R. K. Nikolaev

THE ELASTIC STIFFNESS MATRIX OF SINGLE-CRYSTAL C60

Abstract The elastic stiffness matrix of the fcc phase of solid C 60 has been determined experimentally on the basis of sound velocity measurements in monocrystalline specimens of different crystallographic orientations. The following values of elastic moduli were obtained: C 11 = 1.49 × 10 10 N/m 2 , C 12 = 0.88 × 10 10 N/m 2 and C 44 = 0.66 × 10 10 N/m 2 . The results are compared with theoretical estimations of elastic moduli and previous experimental data on the elastic properties of solid C 60 .

Elastic moduli of single-crystal C60

The matrix of elastic constants of the fcc phase of solid C60 has been determined experimentally from measurements of the the velocity of 5 MHz ultrasound in single-crystal samples with different crystallographic orientations. The following values were obtained for the elastic moduli: C11=14.9±0.9 GPa, C12=8.8±1.0 GPa, and C44=6.6±0.18 GPa. The results are compared with theoretical estimates of the elastic moduli and data obtained in previous measurements of the elastic characteristics of solid C60.

Oxygen-iodine isomorphism in the Y-Ba-Cu-O ceramics

Abstract The possibility of oxygen substitution by iodine in the interbarium planes of the type Cu(1)-O in the Y-Ba-Cu-O ceramics has been proved by X-ray diffractometry and Mossbauer spectroscopy methods.

Integral and Local Elastic Properties of C60 Single Crystals

Elastie properties of the f.c.c. phase of solid C 60 have been determined experimentally on the base of sound velocity measurements in single crystalline specimens of different crystallographic orientations. Integral elastic properties were measured by the echo-pulse method at 5 MHz and local elastic parameters were measured by the acoustic microscopy technique. The local values obtained have confirmed with a good accuracy the results from measurements of integral elastic properties in single crystals of solid C 60 . From these experimental data the magnitudes of elastic stiffncss constants for the f.c.c. phase of solid C 60 were calculated. The results obtained are in good agreement with the theoretical estimations of elastic moduli made in terms of existing models of intermolecular interaction in C 60 .

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 λp, 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 λp∼d=1.4 nm at temperatures T>200 K and reaches values λp∼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.

Nonmonotonic variation of the electrical conductivity of C60 fullerene crystals dynamically compressed to 300 kbar as evidence of anomalously strong reduction of the energy barrier of C60 polymerization at high pressures

The electrical conductivity of a C60 fullerene crystal is measured under the conditions of quasi-isentropic loading by a diffuse shock wave to a pressure of 30 GPa at room temperature. Nonmonotonic behavior of the conductivity of the samples with an increase in pressure is observed: first, conductivity increases by several orders of magnitude and, then, decreases rapidly. An increase in conductivity is explained by a decrease in the band gap under pressure, whereas a decrease in conductivity may be explained under the assumption that the energy barrier of C60 polymerization decreases similarly to the band gap with an increase in pressure. As a result, the rate of C60 polymerization at high pressures increases considerably (by more than seven orders of magnitude).

The temperature dependences of the elastic moduli of solid C60

This paper reports on the determination of the temperature dependences of the complete set of the elastic moduli of solid C60 from sound-velocity measurements made along different crystallographic directions in single-crystal samples within the 100-to 300-K range. Substantial differences in their behavior were revealed, which are accounted for by different relative contributions from relaxation processes to various elastic moduli.

Effect of magnetic field on the microplastic strain rate for C60 single crystals

Microplastic deformation in a magnetic field and in a zero field, as well as after preliminary action of a magnetic field on C60 crystals, is studied with the help of a laser interferometer, which makes it possible to measure the strain rate on the basis of linear displacements of 0.15 µm. It is shown that the introduction of a sample into the field and its removal from a field of 0.2 T directly during sample deformation lead to a change in the strain rate, the decrease in the rate being accompanied by a brief interruption of deformation. The sign of the effect depends on temperature: the magnetic field accelerates deformation at room temperature and slows it down at 100 K. Preliminary holding of a sample in a field of 0.2 or 2 T produces a similar effect on the strain rate. Possible reasons for the observed manifestations of the magnetoplastic effect in C60 and the relation between the sign of the effect and the phase transition at 260 K are considered.

Conductivity of C60 fullerene crystals under dynamic compression up to 200 kbar

The conductivity σ of C60 fullerene crystals is measured under quasi-isentropic loading by a spread shock wave to a pressure of 200 kbar at the initial temperatures 293 and 77 K. A sharp increase in σ by seven to eight orders of magnitude is detected: from 10−6−10−7 Ω−1 cm−1 at normal conditions to 5 Ω−1 cm−1 under pressure from 100 to 200 kbar. The conductivity of samples under pressure decreases with decreasing temperature, which is characteristic of semiconductors. On pressure release, σ regains its initial value.

Long-lived excited states and photoluminescence excitation spectra in single crystals of fullerene C60

Photoluminescence, optical absorption spectra, and photoluminescence excitation spectra were measured on large (2–3 mm), very pure crystals of fullerene C60 at 5 K. It is shown that the main contribution to the photoluminescence of these crystals is from singlet and triplet excitons captured on crystal defects. The concentration of these defects does not exceed 1018 cm−23, and the lifetime of triplet excitons on these defects is about 3 ms. It is shown that the symmetry distortion of the C60 molecules at the defects is rather large and causes the oscillator strength of the zero-phonon optical transitions to be comparable to the most intense optical transitions with the participation of intramolecular vibrations.