Evaluation of the photoelastic constants of ionic crystals

A theoretical model, for evaluating the photoelastic constants of ionic crystals, has been developed by considering the short range repulsive interactions between nearest and next nearest neighbours, the van der Waals potentials and three body interactions as well as taking into account the electronic polarizabilities of cations and anions. The model has been applied to calculate the photoelastic constants of RbC1, RbBr and RbI crystals. The results obtained in the present study are in good agreement with experimental data.     

Calculation of the components of the photoelastic effect in alkali-halide crystals

Calculations are made to determine the contribution of Lorentz-field and Coulomb-field anisotropy in alkali-halide crystals to the photoelastic coefficients p₁₁–p₁₂ and p₄₄ as well as to the piezopolarizability constants K, K, K_x, K_z, and K₀. These calculations are based on Mueller's theory of the photoelastic effect, with data on iron polarizability and photoelastic coefficients. The applicability of Mueller' s theory to the complete series of alkali-halide crystals is analyzed. Criteria are established for the ionicity of bonds in these crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 28–34, December, 1975.     

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.     

The effect of ion-ion correlations on the ionic hall effect in crystals

Abstract

Because an external magnetic field does no work on moving charges, it is argued that independently migrating point defects in an ionic crystal cannot exhibit any Hall effect, to first order. This is in sharp contrast to the cooperative motions displayed by superionic conductors. Experimental results on interstitial migration in AgBr and α—AgI agree with these expectations.     

Transverse dislocation analog of the Nernst-Ettingshausen effect in ionic crystals

We show that in ionic crystals under the action of a temperature gradient and an external magnetic field on the cloud of point defects surrounding a charged dislocation, a phenomenon arises which is analogous to the Nernst-Ettingshausen effect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 80–83, January, 1996.     

Analysis of ionicity parameters and photoelastic behaviour of AB type crystals

Values of ionicity parameters have been obtained using the revised energy gap model due to Grimes and Cowley. We have used a power law as well as an exponential law for representing the dependences of homopolar energies on interatomic distances. Values of fractional ionic character of the chemical bond are calculated in 68 A^NB^8–N type diatomic solids. The heteropolar energy parameters have been found to exhibit systematic relationships with the interatomic separations for diatomic solids with the common cations in I–VII and II–VI crystals. The strain derivatives of electronic dielectric constant related to the photoelastic behaviour of solids have also been studied with the help of the ionicity parameters within the framework of Phillips–van Vechten dielectric theory. The results obtained from the power law and the exponential law are in close agreement with each other.     

The role of auger effect in thermostimulated phenomena in ionic crystals

Absorption and thermostimulated luminescence simultaneously with electric conductivity in KI crystals irradiated with X-rays at L.N.T. were measured. The kinetics of these and many other similar results obtained in other alkali halides are interpreted on the basis of Auger process occurring in consequence of the recombination of free holes with localized electrons inF-centers.     

Possible isotope effect on Frenkel defect production in ionic crystals

An argument is given for the possibility that differing isotope masses for anions in ionic crystals cause a sizeable effect in defect production.     

Analysis of interdiffusion via isolated vacancies in strongly ionic crystals

In this paper, we analyse chemical interdiffusion in strongly ionic crystals for diffusion couples AY _m –BY _m , where A and B have the same charge numbers. We employ the exact sum rule given by Moleko and Allnatt relating the phenomenological coefficients for diffusion in the multicomponent random alloy via the agency of monovacancies. It is shown that the ratio of the intrinsic diffusivities can be expressed very simply in terms of the atom–vacancy exchange frequencies without correlation terms. For the case of an immobile anion sublattice and making use of a highly accurate diffusion kinetics theory due to Moleko et al., it is shown that the interdiffusivity is principally proportional only to the off-diagonal phenomenological coefficient relating the two cations.     

Electrooptical and photoelastic phenomena in ferroelectric crystals (survey)

Relations between optical and ferroelectric properties are discussed as a basis for analyzing and generalizing this author's and other authors' research data pertaining to the electrooptical, the photoelastic, and other effects in ferroelectric crystals.     

Study of dielectric and photoelastic properties of fluorite crystals

Summary The photoelastic constants and strain derivatives of the static and high-frequency dielectric constants of fluorite structure solids have been computed by means of a three-body potential which consists of the long-range Coulomb, three-body interactions, the short-range van der Waals attraction and overlap repulsion operative up to the second-neighbour ions and represented by the Hafemeister and Flygare potential. The agreement between available experimental and our calculated values of photoelastic constants and strain and pressure derivatives of the static dielectric constants is reasonably good.

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Riassunto Le costanti fotoelastiche e le derivate di deformazione delle costanti dielettriche statiche ed ad alta frequenza dei solidi con struttura dei cristalli di fluoruro sono state calcolate per mezzo di un potenziale a tre corpi che è formato dall'interazione di Coulomb e a tre corpi a lungo raggio e dall'attrazione di van der Waals a breve raggio e la repulsione di sovrapposizione operativa fino al secondo ione adiacente e rappresentata dal potenziale di Hafemeister e Flygare. L'accordo tra i valori sperimentali disponibili e i nostri valori calcolati delle costanti fotoelastiche e delle derivate di pressione e deformazione è ragionevolmente buono.

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Резюме Используь трех-частицный потенциал, который содержит длиннодействующее кулоновское и трех-частичное взаимодействие и короткодействующее притяжение ван дер Ваальса и отталкивание вплоть до второго ?соседа?, описываемое потенциалом Хафемайстера и Флигера, вычисляются фотоупрутие постоянные и производные по деформации статических и высокочастотных диэлектрических постоянных для твердых тел со структурой флюорита. Получается удовлетворительное согласие между имеющимися экспериментальными данными и вычисленными нами фотоупругими постоянными и производными статических диэлектрических постоянных по деформации и давлению.

     

Positron-electron doublet in ionic crystals

The states of a positron-electron doublet in ideal ionic crystals are analyzed, taking into account the Coulomb interaction of the particles, with their localization both on the immediately nearest ions of different sign, and on the ions next after them. The exactly-solvable quasi-one-dimensional model of a crystal is considered in detail. The wave functions and the characteristic energies of the doublet are obtained. It is shown that states of two types are realized, one of which has a threshold nature, and arises only for definite magnitudes of the Coulomb integrals. For the ground state of the doublet, one can with good precision take the state corresponding to the computation of the Coulomb interaction for localization of the particles on the nearest ions. The role of the Coulomb interactions of higher order are discussed.Translated from Izvestiya Vysshikh Uchebnyk Zavedenii, Fizika, No. 12, pp. 56–61, December, 1989.     

Metal colloids in ionic crystals

Small metallic particles with diameters in the range 1–100 nm have interesting properties which can sometimes be very different from those of bulk metals. Such colloidal particles play an important part in fields which range from catalysis to radiation damage in compound solids, and also have an intrinsic interest since in some respects they can be regarded as a state of matter intermediate between that of a molecule and a solid. In ionic crystals colloids can be produced either by irradiation or by the introduction of a stoichiometric excess of the metal constituent. In either case the colloids form as a result of the aggregation of fundamental point defects, but it is only fairly recently that a reasonably coherent picture of these processes has emerged and the properties of the colloids have been related to those of small metallic particles studied for other reasons. This review discusses the progress which has been made through the use of a large number of different techniques to study the properties of colloids in ionic crystals and related media such as glasses. Particular emphasis is placed on the optical, magnetic resonance and kinetic properties of colloids, and on the discussion of results in the wider context of the physics of small particles.     

Positron thermalization in ionic crystals

The time required for the thermalization of positrons in ionic crystals is estimated. It is shown that the proton-thermalization time in an ionic crystal is much shorter than the shortest positron lifetime with respect to annihilation, so positron annihilation can be used to study the energy spectrum of valence electrons in ionic crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 68–72, April, 1971.     

Positron annihilation in ionic crystals

Positron lifetime measurements were made on Ca²⁺ doped NaCl, Ba²⁺ doped NaCl and X-ray- and additively-coloured KCl crystals. In the NaCl(Ca) system a correlation which could be approximated by a power equation was found between the dopant concentration and the positron annihilation parameters. In the NaCl(Ba) system the decay parameters do not change significantly as expected and no significant difference in the decay parameters of the coloured and uncoloured crystals could be found. Paper C 4 presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

Charged dislocations in ionic crystals

Experiments and theories concerning charged dislocations in alkali halide crystals are reviewed in detail, with particular attention to the way in which the experiments should be interpreted and to the range of applicability of the sweep-up and various forms of thermal-equilibrium models. Possible effects on mechanical properties and internal friction are analysed. The transient and steady-state effects of plastic deformation on ionic conductivity are described and new interpretations involving charged dislocations are proposed. A survey of results on the scattering of light by alkali halide single crystals leads to the conclusion that charged dislocations do not play an important role.

Evidence about charges on surfaces and on dislocations in AgCl and AgBr is reviewed and compared with that for alkali halides. Comparisons are also made with MgO, the CsCl and CaF₂ structures, semiconductors and ice.     

Electrical breakdown in ionic crystals

A model of formation of an electrical discharge channel in alkali halide crystal is presented. Electrons are generated to the conduction band through cascade Auger transitions in the valence band of the dielectric. The breakdown channel consists of melt and charged layers. The model explains the crystallographic direction and velocity of the discharge channel, as well as generation of prebreakdown current, without resorting to the mechanism of impact ionization of the valence band of the crystal by electrons from the conduction band.     

Nature of the luminescence centers in ionic crystals

The main attention is paid to the following questions: 1. The possible models of the luminescence centers in activated alkali halide crystals, 2. the experimental evidence of the presence of various types of the luminescence centers, 3. the nature of the excitations of the simplest luminescence centers in alkali halides activated by mercury-like ions and by some other ions. Mainly two possibilities of views: the quasi-free ions case on the one hand and the quasi-molecular models on the other are discussed in detail.