The Born repulsive parameters and dielectric behaviour of alkali halides

The dependence of the Born repulsive parameters of alkali halides on elastic and dielectric data has been discussed. The values of hardness parameter in alkali halides have been recalculated using the revised values of van der Waals energies. It is observed that the two sets of hardness parameter corresponding to elastic and dielectric data differ from each other but become compatible if an effective charge parameter for the ions is introduced. Its usefulness has been demonstrated by calculating the strain derivative of static dielectric constant of alkali halides.     

Analysis of the photoelastic effect in ionic crystals

An analysis of the photoelastic effect in ionic crystals has been presented within the framework of Clausius-Mossotti theory of the dielectric constant. The values of the strain derivative of the electronic dielectric constant have been calculated in alkali halides and MgO crystals by taking into account the variation of electronic polarizabilities with compressive stress. The results obtained are found closer to the experimental values. The photoelastic behaviour of MgO crystal is predicted to be of opposite nature to that of alkali halides, in conformity with the experimental observations.     

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.     

Theory of strain derivative of electronic dielectric constant of ionic crysts

A theoretical method for evaluating the strain derivative of the electronic dielectric constant of ionic crystals has been developed. The analysis presented here is based on the shell model and takes account of the exchange charge polarizations. Values of strain derivative of the electronic constant dielectric calculated for 6 alkali halides and MgO show a remarkable agreement with experimental data on photoelastic constants.     

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.     

Structural and dynamical properties of silicon in a «covalent» huggins-mayer scheme

Summary A rereading of the Fumi and Tosi paper on the ionic sizes and Born repulsive parameters in the alkali halides family led us to draw a parallelism with recent total-energy calculations for semiconductors, where the Coulombic interactions are substituted by a semi-empirical tight-binding estimation of the covalent attraction and the repulsive term is still of Huggins-Mayer form. We find that for crystalline silicon the hardness parameter ρ is surpirisingly the same as for the alkali halides. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

A new theory of compressible ions — structures of the alkali halides

Ions in ionic crystals are considered to exist in compressible space-filling polyhedral cells analogous to the Wigner-Seitz cell in metals. Repulsion arises from the compression energy of the ions written as a surface integral over the ionic cells. Two adjustable parameters are introduced per ion with the provision that the same parameters can be used in any crystal of any structure in which the ion occurs. The 18 parameters for the 5 alkali and 4 halogen ions have been determined from PV data on the 20 alkali halides. The important successes of the theory are: (i) All the twenty alkali halides are correctly predicted to occur in their observed structures (ii) The thermal transition in CsCl is explained (iii) The pressure transitions in the alkali halides are predicted well (iv) The calculated values of the variation of transition pressures with temperature agree well with experiment. These results are much better than those obtained by earlier theories.     

Anisotropic breathing shell model and phonons in NaCl structures

Callow, Diller and Norgett, in their investigations on the dynamics of alkali halides, have developed analytic potentials for the short range interactions between the anions and cations. These potentials consisting of Buckingham and van der Waals contributions have been determined from the elastic, dielectric constants and the equilibrium conditions of the lattice and are found to satisfactorily explain the defect properties of many halides. We employ the analytic potentials of Catlow et al. and use anisotropic breathing corrections for the computation on phonon dispersions and Debye-Waller factors of all NaCl structure alkali halides. The results of our study show that good agreement can be obtained with these more physical parameters.     

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.     

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.     

The role of ionic quadrupolar deformation in atom-surface potential

We show that the quadrupolar deformation of surface ions gives appreciable contribution to the potential energy between the atom and an ionic surface. Calculations are presented for a number of alkali halides. The validity of Fumi and Tosi crystal radii in the surface region is discussed in connection with the depth and the corrugation parameter.     

Relationship between solid state parameters and melting parameters for alkali halides

In the present communication we report some interesting relationships found between solid state parameters and melting parameters for alkali halides. Values of the melting temperatures and relative changes in volume after melting are found to exhibit systematic trend of variation with the Phillips ionicity parameter defining the amount of fractional ionic character. An empirical analysis is presented for determining the nearest-neighbour distances in alkali halide melts. The predictions made in the present work are shown to be consistent with available X-ray diffraction measurements.     

Analysis of the repulsive softness parameter in alkali halides

An analysis of the repulsive softness parameter for alkali halides has been presented on the basis of the additivity rule. The softness parameters for individual ions have been obtained using data from three different sources viz. overlap data, molecular data and crystal data. The softness parameters so derived are used to evaluate the bulk modulus and its pressure dependence for 20 alkali halides with NaCl and CsCl structures adopting Smith's distortion model for repulsive interactions. Some attractive features of the present approach have been discussed in order to demonstrate its superiority over the previous work.     

A simple shell model calculation of the differential ionic relaxation of the MgO(100) surface

A model previously reported and applied to the calculation of the differential ionic relaxation of the (100) surfaces of the alkali halides has been extended and applied to MgO(100). In particular, mechanically induced polarisations of the surface ions due to asymmetry in the surface short range forces are now included. The new model is tested by repeating the calculation for selected alkali halides. The results for MgO(100), obtained assuming a fully ionic surface, are consistent with the results of a LEED theory-experiment comparison, and indicate that this surface is not relaxed but may be rumpled on a small scale.     

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.     

On the Mott-Littleton dielectric theory for alkali halides

It has been demonstrated that the Mott-Littleton theory of dielectric constant is consistent with the recent experimental data on the pressure dependence of dielectric constant of alkali halides. On the other hand, the Clausius-Mossotti and Drude models yield large deviations from the observed data.     

On the compressed-ion model of cohesion in alkali halides

First-principle local-density calculations of the total energy of ions confined within a sphere were performed The results for positive alkali and negative halogen ions are used to test the validity of the compressed-ion model for the cohesion of alkali halides recently proposed by Narayan and Ramaseshan It was found that the model is not realistic since the calculated ionic compression energies are by far too large to account for the observed equilibrium lattice constants and bulk moduli of alkali halides.     

Zur Theorie kraftfreier Magnetfelder

From the exciton spectrum of the alkali halides it can be shown that theP-function of the alkali atom seems to be necessary, if one wants to construct certain excitons by means of atomic orbital functions. A relation between the ionisation energy of the exciton and the optical dielectric constant has been obtained from the experiments. This is consistent with a hydrogenic model of the exciton, which we have used in an earlier paper.     

Interionic potentials in alkali halides and their use in simulations of the molten salts

After an outline of work on rare-gas systems, which serves as a target for parallel work on alkali halides, and an initial brief survey of those parts of this parallel work for which results have been obtained, interionic potential models for alkali halides are considered in some detail. The rigid ion potentials of Fumi and Tosi are discussed and then a major part of the section is devoted to deriving a new set of polarizable ion potentials, which incorporate the ideas behind the lattice dynamical shell model. Extensions which include many-body terms in the potentials are considered briefly and finally the information which can be obtained from alkali halide diatomic molecules is discussed.

In the third section methods of computer simulation for ionic liquids are outlined, concentrating on the molecular dynamics method, and some of the properties which can be obtained by analysing the ion trajectories are listed. Results from simulations, including some new work on LiF, NaCl and RbI, are reviewed.