The theory of thermionic conductivity in alkaline halide crystals

The kinetic equations developed by the authors are used to explain the phenomenon of thermionic conductivity. The relaxation of A- and K-dipoles in strong fields is considered. The binding energy of the polar complexes is investigated within the framework of elasticity theory. A comparison is made with the experiments of Bucci and Riva.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 16–19, October, 1973.     

The theory of thermionic conductivity in alkaline halide crystals

Kinetic equations are presented which describe the establishment of polarization in ionic crystals of cubic symmetry. A solution is found for the strong field region and the depolarization current density connected with dipole dis orientation is calculated. The theory is compared with the experiments of Bucci and Riva.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 9, pp.65–70, September, 1973.     

Dielectric properties of alkali halide and alkaline earth oxide crystals

The deviations of ionic charge and compressibility from their nominal values in alkali halides and alkaline earth oxide crystals have been discussed and explained in terms of a theory based on shell model and exchange charge interactions originally developed by Dick and Overhauser. The exchange charge polarization parameters have been evaluated in the present study using the appropriate values of overlap integrals. It has been concluded that the second neighbour short range forces have significnant magnitudes in alkaline earth oxides.     

Dielectric effect and electrical relaxation in mica crystals

Results are given of an investigation of the dielectric state and electrical relaxation in crystals of dioctahedral muscovite mica. By comparing experimental and theoretical data for the spectra of thermostimulated depolarization (TSD) currents, the temperature dependences of the conductivity during linear heating log = f(1/T) and results of a study of the transfer of the electrical relief during contact under pressure between polarized and nonpolarized crystals, a conclusion is drawn concerning the drift mechanism of relaxation of the dielectric state in mica.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 99–101, October, 1991.     

Effect of microdefects in crystal lattice on certain luminescence relaxation processes in alkali halide phosphor crystals

A further development of ideas on an ionic mechanism for the activation of certain relaxation processes in recombination luminescence of alkali halide phosphor crystals is presented. It is shown that each stage of the thermal discoloration process in the phosphor crystals corresponds to disruption of the F-band in an isolated microsection of the specimen. The effect of the degree of plastic deformation on the relative proportions of F-center concentrations on various microregions of the crystal was investigated. The effect of impurity ions, of their concentration, arid of the crystal plastic deformation on the photothermal stability of the F-centers and on the thermal quenching of luminescence during photoexcitation of activator color centers in NaCl-Ag phosphor crystals was also studied.     

Healing of cracks in alkali halide crystals

The healing of micro-and macrocracks under local heating and x-ray irradiation of LiF single crystals is studied. The main features revealed in crack healing due to local actions are described. The contribution of the plastic zone formed upon the arrest and healing of a crack to the strength of the crystal is estimated.     

Microhardness studies in alkali halide mixed crystals

Microhardness measurements done in KCl, KBr and in different compositions of KCl-KBr mixed crystals show that it varies nonlinearly with composition. In order to investigate the nature of defects, several techniques such as etching, ionic conductivity and dielectric loss have been employed which showed that the mixed cristals of KCl-KBr are more defective, containing a high concentration of dislocations, low-angle grain boundaries and vacancies as compared to the end products KCl and KBr. These imperfections appear to be responsible for the nonlinear variation of microhardness in mixed crystals. The microhardness studies also revealed that the difference in size of the ions constituting the mixed system are responsible for the internal strains which in turn give rise to imperfections affecting the microhardness of mixed crystals.     

Point defect interaction in alkali halide crystals

Abstract

Molecular statics method is used to determine the energy and configuration peculiarities of vacancy-bivacancy interaction in alkali halide crystals. It is shown that high level of local vacancy supersaturation defines the possibility of cluster formation. The study of carrier traps by luminescence methods is used to reveal vacancy clusters.     

Point defects in alkali halide mixed crystals

Measurements of self diffusion coefficients of ⁴²K, ⁸⁵Rb and ¹²⁵I and measurements of ionic conductivity were carried out on single crystals of the (Rb₂K_1-Z)I solid solutions (z varying from 0 to 1). The self-diffusion coefficient measurements (T=875 K) indicate that two types of cation have the same mobility and that this mobility is about twice that of the anion. All these mobilities are higher for the solid solutions than for the pure components, KI and RbI. The ionic conductivity was analysed using the Schottky defect model. Interactions between defects were taken into account in this analysis. Thus, enthalpies and entropies of formation and migration of vacancies were determined. Examining the variations of these parameters as functions of composition z at a given temperature shows that the density of vacant sites is significantly higher in the case of solid solutions than in the case of pure components (for example, this density is about twice as large for (Rbo.sK_0.5)I as for KI or RbI).     

Anion polarizability functions in alkali halide crystals

Anion polarizabilities in alkali halide crystals are analysed as a function of interionic separation R. The anion polarizability is treated as a function of the anion and cation radii, with its partial derivatives approximated by those with respect to R for fixed cation and anion, respectively. With pressure derivatives of the ionic radii deduced from the crystal compressibility, assuming transferability among crystals, the polarizability derivatives with respect to ionic radius yield pressure derivatives of the polarizability that agree with experiment to within a factor of two. These results offer a useful means of predicting the pressure dependence of dielectric data.     

Proton dechanneling in alkali halide mixed crystals

The effects of the lattice distortion in KCI-KBr mixed crystals on the dechanneling of 1.5 MeV protons are studied by means of backscattering. The dechanneling rate vs. the composition curve has a maximum at about 1:1 mixture of KCI and KBr. The lattice distortion in the mixed crystals is estimated from the dechanneling rate, and the result agrees well with that obtained by an X-ray measurement. The effective cross-section for the dechanneling caused by the lattice distortion is obtained, and compared with the results of a calculation based on a simple model.     

Theoretical study of Ti0 and Pb+ centers in alkali halide and alkaline earth fluoride type crystals

A theoretical study has been carried out on the hfi of Pb⁺ defects in KCl, CaF₂ and BaF₂ using an earlier developed crystal field model. Experimental results can be explained in the same way as was done for Tl⁰ in KCl. However some parameters seem to have a less physical meaning than in the case of Tl⁰. A new method is in progress using the ASW band structure procedure in order to estimate genuine solid state effects. Preliminary results are encouraging.     

Debye temperature of alkali halide crystals

The Debye temperature _D is calculated from the elastic constants for 18 alkali halide crystals. Comparison of the results with calorimetric data _D reveals an agreement within the experimental error. The temperature dependence of _D is determined for NaCl, NaBr, KCl, and KBr crystals from the elastic constants, and this dependence is compared with the calorimetric data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 3, pp. 89–94, March, 1971.     

Optical breakdown of alkali halide crystals

It is shown that the metallization of a dielectric in the region of its interaction with laser radiation is a substantiated mechanism of laser damage of wide-gap dielectrics. Calculations of the radiation pressure produced by high-power laser radiation and the pressure of external bulk compression at which the dielectric energy-gap width becomes zero and which is calculated on the basis of the self-consistent statistical electron theory of ionic crystals have shown that these pressures are of the same order of magnitude.     

Lattice energy of alkali halide crystals

A new repulsive term in the ionic interaction potential ψ(r) = Ar^?ne^?r/gl, is suggested and the three unknown parameters A, λ and n are evaluated. Lattice energies of alkali halide crystals are calculated using this form. The results agree fairly well with the experimental values.     

Field excited phosphorescence in X-irradiated potassium halide crystals

Light emission is observed at room temperature from X-irradiated KC1, KBr and KI samples placed between insulated electrodes under high voltage excitation. The slow response of the effect to the electric field shows that metastable states are involved in the recombination process. The study of the brightness waves and the dependence of the light intensity on the temperature demonstrate that the excitation of the luminescence centers is obtained through a field assisted thermal ionization process.