Analysis of the photoelastic effect in ionic crystals

An analysis of the photoelastic effect in ionic crystals has been presented within the framework of Clausius-Mossotti theory of the dielectric constant. The values of the strain derivative of the electronic dielectric constant have been calculated in alkali halides and MgO crystals by taking into account the variation of electronic polarizabilities with compressive stress. The results obtained are found closer to the experimental values. The photoelastic behaviour of MgO crystal is predicted to be of opposite nature to that of alkali halides, in conformity with the experimental observations.     

Positron states in alkali-halide single crystals

Detailed consideration is given to the annihilatory decay of positron states in real alkali-halide single crystals of defective structure. Representations of the exciton-positron states in the matrix of the crystal and close to a lattice defect are analyzed. On the basis of these representations, expressions are derived for the yield of the three-quantum decay process and the intensities of the components contributing to two-quantum annihilation; the expressions agree with existing experimental data. It is shown that the greatest contribution to the three-quantum decay arises from an exciton-orthopositronium state located in the vicinity of the defect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 76–79, October, 1974.     

Off-center substitutional ions in alkali-halide crystals

The potential wells of alkali and halogen ions as impurities in alkali halides are investigated by means of a deformation-dipole model with single-ion parameters. The calculations indicate that Li⁺ is off center in seven and F^? in six of the sixteen rocksalt-type crystals considered. In about half of the off-center systems the configuration has D_4h symmetry if the impurity is kept at its lattice position. The minimum-energy configuration of the off-center system KCl:Li⁺ is calculated in detail, and the influence of hydrostatic pressure is investigated. Our results are in qualitative agreement with experiment.     

Formation of near-defect excitons in alkali-halide crystals

Pulsed-spectrometric research shows that the presence of defects, including those that are electrically neutral with respect to the crystal lattice, has a significant influence on the distribution of the radiation-induced electron excitations. This is evident in the electron-excitation sink in the vicinity of defects and the formation of localized excitons around impurities in ionic crystals. This influence of defects on the electron-excitation distribution is probably due to deformation of the lattice in the region of the defect. It is shown that, in the region of the defect, there is oscillating potential relief, the presence of which leads to the capture of charge carriers and their localization in this region. Lattice distortion because of deformation in the region of the defect changes the mutual distribution of the ion pairs. This creates conditions for the effective conversion of electronic excitations to localized near-defect dihalide excitons, which are different from those created in an ideal lattice. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 94–108, November, 1996.     

On the mechanism of the electric field effect on dislocation mobility in alkali-halide crystals

The dependence is investigated of the velocity of motion of edge and screw dislocations on the applied shear stress in annealed and quenched NaCl and KC1 crystals with various impurity content under the action of an electric field of up to 50 kV/cm. It is shown that in impure crystals the action of the field is determined by its effect on damping centers, while in pure crystals, by its effect on charged dislocations. The interaction force of an edge dislocation with impurity-vacancy dipoles of various orientation is calculated. The charge density and sign of a dislocation is estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 16–21, May, 1975.     

Bulk modulus of alkali-halide mixed crystals

By using the bulk moduli of the pure and components we calculate, for various compositions, the isothermal bulk modulus of the following mixed crystals: NaCl-NaBr, KCl-KBr and KBr-Kl. The calculated values are in good agreement with the most recent experimental data.     

Theory of the photoelastic effect in ZnO,ZnS and CdS crystals

It has been a general opinion that the Clausius-Mossotti dielectric theory is not capable of explaining the observed photoelastic behaviour of partially ionic crystals.However in the present communication we show that by making account of the variation of polarizabilities under compressive stress within the framework of the Clausius-Mossotti theory it is possible to obtain a reasonable agreement with experimental data on the photoelastic behaviour of ZnO, ZnS and CdS crystals.     

Metallization of alkali-halide crystals under superhigh pressures

An estimation, based on thermodynamical considerations, of the pressure under which the NaCl crystal transforms into the metallic state, was carried out. Using the method of composite atomic orbitals, the thermodynamic potentials of dielectric and metallic phases of the NaCl crystal were calculated. The equations of state were obtained for both phases in a wide range of pressures, from 0 to 2000 kbar. From the point of intersection of the curves of thermodynamic potentials of two phases, the determination was made of the pressure of metallization equal to 1350 kbar. The temperature effects were not taken into account.Translated from Izvestiya VUZ, Fizika, No. 3, pp. 57–62, March, 1973.     

Evidence for a positron-exciton insulator in alkali-halide crystals

The angular correlation of annihilation photons for single crystals of 60% KCl-40% KBr solid solutions and X-irradiated NaBr crystals were measured. For 60% KCl-40% KBr samples the measurements of the angular correlation curves were carried out under an external magnetic field of 13.0 kG. The experimental data and theoretical estimations show that thermalized positrons, captured by free cation vacancies, form the positron centre of F₄-type; if the positrons are localized near the complex defects, containing the cation vacancies, the formation of peculiar “positron-exciton complex” of the positronium-like type is realized.     

Dislocation interaction with radiation defects in alkali-halide crystals

A mechanism of room-temperature irradiation-induced strengthening of alkali-halide crystals (AHCs) is suggested. It is shown that the deformation-stimulated luminescence is a result of the destruction of hole color centers, which strengthen AHCs, by dislocations.     

Positron annihilation in alkali-halide crystals after electron irradiation

The effect of electron beam irradiation on the positron annihilation in KCl, KBr, NaCl single crystals was investigated. The narrowing of the angular correlation curves increases up to the saturation value with the absorbed radiation dose. The shape of the received narrow components differs from the narrow component measured in the additively colored samples. This phenomenon shows the considerable influence of the hole centres upon the positron annihilation in ionic crystals. Paper presented at 3rd Internat. Conf. Positron Annihilation, Otaniemi, Finland (August 1973).