The crystalline and magnetic structure of the nonstoichiometric intermetallic compound Tm 2 Fe 18 has been investigated in the temperature interval from 3 to 300 K using the methods of neutron diffraction, synchrotron radiation, and ultra-small-angle neutron scattering. A theoretical interpretation of the spin-reorientation transition has been given and the value and temperature dependence of the first constants of magnetic anisotropy for the sublattices of iron and thulium have been determined. Based on the results of experiments and theoretical interpretation of small-angle neutron scattering, a conclusion has been made on a reconstruction of the domain structure upon spin-reorientation phase transition.

The problem of developing a dedicated neutron diffractometer for placement on a horizontal reactor channel to measure internal stresses in bulk materials and components under conditions of a limited space is considered. It is shown that the use of a double-crystal monochromator composed of pyrolytic graphite and a focusing bent perfect silicon single crystal is the optimal solution to this problem. The diffractometer with such a monochromator that is installed at the IR-8 reactor of the National Research Center Kurchatov Institute is comparable in luminosity and resolution at a reactor power of 6 MW to modern stress diffractometers at more powerful reactors.

The ultrasmall- and wide-angle neutron diffraction methods are used to study multiscale structures and phase transformations in synthetic opals at different temperatures and pressures (as high as 1500°C and 10 GPa, respectively). Monodisperse colloidal particles of amorphous silica dioxide (a-SiO2) with an average diameter of 150‒1700 nm whose deviation from the mean is less than 5% are synthesized to fabricate the samples. Opal matrices up to 3 cm thick are obtained via the natural sedimentation of a SiO2-globule suspension followed by drying and heat treatment. Neutron-diffraction processes are investigated using a DISK multidetector superposition diffractometer mounted at the IR-8 reactor of the National Research Centre Kurchatov Institute at a neutron wavelength of 1.668 Å. Ultrasmall-angle diffraction experiments are performed in the two-crystal mode of a STOIK spectrometer.

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.

Проведенное исследование свидетельствует о том, что синтетические опалы, как и большинство естественных, имеют не кварцевую, а кристобалитную основу, меняют окраску с белой на синюю после потери водородосодержащего компонента и образуют сверхрешетки. При воздействии температуры и давления испытывают частичную или полную кристаллизацию в соответствующие полиморфные модификации.

The process of self-irradiation of a Pu-Ga alloy has been studied by the neutron-diffraction method with the determination of the crystal structure and root-mean-square atomic displacements 〈u 2〉 (from the data on the Debye-Waller factor). The analysis was carried out at room temperature on the sample with an fcc structure prepared on the basis of a Pu242 isotope feebly absorbing neutrons, in which a quickly decaying Pu238 isotope (1.4 at %) was added to intensify self-irradiation processes; this accelerated the aging processes by four times and allowed achieving the maximum equivalent self-irradiation time of ∼23.5 years. The fcc structure was preserved during all this time interval. An analysis of the small-angle neutron scattering has demonstrated that the sample also contained precipitates with a size of a few hundreds of microns, which did not change during the aging. A change in 〈u 2〉 (due to static displacements) occurs in two stages, i.e., a relatively rapid growth (by about 50%) during the first 5–6 years of self-irradiation, and a slow decrease in the subsequent 6–23 equivalent years to nearly the magnitude that exceeds the initial value by ∼20%. The latter stage can be explained by the sinking of continuously generated point defects to helium bubbles and dislocations loops accumulating with time. The extrapolation of the decrease in 〈u 2〉 to large aging times demonstrates that if the mechanism of point-defect accumulation initiated at the first stage of the self-irradiation does not change with time, the growth of 〈u 2〉 will disappear by about 50 years of equivalent time of self-irradiation.

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 beta-Ce with a double hexagonal closed-packed (dhcp) structure and for gamma-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 beta-Ce and 127 K for gamma-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.

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.