Higher order elastic constants of alkali halides

The Coulomb, van der Waals and repulsive lattice sums occurring in the higher order elastic constants up to sixth order have been calculated for the rocksalt and cesium chloride structures. Numerical values of the static elastic constants up to sixth order based on a rigid ion model with van der Waals and Born-Mayer type central force interaction between first and second nearest neighbors are calculated for several alkali halides representing both structure types. Fair agreement with the available experimental third and fourth order elastic constant data is found.     

Contribution of many body forces to the elastic constants of solids with ZnS structure

A phenomenological model to study the elastic properties of the A^NB^8?N type solids with ZnS structure has been proposed. The model explains many properties like Born relation, internal strain parameter, constancy of reduced mechanical frequencies etc. of these solids quite satisfactorily.     

Elastic constants of solid solutions of single crystals of the alkali halides

On the basis of the Leibfried-Ludwig theory of anharmonic effects in crystals, a theoretical calculation is given of the elastic constants of solid solutions of alkali halide single crystals. The results of the calculations are compared with experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 7, pp. 57–61, July, 1973.     

Temperature derivatives at constant volume of the elastic constants of the alkali halides

Values of dC/dT)_V have been computed from experimental dC/dT)_P and dC/dP)_T for the three elastic constants (expressed as $\text{B}{}_{\mathrm{T}}\text{, C}{}_{44}\text{and}\text{(C}{}_{11}\text{?C}{}_{12}\text{)}\text{2)}$ of each of the 16 Li, Na, K, and Rb halides. The dC/dT)_V measure the explicit dependence of C on T, the effect of thermal expansion having been removed. The dC/dT)_V are all small (compared with dC/dT)_P), are all negative, and vary quite systematically and smoothly with anion, with cation and with the three elastic constants. Negative dB_T/dT)_V is accounted for both thermodynamically and by available lattice dynamical calculations. dC/dT)_V for shear constant is expressed in terms of two vibrational mode parameters, whose values can then be estimated from the observed value, and the trend of dC/dT)_V with Debye temperature.     

On the effective field and optical constants of alkali halides containingF colour centres

Summary The Onsager and Lorentz local fields ofF colour centres are compared and their change as a function of angular frequency and temperature is also considered. The change of these fields is proportional to the change of the refractive index produced byF centres. Some approximations about the explicit temperature dependence of the refractive-index change are also presented.

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Riassunto I campi locali di Onsager e Lorentz diF centri di colore sono confrontati e si considera il loro cambiamento in funzione della frequenza angolare e della temperatura. Il cambiamento di questi campi è proporzionale al cambiamento dell'indice di rifrazione prodotto daF centri. Si presentano anche alcune approssimazioni riguardo alla esplicita dipendenza dalla temperatura del cambiamento dell'indice di rifrazione.

     

Effect of pressure on F-band energies in alkali halides

Variation of peak F-band absorption energy and lattice distortion with pressure has been calculated for lithium halides employing extended ion model and using values of local compressibility in the neighbourhood of the F-centres which include the effect of vacancy and pressure. The calculated results are in very good agreement with experimental results.     

Analysis of first and second order strain derivatives of the dielectric constants of alkali halides

Expressions for evaluating the first and second order strain derivatives of the electronic and static dielectric constants of alkali halides are derived. The strain derivatives of the electronic dielectric constant are calculated by taking account of the effect of the crystalline potential on the variation of electronic polarizabilities with volume. The strain derivatives of the static dielectric constant are evaluated adopting the exponential and the inverse power forms for the short range repulsive potential. Two sets of the Born repulsive parameters derived from ultrasonic data and dielectric data are used to evaluate the lattice contribution to the volume dependence of static dielectric constant of alkali halides.     

Radiolysis of alkali halides

This paper reviews the extensive work pertaining to the creation of colour centres in alkali halides. After a brief historical survey the various mechanisms put forth to account for defect creation are examined in turn and the experimental evidence which supports or belies each of these is presented. After demonstrating from published experimental work that the defects created are of the Frenkel rather than the Schottky type and after consideration of the early stage, extrinsic coloration process, the Valey mechanism and its variants are discussed. Next, the direct displacement model proposed by Balarin is considered and, finally, the recent excitonic mechanism proposed by Pooley and Hersh is examined in detail. The conclusion reached is that, although the excitonic mechanism appears to be much the more preferable model yet proposed, a number of points need careful consideration and experimental study before it can be accepted as having general validity.     

Many body forces and elastic constants of rare gas crystals

The effect of many body Van der Waals forces on the elastic constants of solid rare gases is calculated. These results are combined with contributions from the two body potential and zero point energy. The violation of the Cauchy relation is studied.     

Evaluation of the photoelastic constants of ionic crystals

A theoretical model, for evaluating the photoelastic constants of ionic crystals, has been developed by considering the short range repulsive interactions between nearest and next nearest neighbours, the van der Waals potentials and three body interactions as well as taking into account the electronic polarizabilities of cations and anions. The model has been applied to calculate the photoelastic constants of RbC1, RbBr and RbI crystals. The results obtained in the present study are in good agreement with experimental data.     

Volume dependence of the dielectric constants and the effective charge parameter in alkali halides under hydrostatic pressure

The method often used for evaluating the strain derivatives of the effective charge parameter in ionic crystals is based on the deformation dipole model. In the present communication we show that the strain derivatives of the effective charge parameter calculated from the Phillips-Lawaetz ionicity model are closer to the recent experimental data on the volume dependence of dielectric constants.     

Muonium centers in the alkali halides

Muonium centers (Mu) in single crystals and powdered alkali halides have been studied using the high-timing-resolution transverse field μSR technique. Mu has been observed and its hyperfine parameter (HF) determined inevery alkali halide. For the rocksalt alkali halides, the HF parameter A_μ shows a systematic dependence on the host lattice constant. A comparison of the Mu HF parameter with hydrogen ESR data suggests that the Mu center is the muonic analogue of the interstitial hydrogenH _i ⁰ -center. The rate of Mu diffusion can be deduced from the motional narrowing of the nuclear hyperfine interaction. KBr shows two different Mu states, a low-temperatureMu ^I -state and a high-temperatureMu ^II -state.     

Defect parameters in the alkali halides

Abstract

Methods are available which calculate the free energies of defects in ionic systems within the quasi-harmonic approximation. The Harwell SHEOL code is used to calculate the defect enthaplies and entropies for a number of alkali halides where reliable diffusion data are known. We discuss the trends in the calculations and the comparison with experiment.     

Cathodo-luminescence spectra of alkali halides

Cathodo-luminescence spectra of fourteen alkali halides and of some mixed crystals of NaCl and KCl have been studied. The spectra show broad structureless bands. In some ten halides it is possible to find a strong band whose peak position shifts towards longer wavelengths as the inter-atomic distance increases. For pottasium halides the relationship λ_max=1380d, where “d” is the inter-atomic distance, holds approximately but is not generally true. A mechanism of cathodo-luminescence in alkali halides has been postulated, and an interpretation of the above band and of some NaCl bands has been made on that basis.