H.P. Sharma

Analysis of the photoelastic effect in ionic crystals

Abstract 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.

Analysis of crystal binding of alkali and alkaline earth chalcogenides

Abstract An analysis of the crystal binding of alkali and alkaline earth chalcogenides is presented within the framework of the Born model of cohesion. The contributions of the overlap repulsive interactions between nearest and next nearest neighbours are considered in the evaluation of the cohesive energies of oxides, sulphides, selenides and tellurides of alkali and alkaline earth metals. In addition, the contributions arising from the van der Waals dipole-dipole and dipole-quadrupole interactions are estimated following the method recently proposed by Boswarva. Calculated values of the cohesive energies in alkaline earth chalcogenides present a close agreement with experimental values. Electron affinities of chalcogenide ions estimated in the present study, following the Born-Haber thermochemical process, are in remarkably close agreement with experimental data.

Transverse optic mode Gruneisen parameter and dielectric properties of alkali halides

Abstract The well known Szigeti relations for the dielectric theory of ionic crystals have been critically examined by investigating the volume dependence of dielectric behaviour of alkali halides. It has been predicted that the strain derivative of effective ionic charge parameter should be nearly equal to zero. Values of transverse optic mode Gruneisen parameter for NaCl structure alkali halides have been estimated.

Analysis of polarizabilities of alkali halides using Narayan-Ramaseshan repulsive potential

Abstract The repulsive potential in ionic crystals recently proposed by Narayan and Ramaseshan (NR) can be expressed as the sum of the contributions from the individual ions. In the present paper we show that using this repulsive potential it is possible to divide the polarizability arising from the relative displacement of ions into its ionic constituents. NR have also derived the ionic radii in alkali halides which we have used to estimate the electronic polarizabilities of ions with the help of polarizability-radius cube relation. The electronic polarizabilities of alkali and halogen ions thus evaluated show a good agreement with those deduced from the experimental refraction data.

Effect of hydrostatic pressure on the electronic dielectric constant of ionic crystals

Abstract The strain derivatives of the electronic dielectric constant in LiF, NaCl, KCl, KBr, KI, RbCl and MgO crystals have been calculated with the help of the Clausius-Mossotti model. The variation of electronic polarizabilities with compressive stress has been derived by taking into account the distortion of ions, within the framework of the deformation dipole model. It has been observed that consideration of the variation of polarizabilities with compressive stress improves the agreement between theory and experiment. The photoelastic behaviour of MgO is predicted to be of the opposite nature to that of alkali halides in conformity with the experimental observations.

Theory of the photoelastic effect in ZnO, ZnS and CdS crystals

Abstract It has been a general opinion that the Clausius-Mossotti dielectric theory is not capable of explaining the observed photoelastic behaviour of partially ionic crystals. However in the present communication we show that by making account of the variation of polarizabilities under compressive stress within the framework of the Clausius-Mossotti theory it is possible to obtain a reasonable agreement with experimental data on the photoelastic behaviour of ZnO, ZnS and CdS crystals.

Evaluation of the strain derivatives of electronic polarizabilities and dielectric constants of alkaline earth fluorides

A method recently developed for explaining the photoelastic behavior of alkali halides has been applied to evaluate the strain derivatives of electronic polarizabilities and dielectric constants of alkaline earth fluorides. The calculated values of the strain derivative of electronic polarizabilities present a reasonable agreement with experimental data. Values of the strain derivative of the electronic dielectric constants for CaF2, SrF2, and BaF2 calculated in the present study within the framework of Clausius–Mossotti theory are in agreement with the corresponding experimental values. The sensitivity of the results on the electronic polarizabilities of ions has been discussed.

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

Abstract 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.

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

Abstract 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.

Theory of strain derivative of electronic dielectric constant of ionic crysts

Abstract 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.