Atomic structure of alkali halide surfaces

The atomic structure of surfaces of alkali halide crystals has been revealed by means of high-resolution dynamic force microscopy. True atomic resolution is demonstrated both on steps surrounding islands or pits, and on a chemically mixed crystal. We have directly observed the enhanced interaction at low-coordinated sites by force microscopy. The growth of NaCl films on metal surfaces and radiation damage in a KBr surface is discussed based on force microscopy results. The damping of the tip oscillation in dynamic force microscopy might provide insight into dissipation processes on the atomic scale. Finally, we present atomically resolved images of wear debris found after scratching a KBr surface. PACS 68.37.-d; 68.37.Ps; 75.55.Fv     

On the structure of alkali halide solutions

Summary The adiabatic compressibility of NaCl, NaBr and NaI water solutions has been determined by measuring the sound velocity as a function of both concentration and temperature. A simple model has been employed to interpret the data in terms of size of hydration shells. Reasonably good agreement has been found with neutron and X-ray diffraction results.

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Riassunto La compressibilità adiabatica delle soluzioni acquose di NaCl, NaBr ed NaI è stata misurata determinando la velocità del suono in funzione di temperatura e concentrazione. Per interpretare i dati è stato impiegato un modello semplice che tiene conto delle sfere di idratazione degli ioni. Tale modello risulta in soddisfacente accordo con i dati ricavati da un esperimento di diffrazione di raggi X e neutroni.

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Резюме Определяется адиабатическая сжимаемость водньх растворов NaCl, NaBr и NaI, используя измерения зависимостей скорости звука от концентрации и температурь. Предлагается простая модель для интерпретации данньих в терминах размера сферьі гидратации. Получется хошее согласие с результатами дифракции нейтронов и рентгеновских лучей.

     

On the structure of alkali halide solutions

Summary The structure factors of NaI solutions have been determined by employing both neutron and X-ray scattering. The results have been analysed in terms of the structure of hydration spheres of both Na⁺ and I⁻ ions. No concentration or temperature dependences of the size of the hydration shells have been evidentiated in the present experiment, while the structure of the free water appears to be affected, at least at small distances.

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Riassunto I fattori di struttura delle soluzioni acquose di NaI sono stati misurati impiegando la diffrazione di raggi X e neutroni. I risultati sono stati analizzati per caratterizzare le sfere di idratazione degli ioni Na⁺ ed I⁻. Nessuna dipendenza dalla temperatura o concentrazione è stata evidenziata per le sfere di idratazione, mentre alle piccole distanze si osservano effetti della concentrazione sulla sutruttura dell'acqua libera.

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Резюме Используя дифракцию нейтронов и рентгеновских лучей, определяются структурные факторы растворов NaI. Полученные результаты анализируются в терминах структуры сферы гидратации ионов Na⁺ и I⁻. В проведенных экспериментах не обнаружено зависимостей от концентрации или темиературы для сферы гидратации, тогда как, по крайней мере, на малых расстояниях обнаружено влияние концентрации на структуру свободной воды.

     

Atomic-scale structure of alkali halide solid solutions

A simple model is proposed to explain the crystallographic properties of solid solutions of the alkali halides. The random system is simulated with a periodic superlattice whose energy is evaluated with the Born-Mayer model generalized to include ionic polarization. The resulting average lattice parameter and anioncation distances agree with recent EXAFS data on K_1−xRb_xBr, RbBr_1−xI_x, and KCl_1−xBr_x. The results show that ionic polarization plays a significant role in the determination of both interatomic distances and heats of formation.     

Electronic structure of alkali halide surfaces upon ion irradiation

By a careful analysis of the electron loss spectra of some alkali halide crystals after ion irradiation we have demonstrated that defect generation depends on ion energy nonmonotonously. This effect is connected with inelastic interaction between ion and surface atoms.     

Non-radiative electronic capture rates by a halide vacancy in alkali halides

We treat non-radiative electronic capture rates induced by the non-adiabatic perturbation operator with corresponding transition probabilities. Therefore a dynamical electronic wavefunction is evaluated. By coupling the electron to dressed phonon modes, which connect the local optical mode to the acoustic phonons, exact energy balance in the transition probabilities is possible. The Franck-Condon-Integrals arising there are thermal averaged analytically. The method is applicable to transitions between two discrete electronic levels as well as band to discrete level transitions. Numerical results are given.     

Band structure of selectively adsorbed atoms on alkali halide surfaces

We present calculations for the band structure of selectively adsorbed atoms, i.e. for energies greater than zero on alkali halide surfaces near the regions of degenerated bands for existing experimental data. The calculated splittings of crossing-bands are in good agreement with experimental data. Also splittings between different bound levels appear in the calculation.     

The electronic structure of alkali metal oxides

Crystalline phase total energies, band structures, the distributions of the total and partial densities of electron states, and atomic charges were calculated for lithium, sodium, and potassium oxides, peroxides, superoxides, and ozonides using the CRYSTAL 06 and GAMESS packages in the B3LYP parameterization. For the molecular phases, the geometry was optimized and molecular orbital energies calculated. The results obtained for metal oxides were compared with the experimental photoelectron spectroscopy data and used to analyze their formation and thermal decomposition.     

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.     

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.     

Morphology of alkali halide thin films

Two alkali halide adsorbate-substrate systems were investigated by atomic force microscopy (AFM) working in contact mode. Adsorbate film orientation relative to the substrate was determined from the arrangement of the atomic steps of the substrate and the edges of the forming islands. In this work we present experimental results obtained for systems: NaCl/LiF(0 0 1) and LiF/NaCl(0 0 1), which exhibit a strong tendency of the self-assembly into regular structures.     

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.     

Local structure and lattice dynamics of alkali halide crystals with an anion vacancy

The local structure and vibrations in the region of an anion vacancy are studied using the pair interionic potentials within the shell model for crystals Me ⁺Cl^? (Me ⁺ = Rb⁺, K⁺, Na⁺). The pair potentials are derived from first-principles calculations of different clusters by the Hartree-Fock-Roothaan method with the one-electron states constructed in the form of molecular orbitals as linear combinations of atomic orbitals (MO LCAO). The calculations are performed with the GAMESS program package (US). The correlation corrections are included in the calculations. The validity of the model parameters is verified by comparing the calculated with experimental structural and dynamic properties of ideal alkali halide 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.     

Excitation of electroluminescence in alkali halide compounds

In ionic crystals the concentration of free carriers and the mean free path are usually small and the forbidden band is large. The field energy is in the main spent on heating the crystal. Therefore, it is difficult to accelerate an appreciable number of electrons to the energy necessary for the excitation of electroluminescence.One can overcome these difficulties by removing a thin layer of the luminophore. With the decrease in the layer thickness the development of the cascade breakdown becomes more difficult, and the thermal exchange with the electrodes becomes better. This makes it possible to increase the field up to the values necessary for tunnel ionization of the luminescence centres.We found electroluminescence of the sublimated layer of CsI-Tl.     

Effect of uniaxial strain on the structure of self-localized excitons in alkali halide crystals

We have used luminescence spectroscopy to establish the effect of low-temperature (100 K) elastic uniaxial strain on the configuration of a self-localized exciton (SLE) in alkali halide crystals (AHCs) at the instant of radiative relaxation. In face-centered (fc) alkali halide crystals, redistribution of the luminescence intensity occurs from the asymmetric SLE configuration to the symmetric SLE configuration (type III → II → I), while conversely in body-centered (bc) alkali halide crystals the redistribution occurs in favor of the asymmetric (polarized) SLE configuration (type I → II). External strain along the 〈100〉 direction leads to effective slip of the anions in the alkali halide crystals along the 〈110〉 direction, coinciding with the direction of compression of the SLE, which promotes creation of preferentially the symmetric SLE configuration, while strain along the 〈110〉 direction, acting perpendicularly to the length of the SLE, leads to elongation of the SLE, which promotes creation of the asymmetric SLE configuration with a higher degree of polarization. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 67–72, January–February, 2007.