in the diffusion coefficients, which corresponds to the dominance of the thermally activated diffusion of helium isotopes in fullerite. A further decrease in the temperature to 8–10 K leads to an increase in the diffusion coefficients by more than an order of magnitude. The diffusion coefficients of ³He and ⁴He are independent of the temperature below 8 K, indicating the tunnel character of the diffusion of helium in C₆₀ fullerite. The isotope effect is manifested in the difference between the absolute values of the diffusion coefficients of ³He and ⁴He atoms at the same temperatures.     

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.     

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)     

Electron beam-induced fullerite structure transformation

Auger spectra of thin fullerite (C₆₀) films have been measured under the conditions precluding their electrostatic charging and destruction. The Auger line of these subjects, E _f=268.3±0.2 eV, turned out to lie considerably lower in energy than that of the ion-beam amorphized graphite (E _AG=272.3±0.2 eV) and of pyrographite (E _PG=271.8±0.5 eV). Fullerite was found to convert to a graphitic form under irradiation by low-intensity electron beams used customarily in AES, reflection EELS, and inverse photoemission spectroscopy. It has been established that such beams produce noticeable changes in the fullerite structure already in a few minutes of irradiation. Fiz. Tverd. Tela (St. Petersburg) 39, 187–190 (January 1997)     

Amorphous boron nitride at high pressure

The pressure-induced phase transformation in hexagonal boron nitrite and amorphous boron nitrite is studied using ab initio molecular dynamics simulations. The hexagonal-to-wurtzite phase transformation is successfully reproduced in the simulation with a transformation mechanism similar to one suggested in experiment. Amorphous boron nitrite, on the other hand, gradually transforms to a high-density amorphous phase with the application of pressure. This phase transformation is irreversible because a densified amorphous state having both sp³ and sp² bonds is recovered upon pressure release. The high-density amorphous state mainly consists of sp³ bonds and its local structure is quite similar to recently proposed intermediate boron nitrite phases, in particular tetragonal structure (P4₂/mnm), rather than the known the wurtzite or cubic boron nitrite due to the existence of four membered rings and edge sharing connectivity. On the basis of this finding we propose that amorphous boron nitrite might be best candidate as a starting structure to synthesize the intermediate phase(s) at high pressure and temperature (probably below 800 °C) conditions.     

Boron–Carbon Nanocomposites Fabricated at High Pressures and Temperatures

Interaction of amorphous boron and C₆₀ fullerite is analyzed at pressures of 2.0 and 7.7. GPa and temperatures of 600–1800°C. Effect of pressure and temperature on the material structure is studied, temperatures for synthesis of boron carbide and diamond are found, and the sequence of transformations of the carbon component is determined. Ultrasonic method is used to measure elastic moduli of the samples, and the dependences of the moduli on the structure are analyzed. It is demonstrated that the boron–carbon nanocomposite synthesized at relatively low pressure (2.0 GPa) and temperature (about 1000°C) exhibits high elastic parameters (bulk modulus, B ≈ 75.3–84.0 GPa; Young modulus, E ≈ 108–119 GPa; and shear modulus, G ≈ 43–47 GPa at a density of about 2.2 g/cm³). The results can be used for development of novel nanocomposite materials.