S.Sh. Shilstein

Crystal structure and volume effects in the hydrides of d metals

Abstract We suggest, as the result of a generalization of structural data on the hydrides of d transition metals, a simple model, within the framework of which the structural characteristics of the metal hydrides (hydrogen coordination, volume change and compressibility) are connected with electronic parameters (Fermi energy and density of states at the Fermi level). Volume effects have been considered by using a rigid d-band model. Phase transformations with changing hydrogen coordination have been predicted for a number of hydrides of d metals.

Neutron radiography with refraction contrast

A new method of obtaining images in neutron radiography is proposed. It is based on separation of refracted and unrefracted beams. The required angular resolution is ensured by consecutive reflection of neutrons from two perfect crystals with the object placed between them. The experiments performed have shown that a high contrast is available even in images of weakly absorbing objects. Neutron images of an inhomogeneous magnetic field are obtained.

Hydrogen ordering in the cubic laves phase HfV2

Abstract We investigated the crystal structure of HfV2D4 using a neutron diffraction technique. The high temperature phase is shown to be a disordered solid solution in which deuterium atoms occupy two types of tetrahedral interstices in the cubic lattice of HfV2; moreover, there is a short-range “blocking-type” order in the mutual arrangement of the interstitial deuterium atoms. Below 278 K an order-disorder phase transition occurs, resulting in the formation of a superstructure belonging to space group I4 1 a . We suggest, within the framework of the statistical thermodynamic theory of order-disorder transitions developed for this case, an explanation for the super-structure formation and we reach some conclusions about the nature of regularities of hydrogen behaviour which are common to metals and intermetallic compounds.

SEARCH FOR BIOLOGICAL OBJECTS BY REFRACTION RADIOGRAPHY USING SYNCHROTRON RADIATION OF VEPP-3 STORAGE RING

Abstract The possibility of using synchrotron radiation (SR) for obtaining the X-ray images of biological objects by the method of refraction radiography at the energies of 8.9 and 33.2 keV is shown. The results of experiments at the storage ring VEPP-3 (Novosibirsk) are presented. In comparison with a conventional X-ray source, the exposition time with SR is reduced by a factor of more than a thousand (approximately, down to 1–2 s), and the quality of images is improved.

A neutron diffraction study of ZrD2 up to the pressure of 10 GPa

Abstract A neutron diffraction study of the effect of pressure on the lattice parameters of ZrD2 has been carried out at room temperature using a diamondanvil cell up to a pressure of 10 GPa. It was found that ZrD2 is compressed less than zirconium metal, and the change in the axial ratio c a in ZrD2 is linear with the pressure. These facts are explained within the framework of the electronic structure of the dideuteride. Some possible phase transitions in ZrD2 at higher pressures are considered.

STUDY OF THE MORIN TRANSITION IN NEARLY PERFECT CRYSTALS OF HEMATITE BY DIFFRACTION AND TOPOGRAPHY

Abstract The spin reorientational magnetic Morin transition is investigated on nearly perfect crystals of hematite by combining neutron diffraction, neutron and synchrotron radiation topography, and magnetization measurements. The samples are platelet shaped crystals, with their main surface parallel to the (1 1 1) basal plane. The magnetization measurements show that the main part of the transition occurs over a rather narrow temperature range (about 3 K), whereas this range appears broadened to about 13 K on the neutron diffraction curves. Neutron topographs show the whole crystal image during transition and X-ray topographs show zone contrast when modifying the applied magnetic field. These experimental facts indicate that the interfaces which separate the weakly ferromagnetic and the antiferromagnetic phases are parallel to the main surfaces of the crystals. This is the basis for a very simple model which explains, through severe extinction effects, the apparent contradiction between neutron diffraction and magnetization measurements, and leads to an estimation of the crystal perfection parameters.

Visualization of internal domain structure of silicon iron crystals by using neutron radiography with refraction contrast

Abstract It is shown that visualization of individual domain walls and domain structure inhomogeneities in single crystals of ferromagnets is possible by using the method of neutron radiography with refraction contrast. Images of the internal domain structure of bulk (≈ 1 cm) silicon iron single crystals of different orientations are obtained.

Measurement of the domain wall thickness in silicon iron using the adiabatic spin-flip effect on neutron refraction

Method of the measurement of thickness of the domain walls in iron by comparison of intensities of refraction of neutrons with different wavelengths is described. The refracted intensity depends on the wall thickness through the probability of spin flip of neutrons transmitting the magnetic helix. Thicknesses of the 180° and the 90° domain walls in silicon iron are measured. The measured values are compared to the previous experimental and theoretical results, obtained for different methods of determination of exchange energy in iron.

Coulomb splitting of atomic layers in crystal lattices of layered cuprates and nickelates

Abstract The crystal structure data are collected and analyzed on the splitting of anion-cation layers with non-symmetric electric charge environment. It is shown that the effect is caused by Coulomb interaction with the other charged layers, mainly the nearest neighboring ones. The feasibility is demonstrated of the control of interlayer charge transfer by magnitude of the Coulomb splitting. The correlation between the Coulomb splitting of (BaO) layers, neighboring to (CuO 2 ), and superconducting transition temperature T c is found in yttrium-barium cuprates.

Neutron optical study of the domain structure of ferromagnets

Abstract Neutron optical methods with high angular resolution have been used for determination of internal domain structure characteristics for bulk ferromagnets single and polycrystals. It has been found that several systems of domain boundaries coexist in the whole volume of single crystal multiaxial ferromagnets. The internal domain structure seems to be more stable than the surface pattern of closure domains.