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.     

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.     

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.     

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.     

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.     

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.     

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.     

The fractional ionic character of alkali and silver halide crystals

The fractional ionic character of alkali and silver halide crystals is defined in terms of the deviations from the additivity rule for polarizabilities of ions. The electronic polarizabilities of ions are calculated using an empirical relationship according to which the electronic polarizability of an ion can be assumed to be directly proportional to the cube of its radius. The calculated ionicities indicate that the alkali halides are nearly or more than 90% ionic and silver halides are much less ionic which is also evident from the Phillips ionicity scale.     

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.     

Bombardment of alkali halide crystals with medium-energy atoms

The sputtering characteristics of KCl and NaCl alkali halide single crystals by bombardment with argon and mercury atoms are investigated. The sputtering coefficients are measured in the energy range 0.5–3 keV. Anisotropy is found in sputtering at various incident angles of the beam. The yield of charged particles during bombardment of thin specimens (of the order of 8–20 m) of KCl single crystals is investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 55–59, July, 1984.     

An equation of state for alkali halide crystals

Nonparametric equations of state are obtained for NaBr, KBr, RbCl, and RbBr crystals with B1 and B2 lattices over a wide pressure range. Binding energy, lattice parameters, and free crystal compressibility are calculated together with phase-transition pressures and volume change on transition from B1 to B2 lattice. Calculated results agree well with experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 101–105, January. 1976.