Electronic excitations owing to plastic deformation of ionic crystals

Electron emission and luminescence accompanying plastic deformation of alkali halide crystals are studied. It is shown that the intersection of dislocations can cause electronic excitation. Deformation electron emission and luminescence are produced by relaxation of these excitations. Fiz. Tverd. Tela (St. Petersburg) 41, 900–902 (May 1999)     

Conduction in ionic organic plastic crystals: The role of defects

This paper describes the relationship between the ionic conduction and the characteristics of defects in plastic crystalline phases of N,N-dimethylpyrrolidinium bis(trifluoromethanesulfonyl)amide (P₁₁TFSA). Positron annihilation lifetime spectroscopy (PALS) shows that the phase III to phase II transition involves the generation and expansion of vacancies, which is confirmed by measurement of volumetric expansion. The vacancies expand to match the cation size in phase II and this gives rise to an increase in the conductivity. The relationship between the vacancy volume and the conductivity obeys a Cohen–Turnbull free volume conduction model. The critical volume matches the volume of a cation–anion pair in phase III, which indicates a Schottky mechanism. The critical volume shows a greater value in phase II, which is probably indicative of a pipe diffusion mechanism.     

Acceleration of atoms in crystals under plastic deformation

It is shown that a double-well potential can appear under mechanical loads for atoms in rows parallel to the axis of a screw dislocation and located near the nucleus of a dislocation and under dislocation slip conditions atoms can be accelerated up to energies much higher than the binding energy of atoms in a crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 499–504 (March 1997)     

Simple variational approach to the thermal properties of ionic crystals

The thermal properties of ionic crystals are analysed using the variational principle of classical statistical mechanics. The Einstein and Debye pictures of the lattice vibrations are adopted as trial Hamiltonians. No explicit calculation of the lattice spectrum is needed. The variational result for the thermal expansion in the Einstein picture is identical to that recently derived by Narayan and Ramaseshan by a physically motivated thermal force picture. The agreement with experimental values in the alkali halide family of crystals is surprisingly good, the root mean square error being about 14%. The parameters in the interionic potential used are obtained from the lattice spacings and compressibilities of the crystals and not from anharmonic properties. The Debye picture gives about equally good results for the thermal expansion, but better results for the thermal vibration amplitudes of the ions. It differs from the Einstein picture in incorporating correlated vibrations of atoms and in having an explicit Coulomb contribution to the thermal properties. It is suggested that the theory given in this paper has a useful role to play in studies of thermal expansion and phase stability for large families of ionic crystals when combined with semi-empirical theories.     

Formation of nanodefects in LiF crystals under gamma irradiation

The absorption spectra, photoluminescence spectra, and microhardness of LiF crystals exposed to gamma radiation from a shutdown reactor and a ⁶⁰Co source at a dose rate of 7.65 Gy/s are investigated. The structure of these crystals is determined using x-ray diffraction analysis. It is revealed that the Li sublattice contains not only point and complex radiation-induced defects but also 28-nm LiOH particles induced by gamma radiation. It is shown that the formation of defects occurs more efficiently upon exposure to radiation from a shutdown reactor than from a ⁶⁰Co source.     

Reversible and irreversible magnetic-field-induced changes in the plastic properties of NaCl crystals

It is established that a weak magnetic field with induction B∼1 T gives rise to irreversible changes in NaCl crystals without freshly introduced dislocations, while a strong magnetic field with induction 16<B<35 T gives rise predominantly to reversible changes. It is inferred that there exist two different channels whereby a magnetic field influences the state of point defects. Fiz. Tverd. Tela (St. Petersburg) 40, 2065–2068 (November 1998)     

On the localization of plastic strain under compression of LiF crystals

The plastic flow localization patterns for alkali halide LiF crystals under compression have been investigated. The main spatiotemporal regularities of the strain localization at different stages of deformation hardening in the single crystals have been established. The relation has been traced between the orientation of localized strain zones and the crystallography of slip systems of the test specimens studied simultaneously by the double-exposure speckle photography and photoelasticity methods.     

Defect-density switching wave in crystals under pulsed laser irradiation

A model is proposed for the ignition of a defect-density switching wave in a crystal by powerful laser pulses. It is shown that the switching wave arises as a result of the nonlinear dependence of the activation energy of the defect-formation process on the strain field due to defects. The conditions under which a switching wave arises and the profile, velocity, and propagation direction of the wave are discussed. Zh. Tekh. Fiz. 68, 73–77 (August 1998)     

Self-assembly of nanoparticles on the surface of ionic crystals: Structural properties

With a suitable combination of ligand-stabilised nanoparticle suspension and ionic salt solutions, it is possible to produce microcrystals that are coated with nanoparticles. The self-assembly process of coating microcrystals by gold nanoparticles (NP) is mediated by the crystal lattice. This is the so-called CLAMS process - a generic process for self-organisation of nanoparticles on the surface of crystals [M. Murugeshan, D. Cunningham, J.-L. Martnez-Albertos, R. Vrcelj, B.D. Moore, Chem. Commun. (2005) 2677]. We are exploring here the structural properties of these self-assembled structures by using different imaging techniques.     

Spectroscopy of defects in KBr(NO 2 − ) and KI(NO2 −) crystals under low-temperature UV irradiation

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 53, No. 3, pp. 410–415, September, 1990.     

Analysis of strain hardening of crystals under large plastic strains

A strain-hardening mechanism is considered theoretically for crystals with large shear strains (γ=1–10). According to this mechanism, the emergence of the fourth and fifth stages in the strain-hardening curve for a crystal is associated with geometrically essential dislocations (GEDs) appearing as a result of accommodation processes accompanying large plastic deformations. Geometrically essential dislocations are concentrated in the boundaries of strongly disoriented fragments of the crystal. A comparison of the experimental and theoretical results leads to the conclusion that the density evolution of GEDs with deformation at the fourth and fifth stages is described by the same kinetic equation as for the evolution of the density of statistically random dislocations at the second and third stages, but with lower values of the kinetic coefficients.     

MD simulation of plastic deformation nucleation in stressed crystallites under irradiation

The investigation of plastic deformation nucleation in metals and alloys under irradiation and mechanical loading is one of the topical issues of materials science. Specific features of nucleation and evolution of the defect system in stressed and irradiated iron, vanadium, and copper crystallites were studied by molecular dynamics simulation. Mechanical loading was performed in such a way that the modeled crystallite volume remained unchanged. The energy of the primary knock-on atom initiating a cascade of atomic displacements in a stressed crystallite was varied from 0.05 to 50 keV. It was found that atomic displacement cascades might cause global structural transformations in a region far larger than the radiation-damaged area. These changes are similar to the ones occurring in the process of mechanical loading of samples. They are implemented by twinning (in iron and vanadium) or through the formation of partial dislocation loops (in copper).     

Evolution of InP surfaces under low fluence pulsed UV irradiation

An InP wafer was irradiated in air by a series of UV pulses from a nitrogen laser with fluences of 120 mJ/cm² and 80 mJ/cm². These fluences are below the single-pulse ablation threshold of InP. Over the studied region the distribution of the radiation intensity was uniform. The number of pulses varied from 50 to 6000. The evolution of the surface morphology and structure was characterized by atomic force microscopy, optical microscopy and Raman spectroscopy. The relationship between mound size and the number of pulses starts out following a power law, but saturates for a sufficiently high number of pulses. The crossover point is a function of fluence. A similar relation exists for the surface roughness. Raman spectroscopic investigations showed little change in local crystalline structure of the processed surface layer.     

在具有强非线性效应的离子晶体中光脉冲传播的孤立子特性

从理论上研究了原胞内存在强离子间非线性相互作用的离子晶体,得出光脉冲在其中的传播具有孤立子特性,并分析了当非线性系数g分别为大于0与小于0时形成的亮孤立子与暗孤立子的具体特征.研究表明,不论非线性系数g取正值还是负值,在纵光学模声子频率ωLO以上频区,光脉冲可形成亮孤立子,在g>0时,在横光学模频率ωTO以下频区可形成暗孤立子;值得注意的是在ωTO—ωLO区间,光脉冲可形成超前型暗孤立子;在g<0时,从ωTO往下临近ωTO有一个由超前型亮孤立子向推迟型暗孤立子的转变区 关键词： 孤立子 极化激元 光脉冲     

Influence of electron irradiation on optical properties of scheelite crystals

Optical transmission spectra of nominally pure BaWO₄ and SrMoO₄ and doped BaWO₄:La³⁺ and SrMoO₄:Yb³⁺ crystals are measured after irradiation with a 6-MeV 5-μs pulse electron beam at room temperature. It is shown that the irradiation effect strongly depends on the crystals composition, the type, and the concentrations of doping ions. Possible mechanisms responsible for the spectra transformations and the correspondent formulas of chemical reactions addressing these mechanisms are proposed.     

The effect of special features of forming the imperfect structures in germanium crystals on plastic properties of these crystals

A new technology for forming the structures in semiconductor crystals using plastic deformation in an electric field is suggested. A setup making it possible to implement the suggested method and study the idea of self-organization of dislocation-related dissipative structures is described.