Crystallography Reports最新文献

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.

Extended defects in large synthetic diamonds grown at high pressures and high temperatures (HPHT) using the thermal gradient method were studied by X-ray topo-tomography with a laboratory setup. It is shown that the temperature and growth rate are the principal factors influencing the crystalline quality of the diamonds. Synthesis at high temperatures and low rates allows growth of single crystals with a low density of dislocations, stacking faults, and microtwin lamellae. The annihilation kinetics of the extended defects during annealing at high pressure implies important role of vacancy diffusion. The annihilation of such defects leads to formation of perfect dislocations.

The existence of surface acoustic waves (SAWs) in a semi-infinite substrate with a deposited solid layer is theoretically investigated. The substrate and the layer are not piezoelectrics but may belong to any class of crystallographic symmetry. By writing down the dispersion equation in the form of a condition on the substrate and layer impedance matrices, it is possible to determine, using the properties of impedances, the maximum allowable number of surface waves, depending on the contact type and the relation between the velocities of bulk waves in the substrate and layer materials. A dispersion equation is derived for the symmetric orientation of an orthorhombic substrate with a deposited monoatomic layer, and the possibility of existence of a purely flexure surface acoustic wave in the case of a very hard surface layer, for example, a graphene monolayer on a soft polymer substrate, is shown.

The crystal structures of the L254N mutant of carboxypeptidase T in complexes with stable transition state analogs, such as sulfamoyl-L-glutamate, N-sulfamoyl-L-arginine, N-sulfamoyl-L-valine, and N-sulfamoyl-L-leucine, were determined at 2.05, 1.89, 2.30, and 1.79 Å resolution, respectively. The association constants of these inhibitors and the catalytic efficiency of the corresponding tripeptide substrates ZAAX were found to depend on the distances between the О15, О16, О20, and Т19 atoms of the ligand and the active-site residues N146, Y225, and E277 of the mutant protein. This dependence significantly differs from that found previously for wild-type carboxypeptidase T. These results demonstrate that leucine 254 of the flexible loop of metallocarboxypeptidases is involved in the substrate discrimination by carboxypeptidase T via the induced-fit mechanism.