Yu. A. Osip’yan

Structural features of the polymer-like phase of AC60 fullerides

A new approach is proposed for describing the deformation of the C60 molecule in the polymer-like phase of the fullerides AC60 (A=K, Rb). A discrete basis of orientational states of the molecule is introduced, and orientational ordering is described on the basis of a phenomenological theory. A symmetry analysis of the molecular vibrations in the Fm3m phase is performed. It is shown that small rotations of the C60 can occur at a phase transition.

Effect of a Magnetic Field on the Starting Stress and Mobility of Individual Dislocations in Silicon

The strong effect of a magnetic field on the starting stress and mobility of individual dislocations is discovered in silicon grown by the Czochralski method with a high concentration of dissolved oxygen. It is shown that exposure of dislocations preliminarily introduced into the sample to a magnetic field considerably reduces the starting stresses for the motion of these dislocations. The effect is not observed in samples with a low oxygen concentration. It is assumed that the magnetic field induces singlet-triplet transitions in thermally excited states of silicon-oxygen complexes in the dislocation core, thus stimulating a change in the state (atomic configuration) of oxygen already located at dislocations. As a result, the mean binding energy of oxygen with a dislocation decreases.

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.

Observation of standing x-rays in Bragg diffraction on high-T c superconducting crystals Nd1.85Ce0.15CuO4−δ

Curves of the secondary-fluorescence yield from the constituent elements of high-Tc superconducting crystals Nd1.85Ce0.15CuO4−δ are measured in the dynamic x-ray diffraction regime. The form of the curves attests to the appearance of a standing x-ray wave in the crystal.

Phase transitions and the structure of the C60 crystal doped with lithium by electrodiffusion

The structure and phase transitions of C60 crystals doped with lithium by injecting metal ions from the superionic crystal-C60 single crystal heterojunction under electrodiffusion conditions are reported. The sample experienced irreversible transitions resulting in the virtually complete disappearance of EPR signals and MW conduction in the temperature range 320–370 K. In this temperature interval, a new C60 phase was formed; the phase contained polymeric chains of C60 molecules along the crystallographic c axis and lithium clusters. The structure of this phase was determined. Annealing at 620 K restored the EPR signal and, according to the X-ray data, the initial cubic structure of pure C60. The X-ray pattern, however, contained additional diffraction peaks, which was evidence of the presence of one more phase with a structure yet unknown.

Orientational states of C60 molecule in crystals

Local symmetry of orientational states of the C60 molecule in crystals has been investigated. It was shown that the various orientational phase transitions in different crystals are related to different orientational orbits. The model of orientational phase transitions based on a sequence of orientational states with different symmetry properties has been suggested. We have found that both the local symmetry of C60 molecule and the symmetry of its internal vibrations become higher after a reduction of crystal spatial symmetry at the phase transition. This effect is fairly common and can be observed in the orientational order-disorder phase transitions with wave vectors at the Brillouin zone boundary. Feasible manifestations of the predicted effect in various experiments are discussed.

Cleavage of C60 fullerite crystals

It is found that, under certain conditions, C60 fullerite crystals can be cleaved along cleavage planes that are close-packed planes of the {111} type. Rigid gas-phase grown crystals exhibit good cleavage properties. In experiments with active compressive deformation, these crystals showed a high yield point τy = 2.65 MPa, a “parabolic” stress-strain curve, and brittle fracture after attaining a shear strain of about 8%. The fracture surface was clearly seen to have fragments parallel to the (111) plane. Typical microstructures observed in the cleavage plane are discussed: crystallographic cleavage steps, an indentation pattern, and a dislocation prick rosette. The fact that the activation volume V ≃ 60b3 is small (b is the Burgers vector of a dislocation) and strain-independent indicates the Peierls character of fullerite deformation or dislocation drag in a dense network of local defects.

Features of the optical absorption of crystals of the fullerene C60 in the region of the orientational phase transition

The absorption coefficient of perfect single crystals of the fullerene C60 is measured in the energy range 1.6–2.1 eV at temperatures from 4.2 to 300 K. An absorption fine structure is discovered in the and is assigned to electronic and vibronic transitions with the production of free excitons and excitons localized on structural defects. It is shown that in the region of the structural phase transition from a face-centered cubic structure to a simple cubic structure the absorption coefficient undergoes a jump, which is associated with an energy shift of the free exciton line toward lower energies. It is discovered that spatial inhomogeneity, which is associated with the growth of the new phase from a finite number of nuclei, appears in the crystal at the time of this transition.

CONDUCTIVITY OF C60 FULLERENE CRYSTALS UNDER MULTI-STEP DYNAMIC COMPRESSION UP TO 300 KBAR

In the present work the conductivity of fullerene C60 crystals has been measured under smooth shock wave quasi‐isentropic loading conditions up to 30 GPa at initial temperature T=293 K. Not monotone behavior of conductivity has been revealed under compression of crystal with pressure increasing: ‐at first conductivity grows by many orders then it falls very fast. Conductivity increasing is explained by decreasing of bandgap of C60 under compression whereas conductivity decreasing can be explained on the assumption that the energy barrier of polymerization of C60 reduces with pressure increasing approximately in the same measure, as band‐gap energy.