Yu. A. Dorofeev

Structure of zirconium phosphate gels produced by the sol - gel method

Several zirconium phosphate (ZrP) gels of various water contents produced by a sol - gel method were investigated. They were found to be the polymer-class gels using the method of small-angle neutron scattering. X-ray diffraction analysis revealed that their solid-phase framework was formed from branched fragments (layers and blocks) of -ZrP crystal structure. These gels are strongly fluctuating systems when they are on the scale of . Their fluctuation character is shown to be described by two parameters: a correlation radius and a fractal dimension D. It was found that and almost did not depend on the water content and that D increased from to when the water content decreased from 50% to 0% as the gels were dried to a temperature of . Simultaneously, substantial shrinkage of granules, a decrease in porosity and ion-exchange capability, and formation of blocks from -phosphate layers occurred with decrease in the water of the hydrogel as it dried. A probable description of the evolution of the ZrP-gel structure with variation in the water content during drying is suggested on the basis of the data obtained.

Structural state of expanded graphite prepared from intercalation compounds

The structural state of nanocrystalline samples of expanded graphite is investigated using X-ray diffraction and neutron diffraction analyses. The expanded graphite samples are prepared by a rapid thermal decomposition of intercalation compounds of oxidized graphite based on fluorinated graphite, graphite oxide, and graphite aminofluoride. It is established that the main phase of expanded graphite belongs to the hexagonal crystal system (space group P63/mmc) and that carbon atoms in the structure occupy the 2b and 2c positions. The unit cell parameters and the unit cell volume in the structure of expanded graphite samples are larger than those in the structure of massive graphite.

Phase Separation in Isostructural Systems between Ordering Alloy and Intermetallic Compound

The results of X-ray diffraction, Mossbauer effect and neutron diffraction studies and also magnetic, thermal and kinetic properties measurements of the ternary alloys PdMn x Fe 1-x are presented. It has been shown that the alloys in the 0.2 < x < 0.8 concentration range realize a micro-inhomogeneous state formed by isostructural phases of the constituent compositions: the ferromagnetic PdFe alloy undergoing atomic ordering and the antiferromagnetic PdMn intermetallic compound.

Investigation of the spin-glass state in γ-Fe-Ni-Mn alloys

Abstract The composition region where there are no long-range ferro- and antiferromagnetic order has been determined in γ-Fe-Ni-Mn alloys. In these alloys strong small-angle magnetic neutron scattering has been obtained. The intensity has an unusual temperature dependence which is discussed in the framework of the critical neutron scattering thermodynamic theory. The ground state of these alloys is identified with the spin-glass state in which there are also regions of both short-range ferro- and antiferromagnetic order.

Fine structure and magnetism of the cubic oxide compound Ni0.3Zn0.7O

The fine structure and spin system of the cubic oxide Ni0.3Zn0.7O compound prepared from the initial hexagonal phase by quenching a sample with a high temperature and applying an external hydrostatic pressure to it have been studied using magnetic measurements, synchrotron and X-ray diffraction. It has been revealed that the diffraction patterns of this compound contain a system of weak diffuse maxima with the wave vectors q = (1/6 1/6 1/6)2π/a and (1/3 1/3 1/3)2π/a, along with strong Bragg peaks of the cubic phase. It has been shown that the origin of the diffuse peaks is due to longitudinal and transverse displacements of ions with respect to symmetric crystallographic directions of the {111} type. The reasons for the ion displacement and specific features of the structure of the spin system of the strongly correlated oxide Ni0.3Zn0.7O compound have been briefly discussed.

Size effect in nanocrystalline manganites La1−x A xMnO3 (A=Ag, Sr)

Nanocrystalline samples of the manganites La0.9Ag0.1MnO3, La0.7Ag0.3MnO3, and La0.7Sr0.3MnO3 were synthesized through pyrolysis and isothermally annealed. The atomic, subatomic, and magnetic structures of these manganites were studied using magnetic, x-ray, and neutron diffraction measurements. Increasing the annealing temperature from 600 to 1200°C coarsens the grains from 30–40 to 600–700 nm in size. All the samples studied have rhombohedral structure and are ferromagnets. The Curie temperature decreases for the samples doped by silver and increases for the samples doped by strontium as the anneal temperature is increased. The magnetization of the Mn ions increases with nanoparticle size in all the three systems, which indicates the presence of a size effect.

Magnetic properties of Ni3AlxMn1 − x alloys

Paramagnetic susceptibility of alloys Ni3AlxMn1 − x in the field of the transition between the ferromagnets described in the band theory (Ni3Al) and in the spin-localized (Ni3Mn) model of magnetic moments has been investigated. The concentration dependences of the paramagnetic Curie temperature, effective magnetic moment, and temperature-independent component of magnetic susceptibility have been determined.

Magnetic-field-induced spin-reorientational transition in TbMn6Sn6

Magnetic state of the intermetallic TbMn6Sn6 compound was studied by neutron diffraction analysis in external magnetic fields of H=0, 4, and 5 kOe. Magnetic moments and their orientations were determined for the Tb and Mn sublattices. Magnetic fields higher than 4 kOe were shown to induce spin-reorientational transition at room temperature from the c axis to the basal plane.

Small-angle neutron scattering from samples of expanded carbon

The subatomic structure of expanded graphite has been investigated by small-angle neutron scattering. Samples were synthesized during quick thermal decomposition of intercalated compounds based on oxidized graphite. They had a low bulk density (up to 0.1 g/cm3) and were characterized by considerable small-angle scattering. It has been established that majority of the volume of expanded graphite samples is occupied by participles with characteristic sizes in two ranges: from 6 to 8 nm and from 20 to 30 nm. Small particles have properties of a surface fractal with the dimension Ds = 2.4–2.6, whereas the larger particles are mainly smooth and have the dimension Ds = 2.0–2.1. The specific surface of the samples studied was determined from the small-angle scattering data.

Magnetic phase transitions in TbNi5 single crystal : Bulk properties and neutron diffraction studies

Magnetic susceptibility, heat capacity, and neutron diffraction measurements have been carried out in order to study the unusual magnetic ordering of TbNi5 using a single crystal over the temperature region of 2–30 K. Two spontaneous magnetic transitions have been observed: one is a second-order transition from a paramagnetic state to an incommensurate structure (Tp = 24 K), and the other, a first-order transition from the incommensurate structure to a lock-in phase (Tfc = 10 K). We also found an irreversible magnetic field-induced transition from the modulated structure to the ferromagnetic collinear one.