M.P. Verma

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.

Application of three body force shell model to the lattice dynamics of transition metal oxides MnO, CoO and NiO

Abstract Lattice dynamics of some transition metal oxides has been studied using the three body force shell model. Reasonably good fits with the experimental phonon dispersion curves have been obtained by choosing values of molecular electronic polarisability substantially lower than those derived from the dielectric constant. This result is similar to those obtained in many other crystals which are not ideally ionic.

Correlation between electronic polarizabilities and ionic radii in alkali halides

Abstract 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.

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.

Dielectric properties of alkali halide and alkaline earth oxide crystals

Abstract The deviations of ionic charge and compressibility from their nominal values in alkali halides and alkaline earth oxide crystals have been discussed and explained in terms of a theory based on shell model and exchange charge interactions originally developed by Dick and Overhauser. The exchange charge polarization parameters have been evaluated in the present study using the appropriate values of overlap integrals. It has been concluded that the second neighbour short range forces have significnant magnitudes in alkaline earth oxides.

Evaluation of strain polarizability parameter in alkali halides

Abstract 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.