Modeling of the evolution process of primary Frenkel pair defect formation in alkali halide crystals

The evolution process of primary Frenkel pair defect formation is modeled in alkali halide crystals by the Monte Carlo method. A systematic analysis is carried out of the temperature and time dependence of the efficiency of Frenkel pair accumulation in ionic crystals. The effect of the local heating of a crystal microvolume and of the interaction energy between the components of a F, H-pair on the defect-formation process of primary Frenkel pairs is investigated. It is shown that the local temperature and interaction energy between the F, H-pair being formed are basic factors which determine the relaxation process of correlated Frenkel pairs in ionic crystals following pulsed irradiation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenni, Fizika, No. 2, pp. 19–23, February, 1985.     

Relaxation kinetics of correlated neutral Frenkel pairs of defects in alkali-halide crystals

A modelling method is developed that permits qualitative investigation of the evolution of genetic pairs of Frenkel defects in alkali-halide crystals (AHC). The relaxation kinetics and the form of the low-temperature annealing curves of F-, H-pairs are determined by initial mutual distribution functions (IDF) of the pairs being formed up to the time of excess energy dissipation by an H-center. It is shown that, because of discreteness of the lattice for AHC, interactions between the F- and H-centers are characteristic of IDF with a nonuniform spacing distribution of the F- and H-pairs. Fundamental regularities of Frenkel defect relaxation after pulse exposure and of F-, H-pair annealing after low-temperature AHC exposure are discussed on the basis of modelling results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 16–21, March, 1989.     

Evolution of a microstructure of materials under irradiation

The evolution of a microstructure of metals (or alloys) under irradiation resulting in swelling of the material is considered for the case with the formation of Frenkel pairs. A closed system of equations describing the evolution of the microstructure of the material exposed to irradiation is obtained, and relationships for the swelling rate are derived. It is shown that the swelling rate varies linearly with time for a stationary source of point defects (the number of Frenkel pairs per lattice site). An expression for the swelling rate is deduced for a radiation source operating in a more realistic pulsed mode.     

Intrinsic stability of quasicrystals under the generation of Frenkel pairs

Under irradiation metastable quasicrystals undergo a phase transition to an amorphous state. This transition can be reversed by annealing. As in normal crystalline materials the phase transition is considered to be triggered by generation and recombination of vacancies and interstitial atoms (Frenkel pairs). We have classified the possible Frenkel defects in a metastable monatomic quasicrystal with respect to geometric and energetic properties. With numerical simulation we have studied the behaviour of the quasicrystal under a load of Frenkel defects for various defect concentrations. We find three ranges of behaviour: up to 5% defects per atom the structure remains icosahedral, in a middle range it stays disordered icosahedral or it becomes either disordered or perfect crystalline, depending on the implementation of the defects. If there are more than 10% defects the structure becomes irreversibly amorphous. We finally compare our results with experimental data.     

PAC-study of defects in n-type germanium

Using perturbed angular correlations of γ rays the behaviour of Frenkel defects in n-type germanium is studied.¹¹¹In/¹¹¹Cd probes either serve as primary knock-on atoms in the production of Frenkel defects by neutrino recoil or as trapping agents for defects produced by electron irradiation. The neutrino recoil process leads to a defect which is characterized by vQ=e²qQ/h=42(2) MHz (η=0.6), while a defect with vQ=423(1) MHz (η=0) is observed after electron irradiation and thermal annealing.     

Transient optical absorption and luminescence in <Emphasis Type="Italic">A</Emphasis>Pb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> (<Emphasis Type="Italic">A</Emphasis> = K,Rb) crystals

This paper reports on a study of transient optical absorption and pulsed cathodoluminescence in APb₂Cl₅ (A = K, Rb) in the visible and ultraviolet spectral regions. The measurements performed by absorption optical spectroscopy with nanosecond time resolution showed the transient optical absorption of APb₂Cl₅ to derive from optical transitions in hole centers, and that the optical density relaxation kinetics is mediated by interdefect tunneling recombination in complementary pairs which involves Frenkel defects on the cation sublattice and self-trapped carriers. The slow components in the transient optical absorption decay kinetics, with characteristic times ranging from a few ms to seconds, have been assigned to diffusion-mediated annihilation of interstitial atoms with alkali metal vacancies. The mechanisms underlying creation and relaxation of the short-lived Frenkel defects on the cation sublattice and self-trapped carriers have been analyzed.     

Electron microscope studies of the influence of electrons with subthreshold energies on the formation and annealing of structural defects in ZnTe and CdZnTe

The influence of subthreshold-energy electrons on the formation and annealing of structural defects in ZnTe and on the annealing of dislocation loops preinjected in the sample by bombardment with argon ions is investigated by transmission electron microscopy. The final form and density of structural defects formed after electron irradiation is observed to depend on the total electron flux, whereas the rate of formation of defects and annealing of dislocation loops increases linearly with the intensity of electron irradiation. A comparison is drawn with CdZnTe, Si, and GaAs irradiated under similar conditions. The results are attributed to subthreshold defect nucleation mechanisms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 111–115, January, 1996.     

Molecular dynamic simulation of disorder induced amorphization in pyrochlore

The defect accumulation mechanism of amorphization has been studied for the La2Zr2O7 pyrochlore by means of classical molecular dynamic simulations. Present calculations show that the accumulation of cation Frenkel pairs is the main driving parameter for the amorphization process, while the oxygen atoms simply rearrange around cations. Under Frenkel pair accumulation, the structure follows the pyrochlore-fluorite-amorphous sequence. Present results consequently provide atomic-level interpretation to previous experimental irradiation observations of the two-step phase transition.     

Effects of Nd:YAG laser irradiation on structural, morphological, cation distribution and magnetic properties of nanocrystalline CoFe2O4

The cobalt ferrite nanoparticles of 20 nm size were synthesized by sol-gel auto-combustion technique. The samples were irradiated with Nd:YAG laser to understand the effects of irradiation on structural, cation distribution and magnetic properties. The virgin and irradiated samples were characterized by X-ray diffraction technique. The X-ray diffraction studies at room temperature shows that defects were created in the lattice after irradiation which causes effects on structural, cation distribution and magnetic properties. The energy dispersive analysis of X-rays (EDAX) showed the chemical composition is as per the expected stichiometry. The lattice constant observed from XRD data for virgin and irradiated samples shows increasing trend after irradiation. Cation distribution was investigated by using X-ray diffraction method. We observe decrease in magnetization of the samples after irradiation. The observed reduction in the saturation magnetization after irradiation can be understood on the basis of the partial formation of paramagnetic centers and rearrangement of cations in the lattice.     

The Effect of Point Defects on the Activation Processes in Semiinsulating GaAs Irradiated by Si28 Ions

The layered concentration, concentration profile, and mobility of electrons in the Si²⁸ ion-implanted layers (IIL) of semiinsulating GaAs are investigated. The specific resistance of the latter is also studied upon radiation annealing (RA) in the temperature range 590–800 °C using electron energies higher than the threshold energy of defect formation. The IIL are shown to form during RA at much lower temperatures. The layers exhibit high electrical activation of Si²⁸ ions, with the electron-concentration profile corresponding to the calculated one, and a low concentration of residual defects limiting the electron mobility. The radiation annealing increases the resistance of semiinsulating GaAs. The calculations show that these effects are due to the Frenkel pairs (FP) generated by radiation. A high degree of ionization of GaAs atoms significantly reduces energies of potential barriers of diffusion, FP recombination, and electrical impurity activation.     

Evolution of Anion and Cation Excitons in Alkali Halide Crystals

The manifestations of the existence of free anion excitons, the processes of their self-trapping, and the coexistence of mobile and self-trapped excitons (STEs) in wide-gap alkali halide crystals are reviewed. The radiative channel of decay of anion excitons, yielding luminescence, and a particular type of nonradiative channel with the creation of elementary Frenkel defects (FDs) are considered. We analyzed the criteria for the efficiency of this channel for defect formation, possible mechanisms for the decay of self-trapped excitons with the production of neutral and charged anion Frenkel defects, and the processes of multiplication of electronic excitations in alkali halide crystals. Particular attention is paid to the decay of cation excitons, including from the point of view of the possibility of the low-temperature creation of elementary Frenkel defects in the cation sublattice of alkali halide crystals.     

Luminescence and defect creation in LiF and LiF-Na

The topic is the radiative and nonradiative decay of electronic excitations with the creation of radiation defects in LiF. A luminescence band with a peak at 3.45 eV is observed in the x-ray luminescence spectrum at 4.2 K; it may be assigned to the luminescence of a self-localized exciton. In LiF-Na, additional luminescence bands with peaks at 2.0 and 2.4 eV are observed; these may be attributed to the luminescence of electron excitations localized around the Na' ion. The formation of Frenkel defects at 4.2 K in LiF and LiF-Na is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 28–32, December, 1994.     

Hybrid excitons in organic–inorganic semiconducting quantum wells in a microcavity

We investigate theoretically the optical properties of composite organic–inorganic semiconductor quantum wells. These properties are dominated by hybrid Frenkel (or charge-transfer) and Wannier–Mott excitonic states. An important effect is the possibility of using the Stark shift to tune the resonance between Frenkel and Wannier–Mott excitons. This fact is very important from a practical point of view because it may be difficult to grow such a structure exactly at resonance. We also discussed the coupling of Frenkel or charge transfer and Wannier–Mott exciton through a microcavity photon. We evaluate the hybrid exciton-polariton Rabi splitting. In the strong coupling regime the Rabi splitting depends essentially on the oscillator strength of the Frenkel or charge-transfer exciton.     

Photoannealing of defects in irradiated zinc oxide

The change in the absorption and luminescence spectra of polycrystalline ZnO at 100°K following electron irradiation and photoannealing with. F^– and F^+-light was investigated. It was found that following irradiation, absorption bands of intrinsic point defects appear which after photoannealing are converted into bands of lower energy, tentatively attributed to complex electron centers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 3–7, September, 1985.The authors wish to express their thanks to E. V. Komarov for invaluable help in the investigation.     

Quantum-chemical simulation of Frenkel pairs separation in a LiF crystal

Semiempirical quantum-chemical (INDO) simulation of the creation of the primary Frenkel pairs of defects in a LiF crystal based on the Pooley-Smoluchowski mechanism of the self-trapped exciton annihilation has been undertaken. The conclusion has been drawn that this mechanism can be operative from the viewpoints of both the time and the energy needed to create the F, H pair in terms of this mechanism. Unlike the Toyozawa's model an initial vibrational excitation is not bottleneck of the decay process (cf. [8]).     

Influence of Oxygen Vacancies on the Luminescence Spectra of Y2O3 Thin Films

Luminescence spectra of Y₂O₃ thin films annealed in air and in vacuum are investigated. It is established that the presence of oxygen vacancies leads to a decrease in the intensity of the luminescence band with a maximum at 3.4 eV (related to emission of selflocalized Frenkel excitons describing the excited state of a molecular ion (YO₆)^9–) and of the luminescence band with a maximum at 2.9 eV (related to the anion sublattice). It is revealed that the oxygen vacancies also lead to a decrease in the luminescence intensity in the 2.60, 2.35, 2.10. 1.90, and 1.70 eV bands that are related to radiative recombination in the donor–acceptor Y³⁺–O^2– pairs. The donor–acceptor distances are calculated.     

Spectral-Luminescent Properties of Cationic Indotricarbocyanine Dyes in Cancerous Cells and Solutions

The spectral-luminescent properties of symmetrical indotricarbocyanine dyes with the same cation and different anions (Br^–, BF₄ ^–) in cancerous HeLa cells, an isotonic solution of NaCl, and organic solvents have been investigated. It is shown that when these dyes are present in cells in nontoxic concentrations, they are in a monomeric state, they are localized in the region with a low dielectric permeability, bonded to biological macromolecules, and do not make contact with an aqueous medium. It has been established for the first time that the molecules of the polymeric dyes are present in cells predominantly in the form of contact ion pairs, and in a dye with the Br^– anion the presence of free cations or solvately separated ionic pairs is revealed. It is shown that the shift of the ion equilibrium toward contact ion pairs for the dyes in the cells (as compared to the low-polarity organic solvents) can be due to the fairly high concentration of salts in the biosystem.     

A defect model for undoped and tellurium doped gallium arsenide

The introduction of two new defects, both 〈100〉 split-interstitials, can explain the experimental results obtained from annealing studies on undoped and tellurium doped gallium arsenide. This model concludes that the arsenic and gallium Frenkel reactions only occur at annealing temperatures above 4&#x0303;00 and 9&#x0303;00°C respectively and suggests that gallium diffuses by occupying split-interstitial sites. This model also suggests that interestitial arsenic may occupy the hexagonal interstitial sites.     

Influence of helium on the evolution of irradiation-induced defects in tungsten: An object kinetic Monte Carlo simulation

Understanding the evolution of irradiation-induced defects is of critical importance for the performance estimation of nuclear materials under irradiation. Hereby, we systematically investigate the influence of He on the evolution of Frenkel pairs and collision cascades in tungsten (W) via using the object kinetic Monte Carlo (OKMC) method. Our findings suggest that the presence of He has significant effect on the evolution of irradiation-induced defects. On the one hand, the presence of He can facilitate the recombination of vacancies and self-interstitial atoms (SIAs) in W. This can be attributed to the formation of immobile He-SIA complexes, which increases the annihilation probability of vacancies and SIAs. On the other hand, due to the high stability and low mobility of He-vacancy complexes, the growth of large vacancy clusters in W is kinetically suppressed by He addition. Specially, in comparison with the injection of collision cascades and He in sequential way at 1223 K, the average sizes of surviving vacancy clusters in W via simultaneous way are smaller, which is in good agreement with previous experimental observations. These results advocate that the impurity with low concentration has significant effect on the evolution of irradiation-induced defects in materials, and contributes to our understanding of W performance under irradiation.