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).     

Point defect generation,nano-void formation and growth. I. Validation

The volume fraction of point defects generated as a function of plastic shear strain squared, γ², was derived from crystal plasticity concepts. The evolution was determined from the stress–strain values using a new constitutive relation which replicates the measured behavior with at least two fitted loci. Assuming that nano-voids form by clustering of vacancies, the nano-void diameter was found to be proportional to their spacing and shear strain with the constant being characteristic of point defect production during deformation. The predicted amount of point defect generated was validated using the previously determined resistivity of [100] copper single crystals deformed at 4.2?K and annealed at 296?K. Similar analysis of super-pure polycrystalline copper data affirmed that the dynamic annihilation parameter extrinsically incorporated in the new derivation is larger due to formation of slip clusters. Moreover, the temperature dependence of the mean slip-distance to inter-forest spacing ratio at Stage II to III transition indicates that the thermally activated drag of vacancy-creating jogs occurs above 150?K. For polycrystalline aluminum deformed at 296?K, it was concluded that the nuclei of the nano-voids were not part of the evolving dislocation array but were embedded in the grown-in microstructure. This hypothesis is pursued in the accompanying paper, Part II, and its prediction results in a criterion for ductile failure.     

Nonlinear polarization spectroscopy of ions interaction potential in alkali halide crystals

Anisotropic part of optical cubic nonlinear susceptibility in several alkali halide crystals was measured by new polarization technique and this data were used for defining the role of short-range forces in ions interaction. The model of ions interaction potential is discussed.     

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.     

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.     

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.     

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.     

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 states of a hollow cylinder in two-dimensional phononic crystal

Point defect states in two-dimensional phononic crystal of a hollow mercury cylinder in a water host are studied.An improved plane expansion method combined with the supercell technique is used to calculate the band gaps and the pressure distribution at the defect position.The sonic pressure of defect modes shows that the waves are localized at or near the defect.As the filing fraction increases,more defect modes appear in the band gaps.     

Modeling of the evolution process of primary Frenkel pair defect formation in alkali halide crystals

The evolution process of primary Frenkel pair defect formation is modeled in alkali halide crystals by the Monte Carlo method. A systematic analysis is carried out of the temperature and time dependence of the efficiency of Frenkel pair accumulation in ionic crystals. The effect of the local heating of a crystal microvolume and of the interaction energy between the components of a F, H-pair on the defect-formation process of primary Frenkel pairs is investigated. It is shown that the local temperature and interaction energy between the F, H-pair being formed are basic factors which determine the relaxation process of correlated Frenkel pairs in ionic crystals following pulsed irradiation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenni, Fizika, No. 2, pp. 19–23, February, 1985.     

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.     

Temperature and impurity concentration dependences of the efficiency of Frenkel defect accumulation in alkali halide crystals

A simple phenomenological model of the efficiency of accumulation of F centers in alkali halide crystals as affected by temperature and homologous cation impurity is considered. The conclusion is drawn that the temperature dependence is probably due to elastic interaction of primary Frenkel defects — F , H centres affecting their diffusion-controlled separation (similar to metals) whereas the impurity concentration dependence is due to the tunneling recombination of F , H_A pairs.     

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.