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)     

Copper-oxygen substructures in carbon allotropes (graphite and fullerites)

A comparative study of crystalline graphite and copper-doped fullerite membranes is reported. It is assumed that C₆₀ clusters form complexes with oxygen and copper similar to those known to exist in graphite. Above room temperature, these complexes, first, change the symmetry of the fullerite lattice and, second, are responsible for the nonmonotonic temperature dependence of the electrical resistance. Fiz. Tverd. Tela (St. Petersburg) 41, 748–750 (April 1999) Deceased.     

Electronic structure of C60 fullerite

The complete systems of fundamental optical functions of single crystals and polycrystalline films of fullerite (C₆₀) are calculated on the basis of known reflection spectra and the imaginary and real parts of the permittivity. The integrated permittivity spectra are decomposed into elementary components. The three basic parameters of each component (the energy of the maximum, the half-width, and the oscillator strength) are determined. The nature of these components of the permittivity is discussed on the basis of existing theoretical calculations of fullerite bands. Fiz. Tverd. Tela (St. Petersburg) 41, 1124–1125 (June 1999)     

Observation of the two-photon photoelectric effect in low-intensity optical fields during photostimulated fractal aggregation of colloidal silver

The two-photon photoelectric effect enhanced by the characteristic local fields of fractal nanostructures is observed in optical fields with intensity of the order of 10⁻³ W/cm². Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 97–100 (25 July 1997)     

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)     

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)     

Electronic structure of C60 films

A complete set of fundamental optical functions of fullerite (C₆₀) films in energy ranges of 2.5–5.0 and 4.0–9.0 eV is calculated using the known spectra of the imaginary and real parts of the dielectric constant. An integrated spectrum of the dielectric constant is decomposed into elementary components. Three basic parameters of each component (the maximum and halfwidth energies and oscillator strength) are determined. Based on the known theoretical calculations of fullerite zones, a scheme of the nature of these components of the dielectric constant is suggested. Udmurt State University, 71, Krasnogeroiskaya Str., Izhevsk, 426034, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 227–232, March–April, 1999.     

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)     

Permutation-inversion symmetry of C70 fullerite in the high-temperature phase

The permutation-inversion symmetry group of C₇₀ fullerite in its high-temperature phase is constructed with allowance for the rotation of its constituent molecules, and the local symmetry group of a rotating molecule in the crystal is identified. Irreducible representations of these groups are constructed that are compatible with the principle of wave-function symmetry with respect to permutations of identical nuclei. A group-theoretic classification is made of the quantum states of a rotating molecule and of the crystal in the high-temperature phase of C₇₀ fullerite. Selection rules are derived for electronic, vibrational, and rotational spectra in terms of irreducible representations of the permutation-inversion symmetry group of the crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1895–1901 (October 1997)     

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)     

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)     

Fractal structure of ultradisperse-diamond clusters

Ultradisperse-diamond clusters are shown to be fractal objects, and the character of variation of the fractal dimension in the course of the diamond-graphite phase transition under annealing in an inert atmosphere is studied. Fiz. Tverd. Tela (St. Petersburg) 40, 776–780 (April 1998)     

Formation and magnetic properties of fractal aggregates of cobalt

Macroscopic fractal aggregates of cobalt are obtained by thermal evaporation of cobalt metal in an argon atmosphere and subsequent deposition on a silicon substrate heated to 1000 K. It is established that the fractal structure is formed by diffusion-limited aggregation of cobalt particles. The macroscopic fractal cobalt aggregates are ferromagnetic. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 8, 556–558 (25 October 1997)     

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)     

Study of cluster materials obtained by selective laser photochemistry

This paper presents experimental results from the study of the characteristics of cluster materials obtained by resonance laser photochemistry. The effect of the redox reagents used in the photoprocessing on the size and shape of cluster structures of gold has been studied. The minimum size of the structures, measured by electron microscopy, is 30–50 nm. When organic acids are used as redox reagents, fractal structures formed from flat scales are synthesized. Redox reagents based on alcohols form cluster structures of fractal type consisting of spherical clusters. Fiz. Tverd. Tela (St. Petersburg) 40, 2236–2238 (December 1998)     

Synthesis, structure, and dielectric properties of KH2PO4 fractal aggregates

Macroscopic fractal aggregates of KH₂PH₄ (KDP) measuring up to 500 μm have been obtained. The fractal structure forms as a result of the precipitation of KDP particles from a supersaturated aqueous solution in the presence of a temperature gradient followed by a diffusioncontrolled mechanism of aggregation. The electron-microscopic analysis performed has shown that the fractals are formed predominantly from crystallites of the tetragonal modification measuring ∼1 μm. The dielectric constant (ɛ) of fractal KH₂PO₄ has been measured in the temperature range 80–300 K. A characteristic anomaly has been discovered on the ɛ(T) curve in the vicinity of 122 K, which attests to a ferroelectric phase transition. The absolute value of ɛ is significantly smaller than the components ɛ ₁₁ and ɛ ₃₃ for KH₂PO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 2059–2061 (November 1999)     

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.     

Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C₇₀ at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Grüneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the d ω/dP value for certain phonon modes abruptly changes at pressures of P ₁≈2 GPa and P ₂≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C₇₀ crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C₇₀, which is about 2.1±0.1 eV. Zh. éksp. Teor. Fiz. 111, 262–273 (January 1997)     

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)     

Structure, internal friction, and Young’s modulus of fractal carbon deposits

We have observed that carbon deposits obtained by atomizing graphite in an electric arc in an atmosphere of argon and helium exhibit porosity and possess fractal structure. The results of measurements of the internal friction Q ⁻¹ and the effective Young’s modulus E as a function of temperature are presented. Data on the resistivity, density, and microhardness of the carbon deposits are presented. A possible mechanism for formation of fractal structures in carbon deposits is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 584–586 (March 1998)