Evaluation of strain polarizability parameter in alkali halides

The strain polarizability parameter λ for alkali halides under the effect of hydrostatic pressure has been evaluated. The correlations of λ with fractional ionic character, effective ionic charge and the overlap repulsive energy have been pointed out.     

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.     

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.     

Evaluation of the second order strain derivatives of the electronic dielectric constant of alkali halides

We have evaluated the second order strain derivatives of the electronic dielectric constant of alkali halides employing two different theories viz. the Clausius—Mossotti theory of polarizability and the Penn model of energy gap. The results obtained from the two theories are in fair agreement with each other. It has been emphasised that the present calculations can be used to separate the electronic and ionic contributions to the second order strain derivative of the static dielectric constant.     

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.     

Effect of many body forces on the photoelastic constants of the alkali halides

A simple phenomenological lattice theory based on the Lundqvist potential of ionic cohesion (many body theory of elastic dielectric), to explain photoelastic behaviour of the solids with rock-salt structure, is presented and shown to apply within reasonable errors to the known experimental constants. The predicted values of the photoelastic constants of the alkali halides are very close to the experimental values and are better than obtained by all earlier workers.     

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.     

Study of strain optical constants of mixed lithium halides

The theory of photo-elasticity of mixed alkali halides has been used to analyse the strain optical constants and volume derivative of the electronic dielectric constant of the mixed crystal, LiF_0.5Cl_0.5.     

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.

     

Correlation between electronic polarizabilities and ionic radii in alkali halides

An analysis of the electronic polarizabilities and sizes of ions in alkali halides has been carried out by considering a relationship according to which the electronic polarizability of an ion should vary as the n th power of its radius. The values of crystal radii, polarizabilities and the exponent “n”have been calculated using the free ion data reported by Pauling. The values of “n” are found to be nearly 3 in most of the crystals suggesting the validity of the polarizability radius cube relation. The polarizabilities and crystal radii calculated in the present study agree well with those estimated by earlier investigators.     

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.     

A resonance of the electronic polaron appearing in the optical absorption of alkali halides

Excited states (scattering states) of free and bound electronic polarons in non metals are introduced and investigated in the continuum approximation. It is suggested that transitions to these states might lead to prominent resonances in the optical absorption at energies approximately twice the bandgap energy. A shift towards higher energies of the corresponding resonances in the energy loss function is calculated. Such resonances and the predicted shift are found in the experimental data for alkali halides; previously they have generally been attributed to plasma excitations. Limitations of the present model, due to the continuum approximation, (and related to the oscillator strength of the transitions) are discussed. The electronic polaron coupling constant a is calculated and tabulated for a number 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.