Crystallography Reports最新文献

The results of studying aqueous solutions of guanilurea hydrogen phosphite (GUHP), in particular, the isothermal cut of the phase diagram in the system of initial reagents for the synthesis of GUHP salt at 25 and 45°C, are presented. The solubility of GUHP in water was studied as a function of pH. The conditions for bulk crystal growth are determined. The indexing of GUHP crystal faces was performed. The features of the real structure of GUHP single crystals were studied in dependence of the growth conditions for the first time.

A three-dimensional reconstruction of the microstructure of the defects associated with the formation of fatigue cracks in samples destroyed during low-cycle fatigued tests has been performed by serial-section focused ion beam tomography. The geometric parameters of defects containing Hf, Nb, Ti, Al, and Ni, identified during 3D reconstruction, were determined. The morphology of individual particles is presented by a set of shapes that form flat (carpet-like) conglomerates up to several tens of microns in size, which cannot be detected by nondestructive testing methods. The revealed morphological features make it possible to propose a complex of measures aimed at increasing the service life of parts made of granulated heat-resistant nickel alloy EP741NP; this is an important practical result.

Nanosized polysubstituted solid solutions, containing from three to eight di- and trivalent cations in different quantitative ratios, with the general formula (M_{{1 - x}}^{n}R_{x}^{m}{{{text{F}}}_{{2 + x}}}), where M = Ca, Sr, Ba, Pb, R = La, Gd, Dy, Yb, n = 3 and 4, m = 0, 1, 2, and 4, and х varies from ~0.07 to ~0.3 for different cationic compositions, have been obtained by coprecipitation from aqueous solutions of nitrates. They all retain the fluorite structure (structural type CaF₂, space group (Fmbar {3}m)). The possibilities of precipitation of mixed solid solutions with different combinations of cations and using different fluorinating agents were demonstrated. It is shown that the obtained solid solutions are medium- and high-entropy phases.

Many structural metallic materials (in particular, hafnium, zirconium, and titanium alloys) exploited in hydrogen-containing media tend to form brittle hydride phases when absorbing hydrogen, which deteriorates their mechanical properties. Determination of the terminal solid solubility (the hydrogen concentration in a metal at which hydride formation occurs) is an important and time-consuming experimental task. The purpose of this study is to develop a new approach based on the hydrogen permeation method, which makes it possible to find the terminal solid solubility at isothermal saturation without using an expensive equipment. The technique is based on the change in the character of hydrogen permeation through a membrane from a material studied as a result of the hydride phase formation; this technique imposes certain limitations on the experimental conditions. They are related to the parameters of the surface and bulk processes of hydrogen interaction with the material. The importance of the correct choice of experimental conditions for determining the terminal solid solubility of hydrogen in metals was demonstrated using computer simulation, the possibility of applying the technique to hafnium was analyzed, and the protocol and parameters of the experiment were substantiated.

Memory effects in magnetoplasticity of NaCl crystals with different impurity content have been studied. Dislocation paths and microhardness of crystals after their exposure to a constant magnetic field or to crossed ultralow magnetic fields are measured. In two crystals, noticeable relaxation displacements of dislocations, introduced after exposure, are found. In two other crystals with a lower impurity concentration, the paths remain at the background level, but in one of them the exposure causes an increase in the density of mobile dislocations. Similar magnetic exposure also leads to a decrease in the microhardness of crystals, but to different extent. Interpretation of observations is reduced to a spin-dependent transformation of impurity centers in a magnetic field, which plastisizes the crystal. Introduction of dislocations after magnetic exposure leads to their relaxation displacements from unstable positions, and at strongly weakened pinning centers dislocation immediately occupy positions close to equilibrium.

The irradiation creep in metals with cubic crystal lattices at low stresses (below the yield strength) was studied within the framework of multiscale modelling. The modelling combines theoretical (dislocation theory of crystal plasticity, diffusion theory, anisotropic theory of elasticity, chemical kinetics) and computational (molecular statics, molecular dynamics, object kinetic Monte Carlo method) approaches. The values of the rate and modulus of irradiation creep were determined in metals with bcc (Fe, V) and fcc (Cu) crystal lattices containing rectilinear dislocations with the Burgers vectors 1/2〈111〉, 〈100〉 (bcc), and 1/2〈110〉 (fcc), uniformly distributed over possible families of their slip systems. The obtained calculated and theoretical values of the irradiation creep rate and modulus are in good agreement with the results of reactor experiments.

The magnetoresistance effect has been revealed in a superionic conductor: a Pb_0.66Cd_0.34F₂ single crystal. The effect is apparently related to the action of the Lorentz force on the system of mobile F^– ions. The effect is approximately 1% at a magnetic field with induction of about 1 T.

Critical phenomena in cubic helimagnets with nonequivalent magnetic atoms are investigated within the framework of the Weiss mean-field theory. The reason for the occurrence of temperature dependences of elastic moduli and the pitch of the magnetic helicoid is found, and the form of these dependences, determining the change in the conditions for the appearance of magnetic skyrmions in the type-II multiferroic Cu₂OSeO₃, is predicted.

The kinetics of kinks and domain walls in quasi-one-dimensional systems is described within the framework of a model intermediate between the sharp kink model and the continuum model of an elastic string. The effects resulting from the discrete structure of crystalline materials are considered, including the periodic inhomogeneity of the energy relief for kink migration. Within a transparent approximation using a minimum number of internal variables, the dependence of the Peierls barriers on the driving force is calculated, and the transition between static and dynamic regimes is described. The theory is based on the universal Frenkel–Kontorova model and can be applied to extended systems of various nature.

The growth of a repolarization nucleus in an electric field is hindered by cohesive forces acting near its tips on the adjacent domain walls. They can take large values when the distance between domain walls becomes comparable to their thickness. It is shown that the cohesive forces are expressed via the coefficients of the Ginzburg–Landau energy expansion, which includes a gradient contribution. For a uniaxial ferroelectric, the maximum internal field associated with the gradient interaction of the domain walls has been estimated. It is related to the internal coercive field E_c0 of the Ginzburg–Landau theory as ({{E}_{{{text{max}}}}}{text{*}})/E_c0 = 3√3/8 ≈ 0.65.