S. S. Agafonov

Polyamorphic transition in amorphous fullerites

Samples of amorphous fullerites have been prepared by mechanoactivation (grinding in a ball mill), and their structure has been studied using neutron diffraction. It has been found that the amorphous fullerites subjected to high-temperature (600–1700°C) annealing undergo a polyamorphic transition from the molecular glass to the atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles.

Neutron studies of structural changes in amorphous C60 fullerite under temperature, pressure, and thermobaric effects

The structural behavior of amorphous fullerites obtained as a result of mechanical activation and thermal, baric, and thermobaric effects is studied via neutron diffraction. It is shown that the phase transition between the molecular crystal (fullerite) and atomic crystal (graphite) phases in the nanoscale state occurs through intermediate amorphous phases.

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.

Polyamorphous transition in amorphous fullerites C70

Samples of amorphous fullerites C70 have been obtained by mechanical activation (grinding in a ball mill). The structure of the samples has been investigated by neutron and X-ray diffraction. The high-temperature (up to 1200°C) annealing of amorphous fullerites revealed a polyamorphous transition from molecular to atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles. Possible structural versions of the high-temperature amorphous phase are discussed.

Short-range order in irradiated diamonds

Using the neutron-diffraction method, it has been established that, when density in irradiated diamonds varies, a transition from a diamond-like amorphous structure to a graphite-like structure occurs. The transition occurs at a density ρ ≈ 2.7–2.9 g/cm3 and is accompanied by a sharp change in resistivity.

X-Ray diffraction study of the microstructure of C60 fullerite powder after grinding in a planetary ball mill

The profiles of diffraction peaks of C60 fullerite were used to follow changes in the microstructure of C60 powder during grinding in a planetary ball mill at a rotation rate of 500 rpm and milling time of 8 h. The results demonstrate that grinding for just 1 h leads to a significant decrease in crystallite size. With increasing milling time, the average crystallite size varies little, whereas the lattice strain increases considerably.

Nonmonotonic structural changes in HTSC Bi-2201 ceramics depending on oxygen nonstoichiometry

Effect of the oxygen content in ceramic high-temperature superconductors Bi2Sr2CuO6 + δ on their phase composition, crystal structure, and charge state of cations has been studied. As the oxygen content changes, the lattice parameters were shown to exhibit nonmonotonic variation; no changes in the phase composition and charge state of cations take place. According to neutron diffraction data, the overstoichiometric oxygen atoms are introduced only into the BiO1 + δ layers. The nonmonotonic variations observed are explained assuming that, at a certain oxygen content, the hole charge carriers induced by introduced oxygen are localized in the BiO1 + δ layers rather than “flow down” into superconducting CuO2 layers.

Crystal Structure of TbNiD3.3

A TbNi-based deuteride has been prepared by hydriding TbNi at a temperature of 297 K and a deuterium pressure no higher than 0.25 MPa. The structure of TbNiD3.3 differs from that of the parent intermetallic compound, indicating that deuteration causes structural changes in the metallic sublattice. The deuteride has an orthorhombic structure (CrB type, sp. gr. Cmcm) in which the deuterium atoms occupy three positions, 4c, 8f, and 4b, with [Tb3Ni2], [Tb3Ni], and [Tb4Ni2] nearest neighbor environments, respectively.

Study of the structure of synthetic opals affected by temperature and pressure

It is demonstrated that synthetic opals, like most natural ones, have a cristobalite rather than quartz basis, change their color from white to blue after losing their water-containing component, and form superlattices. Being affected by temperature and pressure, they undergo partial or complete crystallization to the corresponding polymorphic modifications.

Thermal vibrations and polymorphic β → γ transition in cerium

Method of neutron diffraction was used to determine the temperature dependence of the Debye-Waller factor and the related thermal atomic displacements for two polymorphic modifications of cerium, namely, for β-Ce with a double hexagonal closed-packed (dhcp) structure and for γ-Ce with a face-centered cubic (fcc) structure. It has been shown that the phase transition does not lead to substantial changes in the root-mean-square thermal atomic displacements and that the Debye temperatures of the two modifications are close: 131 K for β-Ce and 127 K for γ-Ce. However, the relative (with respect to the lattice parameters) displacements along the axes change considerably. The transition from the anisotropic hexagonal to the isotropic cubic modification leads, because of a redistribution of thermal atomic displacements along the crystallographic axes, to a decrease in the maximum values of these quantities and to a weakening of their temperature dependence. It has also been shown that a change in the thermal atomic vibrations and in the vibrational contribution to the entropy of the polymorphic transformations is connected with the sign of the volume effect of the transformation (stronger upon a positive effect and weaker, upon a negative one). The reasons for this behavior are discussed.