I. A. Bobrikov

Structural phase transition in CuFe2O4 spinel

A structural transition with a reduction in symmetry of the high temperature cubic phase (sp. gr. Fd3m) to the tetragonal phase (sp. gr. I41/amd) and the appearance of a ferrimagnetic structure occur in CuFe2O4 copper ferrite at T ≈ 440°C. It is established by an experiment on a high-resolution neutron diffractometer that the temperature at which long-range magnetic order occurs is higher than that of tetragonal phase formation. When cooling CuFe2O4 spinel from 500°C, the equilibrium coexistence of both phases is observed in a fairly wide temperature range (∼40°C). The composition studied is a completely inverse spinel in the cubic phase, and in the tetragonal phase the inversion parameter does not exceed few percent (x = 0.06 ± 0.04). At the same time, the phase formed upon cooling has a classical value of tetragonal distortion (γ ≈ 1.06). The character of temperature changes in the structural parameters during the transition from cubic to tetragonal phase indicates that this transition is based on the Jahn-Teller distortion of (Cu,Fe)O6 octahedra rather than the mutual migration of copper and iron atoms.

Investigation of the crystal and magnetic structures of BaFe12 - xAlxO19 solid solutions (x = 0.1‒1.2)

The structure of barium ferrite BaFe12 - xAlxO19 solid solutions (x = 0.1‒1.2) with iron partially replaced with diamagnetic aluminum ions has been studied by neutron diffraction. Experimental data have been collected at room temperature on a high-resolution diffractometer, which yielded precise information about the changes in the crystal and magnetic structures and data on the behavior of the sample microstructure. Barium hexaferrite retains a magnetoplumbite structure in the entire range of aluminum concentrations under study, and its magnetic structure is described within the Gorter model, with moments orientated along the hexagonal axis. The total magnetic moment per formula unit decreases while diamagnetic aluminum ions substitute for iron ions. Microstrains in crystallites increase with an increase in the diamagnetic ion concentration, which is related to the difference in the ionic radii of iron and aluminum ions.

Structural phase transition in CuFe{sub 2}O{sub 4} spinel

A structural transition with a reduction in symmetry of the high temperature cubic phase (sp. gr. Fd3m) to the tetragonal phase (sp. gr. I41/amd) and the appearance of a ferrimagnetic structure occur in CuFe2O4 copper ferrite at T ≈ 440°C. It is established by an experiment on a high-resolution neutron diffractometer that the temperature at which long-range magnetic order occurs is higher than that of tetragonal phase formation. When cooling CuFe2O4 spinel from 500°C, the equilibrium coexistence of both phases is observed in a fairly wide temperature range (∼40°C). The composition studied is a completely inverse spinel in the cubic phase, and in the tetragonal phase the inversion parameter does not exceed few percent (x = 0.06 ± 0.04). At the same time, the phase formed upon cooling has a classical value of tetragonal distortion (γ ≈ 1.06). The character of temperature changes in the structural parameters during the transition from cubic to tetragonal phase indicates that this transition is based on the Jahn-Teller distortion of (Cu,Fe)O6 octahedra rather than the mutual migration of copper and iron atoms.

Correlation Fourier diffractometry: 20 Years of experience at the IBR-2 reactor

The high-resolution Fourier diffractometer (HRFD) was commissioned at the IBR-2 pulsed reactor at FLNP JINR in 1994. The specific feature of the HRFD design is the use of fast Fourier chopper for modulating the primary neutron beam intensity and the correlation method of diffraction data acquisition. This allowed to reach with HRFD extremely high resolution (Δd/d ≈ 0.001) over a wide range of inter-planar spacings at a relatively short flight path between chopper and sample (L = 20 m). Over time, a lot of diffraction experiments on crystalline materials, the main goal of which was to study their atomic and magnetic structures, were performed at HRFD. Successful implementation of the Fourier diffractometry technique at the IBR-2 reactor stimulated the construction of yet another Fourier diffractometer intended for internal mechanical stress studies in bulk materials (FSD, Fourier Stress Diffractometer). In this paper the experience of using this technique at the IBR-2, which is a long-pulse neutron source, is considered, the examples of HRFD studies are given, and possible solutions for existing technical problems of using correlation diffractometry and ways of increasing the intensity and resolution of HRFD are discussed.

Study of the crystalline and magnetic structures of BaFe 11.4 Al 0.6 O 19 in a wide temperature range

The structural study of barium hexaferrite (BaFe12O19) whose iron ions are partly replaced by diamagnetic aluminum ions (x = 0.6) is carried out with the help of the neutron-diffraction method. Experimental data are acquired in a wide temperature range of 4–730 K via a high-resolution diffractometer, which makes it possible to obtain both precise information on changes in crystalline and magnetic structures and behavioral data on the sample microstructure. BaFe11.4Al0.6O19 retains the magnetoplumbite structure in the whole temperature range and exhibits the “invar effect,” according to which its bulk thermal-expansion coefficient is close to zero at low temperatures (<150 K). The magnetic structure is defined by the Gorter model with magnetic moments oriented along the hexagonal axis. For all nonequivalent crystallographic positions, the magnetic moment of F3+ is close to 4 μB at 4 K. The total magnetic moment per formula unit is 15.6 μB, i.e., less than the nominal value of 20 μB. This is caused by the fact that diamagnetic Al ions are included in the composition. Crystallite microstrains increase insignificantly with temperature due to increasing influence of the magnetic subsystem.

The effect of oxygen isotope substitution on the phase diagram of nearly half-doped R1-xSrxMnO3 manganites (R = Sm, NdTb, NdEu)

The effect of isotope substitution on the electrical resistance, magnetic susceptibility and neutron diffraction patterns of nearly half-doped R1?xSrxMnO3 manganites was studied for ceramic samples with R = Sm (x = 0.425?0.525), Nd/Tb (x = 0.45) and Nd/Eu (x = 0.45). In Nd/Tb and Nd/Eu manganites, the composition was chosen to yield the same mean ionic radius as in the Sm compound, but the samples differed in the cation disorder parameter ?. The oxygen isotope substitution leads to fundamental changes in the phase diagram of the manganites studied, favouring enhanced inhomogeneity, stabilizing the insulating antiferromagnetic (AFM) state and even causing a metal?insulator transition (MIT). The MIT induced by the oxygen isotope substitution was found here for the first time in manganites with A-type AFM; earlier this unusual phenomenon was observed only for compounds with the CE-type AFM structure. This suggests that such a MIT is closely related to the ferro?AFM crossover independently on the specific nature of the AFM phase and related orbital order.

Neutron scattering for analysis of processes in lithium-ion batteries

The review is concerned with analysis and generalization of information on application of neutron scattering for elucidation of the structure of materials for rechargeable energy sources (mainly lithium-ion batteries) and on structural rearrangements in these materials occurring in the course of electrochemical processes. Applications of the main methods including neutron diffraction, small-angle neutron scattering, inelastic neutron scattering, neutron reflectometry and neutron introscopy are considered. Information on advanced neutron sources is presented and a number of typical experiments are outlined. The results of some studies of lithium-containing materials for lithium-ion batteries, carried out at IBR-2 pulsed reactor, are discussed. The bibliography includes 50 references.

Disordering effects in the atomic structure of fine-crystalline HTSC YBa2Cu3Oy

The atomic structure of YBa2Cu3Oy fine-crystalline HTSC samples with various average crystallite sizes 〈D〉 ranging from 0.4 to 2 μm and an oxygen concentration y close to the optimal value for superconductivity (y ≈ 6.93) is investigated by the neutron diffraction technique. We have found some effects associated with the redistribution of cations and oxygen atoms and with variations in the positions of atomic layers in the unit cell, which are not observed in macrocrystalline samples. In all probability, these effects appear due to nonequilibrium conditions of synthesis required for obtaining this compound in the fine-crystalline state. The results have made it possible to explain the peculiar physical properties of fine-crystalline YBa2Cu3Oy samples (in particular, the coexistence of high superconducting transition temperatures Tc and noticeably lower values of magnetization in strong magnetic fields for T < Tc). It is shown that a nanoscale structural inhomogeneity exists in fine-crystalline YBa2Cu3Oy samples with the optimal oxygen content and changes the fundamental superconducting parameters, viz., the magnetic field penetration depth and the coherence length.

Low-temperature structural anomalies in Pr0.5Sr0.5CoO3

The magnetic and crystal structures of the Pr0.5Sr0.5CoO3 metallic ferromagnet have been studied by the neutron diffraction technique. It is demonstrated that below 150 K, the compound is mesoscopically separated into two crystalline phases with different spatial symmetries and with different directions of the magnetic anisotropy. The phase separation exists down to 1.5 K, and at temperatures below 90 K, the low-symmetry phase occupies about 80% of the sample volume. The main structural difference between the phases is the configuration of oxygen atoms around praseodymium and, to a certain extent, around cobalt. The ferromagnetic structure with the magnetic moment lying in the basal plane of the structure (μCo ≈ 1.7 μB at 1.5 K) arises at 234 K, whereas the component directed along the long axis of the unit cell appears at 130 K. The formation of the new structural phase and change in the orientation of the magnetic moment give rise to the anomalies of the physical and magnetic characteristics of this compound observed earlier at temperatures about 120 K.

Comparative study of structural phase transitions in bulk and powdered Fe–27Ga alloy by real-time neutron thermodiffractometry

Phase transformations in an iron–gallium alloy have been analyzed by in situ real-time neutron diffraction in the temperature range from 293 to 1223 K. Two compositionally identical samples were studied: the first was in the as-cast bulk state, and the second was ground into a powdered state. In both samples, the same sequence of structural transitions was recorded on heating with a constant heating rate (D03 → A2 → L12 → D019 → A2), and the same structural state (D03 + L12) was recorded after slow cooling to room temperature. Owing to strong texture in the bulk sample, only diffraction patterns of the powdered sample were treated with the Rietveld method to determine the volume fractions of the coexisting phases, the coefficients of thermal expansion, and the thermal and static atomic disorder parameters. The occupancy of Ga positions and the ordered iron magnetic moment were refined at selected temperatures. The level of microstrain in the crystallites in the initial as-quenched state is small, but it sharply increases in the course of phase transitions when the alloy is heated. The microstrains are high and strongly anisotropic after slow cooling. Generally, phase transformations occur similarly in the powdered and bulk samples, but with a noticeable difference in details. The fulfilled analysis of the bulk and powdered samples allowed the real possibilities of the quantitative neutron diffraction analyses of phase transitions in ferromagnetic ordered alloys to be assessed.