The association energy of aggregates of divalent impurities and vacancies in NaCl-type crystals (point charge approximation)

The formation and association energies for the different aggregates of divalent metal impurities and cation vacancies are derived by considerations of the sums over all lattice vectors. This gives a simple formula in the point charge approximation if an infinite lattice is assumed. Because of the neglection of the polarization of the ions this method yields a higher value for these energies, as a comparison with the literature for the case of dipoles shows. But nevertheless it is a first possibility for a correct estimation of these energies also for higher aggregates.     

Nematic-isotropic phase transition in polar liquid crystals. 1. Statistical theory

The theory of transition from the nematic to the isotropic phase for liquid crystals in the system of rodlike particles with large longitudinal dipoles has been developed with due regard for the equilibrium between monomers and antiparallel molecular pairs—dimers. The order parameters of monomers and dimers are determined as well as the dimer fraction. It is shown that, in accordance with the results obtained earlier, for low values of dipole moments, the temperature of the nematic-isotropic phase transition increases with an increase of the dipole moment. However, for large dipoles, the transition temperature starts decreasing with an increase of the dipole moment because of higher dimer concentration. This provides the interpretation of the recent computer simulation, which showed a destabilization of the nematic phase in the system of rigid rods with pronounced central dipoles. The temperature dependence of the dimer fraction is also studied. The qualitative relation between the sign of the jump in the dimer fraction at the transition point and the effect of dimerization on the transition temperature are established.     

基于偶极子点光源模型光子晶体LED出光效率的研究

利用三维时域有限差分法(FDTD)对光子晶体LED出光效率的影响因素进行分析,比较了电偶极子和磁偶极子点光源模型对LED出光效率的影响,研究不同极化角偶极子点光源下光子晶体LED中的出光效率。数值计算结果表明:极化角越小,偶极子点光源在LED出光效率增强因子越大,磁偶极子点光源模型与电偶极子点光源模型相比,极化角对出光效率增强因子的影响明显减小。基于磁偶极子点光源模型,考虑极化角的影响优化设计一种空气孔三角晶格光子晶体LED结构,其出光效率增强因子高达4.5。     

X-ray diffraction study of the structure of detonation nanodiamonds

The spatial structure of aggregates formed by detonation nanodiamonds is investigated using the wide-angle and small-angle X-ray scattering techniques. The effective sizes of crystallites and the crystallite size distribution function are determined. The shape of scattering aggregates is restored from the small-angle X-ray scattering data. An analysis of the results obtained allowed the conclusion that the nanodiamond aggregates have an extended spatial structure composed of nine to ten clusters, each involving four to five crystallites with a crystal lattice of the diamond type.     

Possible dynamical mechanism of dislocation drag in metals in the easy slip stage

The slip of an edge dislocation through a system of parallel immobile dislocation dipoles oriented parallel to it has been investigated. A new mechanism of dislocation drag (irreversible transformation of the kinetic energy of a moving dislocation into the energy of natural vibrations of a pair of edge dislocations (forming a dipole), excited by the elastic field of the moving dislocation) is proposed and analyzed. The dynamic drag force of moving dislocation caused by this mechanism is calculated. It is shown that this force is inversely proportional to the slip velocity of mobile dislocations.     

Diffusion of self-point defects in body-centered cubic iron crystal containing dislocations

The energetic, crystallographic, and diffusion characteristics of self-point defects (SPDs) (vacancies and self-interstitial atoms (SIAs)) in body-centered cubic (bcc) iron crystal in the absence of stress fields have been obtained by the molecular statics and molecular dynamics methods. The effect of elastic stress fields of dislocations on the characteristics of SPDs (elastic dipoles) has been calculated by the methods of the anisotropic linear theory of elasticity. The SPD diffusion in the elastic fields of edge and screw dislocations (with Burgers vectors 1/2 〈111〉 and 〈100〉) at 293 K has been studied by the kinetic Monte Carlo method. The values of the SPD sink strength of dislocations of different types are obtained. Dislocations are more effective sinks for SIAs than for vacancies. The difference in the sink strengths for SIAs and vacancies in the case of edge dislocations is larger than the screw dislocations.     

Effect of some Me2+ impurity ions on the lattice parameter of KCI crystals

The effect of concentration of substitutional Ca²⁺, Sr²⁺, Ba²⁺ and Pb²⁺ ions on the lattice parameter of KCI crystals is examined. The amount of lattice contraction induced by the alcaline earths ions qualitatively correlates with the pertinent ionic radii values (relative unit cell contraction induced by one impurity dipole amounts to −0.905, −0.110 and −0.020 for Ca²⁺, Sr²⁺ and Ba²⁺ ions, respectively). The initial drop of lattice parameter observed at low Me²⁺ concentration is tentatively ascribed to the drop of dislocation density. In Pb²⁺ doped crystals except for the above initial drop, at higher dopant concentration no effect upon the lattice parameter is found.     

The role of Mg2+-V-dipoles: In the thermoluminescence of LiF2:Mg single crystal

The thermoluminescence and radiation or thermally induced changes of the concentration of magnesium dipoles have been studied in LiF:Mg²⁺ crystal grown in an inert atmosphere. The evolution of the TL spectrum with γ-radiation dose shows that the glow peaks observed at the beginning of irradiation decrease at high doses whereas new peaks, positioned at higher temperatures, appeare. Moreover, it has been found that the thermally induced recovery of the dopant dipoles, destroyed during the irradiation, strongly depends on the dose applied. The recovery curves obtained for a fixed irradiation dose dose not exhibit thermal annealing steps at temperatures, at which glow peaks occur. Further, the number of the recovered magnesium dipoles is not correlated with the quantity of light emitted during heating the irradiated crystal what is expected in respect to some of the previously proposed models. On the basis of results obtained it is proposed that the decay of the magnesium dipoles is mainly due to the dopant aggregation induced by the γ-radiation. The role of the magnesium ions in thermoluminescent processes is discussed.     

Influence of the model parameters on the Monte Carlo simulation of the initial aggregation of point defects in solids

The kinetic process to reach the equilibrium state of lattice defects is characterized by an aggregation of these defects. The formation of the precursors of the aggregates, the so called critical nuclei, can be investigated by Monte Carlo simulation. But in many cases the exact interaction potential between the movable defects in the lattice and the nucleus is unknown. Therefore such simulations were carried out for different typical interaction parameters. Conclusions concerning the general behaviour were derived. The calculations allow especially an estimation of the influence of strength of the potential and temperature on the number of defects forming a critical nucleus.     

Transitions to the relaxor and dipole-glass states in mixed crystals of the potassium dihydrogen phosphate family

Regularities of the formation of the relaxor and dipole-glass states in the systems of mixed ferroelectric-antiferroelectric crystals of the potassium dihydrogen phosphate family are discussed. The problems related to the crystal structure, lattice dynamics, and relaxation processes in dipole glasses and relaxors implemented in systems of this type are considered.     

Electron diffraction study on the electrostriction of ruby

The pseudo-cubic lattice of ruby was so deformed, when immersed in a strong electric field, that it showed an induced superlattice structure along the [110] axis. The field strength was about 2 × 10⁴ kV/cm at the specimen. Electron diffraction pattern from the superlattice informed on the orientation of the Al₂O₃ dipoles in ruby. The ruby deformed by electron bombardment was accompanied by an emission of red light (wavelength: 6000 Å).     

The coloration effect in KCl: Eu2+ crystals γ-irradiated at 200 K

The formation of colour centers at 200 K in γ-irradiated KCl crystals doped with Eu²⁺ ions in a wide concentration range is studied by optical absorption, EPR, and ITC method. The EPR and ITC measurements reveal the drastic decrease of concentration of isolated europium dipoles induced by irradiation at 200 K. Moreover, no correlation between concentration changes of colour centers and Eu²⁺ dipoles induced by irradiation and postirradiated annealing has been found. The results obtained are explained by radiation stimulated aggregation of Eu²⁺ dipoles into small clusters at 200 K.     

Simulation of the OH<Superscript>−</Superscript> group location in the cubic phase of BaTiO<Subscript>3</Subscript> crystals

An investigation of the isolated OH⁻ group location in the nonpolar phase of barium titanate crystals and an analysis of lattice local strains have been performed using computer simulation. It is shown that the presence of the OH⁻ group leads to the formation of a polarized cluster with a rather high electric dipole moment.     

Nematic-isotropic phase transition in polar liquid crystals. 2. Role of dispersion interactions

The present study furthers the development of the theory of the nematic-isotropic phase transition in the system of rodlike particles with large longitudinal dipoles. The effect of dispersion interactions between single molecules (monomers) and molecular pairs (dimers) on the transition temperature is considered. The crucial point of the approach used is existence of dimers in the system. Dispersion interactions are estimated within the framework of the model in which molecules consist of spherical blocks interacting according to the Lennard-Jones law. The direct pair-correlation functions are approximated by spherical invariants forming a complete system of functions. Then, these correlation functions are varied in order to study the behavior of the phase-transition temperature depending on the choice of the molecular model. It is shown that, depending on the molecule shapes, dimers can either destroy or stabilize the nematic order. It is also shown that the presence of dimers in the system decelerates an increase of the transition temperature (and, in many instances, can even reduce it) with an increase in the value of the molecular dipole.     

Twinning of quasicrystals and related crystals

Electron microscopy and electron diffraction have been employed to bring out the intimate relationship between icosahedral quasicrystal and various types of crystalline phases with emphasis on aluminium transition metal alloys. The relationship between quasicrystal and twinned rational approximant structures will be treated in the context of irrational twinning. The twinned aggregates show an icosahedral like symmetry in electron diffraction patterns. The crystallography of irrational twins has been discussed in terms of various ways of representing a twin relationship in a cubic lattice.     

Resonance absorption in the amorphous semiconductor Si12Ge10As30Te48

The complex permitivity of the amorphous semiconductor Si₁₂Ge₁₀As₃₀Te₄₈ has been measured at frequencies from 1 to 4 GHz and at temperatures from 13 to 42°C. The results show that there is resonace absorption at the resonace frequency of 1.6 GHz corresponding to a relaxation time of 2.45 × 10^?9 sec at 20°C, and that this resonance frequency increases with increasing temperature. On the basis of the model that the power loss is due to the transitions of dipoles between their equilibrium positions, the computed results are in good agreement with experimental ones. The size and the possible formation of such dipoles are also discussed.     

AFM studies of the nucleation and growth mechanisms of macromolecular crystals

Atomic force microscopy (AFM) has been used to visualize events arising from the formation of intervening metastable phases at the surfaces of macromolecular crystals growing from solution. Crystals investigated were of the proteins canavalin, thaumatin, lipase, xylanase, and catalase, crystals of transfer RNA, and crystals of satellite tobacco mosaic virus. The appearance of aggregates on crystal surfaces was observed. The aggregates we infer to originate from liquid-protein droplets. These were particularly evident in freshly mixed mother liquor solutions. Droplets, upon sedimentation, have two possible fates. In some cases they immediately restructured as crystalline, multilayer stacks whose development was guided by, and contiguous with the underlying lattice. These contributed to the ordered growth of the crystal by serving as sources of growth steps. In other cases, liquid-protein droplets formed distinct microcrystals, somehow discontinuous with the underlying lattice, and these were subsequently incorporated into the growing substrate crystal. Scarring experiments with the AFM tip indicated that, detached from the crystal, molecules do not dissolve in the fluid phase but form metastable liquid-protein droplets with a potential to rapidly crystallize on the crystal surface. The molecular structure of the growth steps for thaumatin and lipase protein crystals were deduced. There is no step roughness due to thermal fluctuations, and each protein molecule which incorporated into the step edge remained. Growth steps propagate by addition of individual molecules which form subkinks of different size on the step edge.     

An Insight on the Polyacetylene Properties Through Resonant Raman Scattering Spectra Analysis

Resonant Raman Scattering spectra of trans polyace-tylene (CH)_X and (CD)_X are interpreted in terms of a model based on a bimodal distribution of conjugation length segments. Such a theory takes into account the electronic and lattice dynamics properties of different conjugation length segments, together with the electronic dipole moments from the transitions and the electron vibration interactions in the excited electronic states. This model accounts also for the Resonant Raman Scattering spectra taken during the isomerization process and from cis rich samples.     

The Dielectric and Optical Properties of the Homologous Series of Cyano-Alkyl-Biphenyl Liquid Crystals

The electric permittivities, refractive indices and densities of the homologous series of alkyl-cyano-biphenyls (C_nH_2n+1.φ.φ.CN) have been measured as a function of temperature in the nematic phases for n = 5 to 9, and in the smectic phases for n = 8 and 9. The results have been analysed in terms of the Maier and Meier theory to yield values for the molecular dipole moments and polarizabilities. Kerr effect measurements on dilute solutions of 44′-n-pentyl-cyano-biphenyl at different temperatures are reported, and in conjunction with similar dielectric and optical measurements have provided results for the free molecule dipole moment and polarizabilites; values for the higher homologues are predicted using a bond additivity model. Bordewijk's theory is applied to the permittivity measurements, and using appropriate molecular parameters, dipole-dipole correlation factors are calculated.