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.     

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.     

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.     

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.     

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).     

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.     

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.     

Electronic polarisabilities of ions in alkali halide crystals

Tessman, Kahn and Shockley calculated the electronic polarisabilities of ions in alkali halide crystals using the long wavelength limiting values of the visible light dielectric constants. We have recalculated these widely used polarisabilities using the more accurate room-temperature dielectric constant data of Lowndes and Martin and a better minimisation procedure of Pirenne and Kartheuser. We have also calculated for the first time the low temperature values of these polarisabilities. The computed values of the polarisability in ų are Li⁺ 0·029, Na⁺ 0·285, K⁺ 1·149, Rb⁺ 1·707, Cs⁺ 2·789, F^? 0·876, Cl^? 3·005, Br^? 4·168, I^? 6·294 at 300°K and Li⁺ 0·029, Na⁺ 0·290, K⁺ 1·133, Rb⁺ 1·679, Cs⁺ 2·743, F^? 0·858, Cl^? 2·947, Br^? 4·091, I^? 6·116 at 4°K. The relative standard deviations for all the alkali halides are 1·20 and 1·43 per cent at 300°K and 4°K respectively justifying the additive nature of the individual ion polarisabilities.     

Photoelectric work function changes during heterogeneous adsorption of alkali halide molecules on alkali halide crystals

The changes of photoelectric work function during heterogeneous adsorption of alkali halide crystals had been observed. They were attributed to the non-zero effective dipole moment of the adsorbed layer, and were estimated using the Helmholtz formula. Measurements of this effect showed semi-quantitative agreement with the theoretical model. The divergences between experimental data and theoretical calculations were briefly discussed.     

Four-wave mixing in alkali halide crystals and aqueous solutions

Noncollinear phase-matched nonresonant four-photon frequency mixing _p+ _p– _{L s} in crystals and aqueous solutions of LiCl, CsCl, KF, and KI is studied. The concentration of the aqueous solutions is varied between 0.5 mol/l and saturation. Picosecond laser pulses of a mode-locked Nd-glass laser are applied as pump pulses. The energy conversion of laser light at frequency _L to frequency _S is measured and the nonlinear susceptibilities ⁽³⁾ are calculated. The dependence of the hyperpolarizabilities on concentration is analysed and gives information on the solute-solvent interaction.     

Low temperature space charge polarization in alkali halide crystals

Evidence is presented to indicate the presence of linear space charge polarization in single crystals of KCl and NaCl in the temperature range between liquid nitrogen temperature and 350 K. Polarization phenomena, both dipolar and space charge, were studied by means of TSPC, ITC and by the testing of Ohm's law and the Superposition principle. The results are interpreted in terms of the formation of a linear space charge, possibly caused by the trapping of ionic charge carriers within the bulk of the material.