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Contents Zntroduction Fqsee-ion polurixabilities Crgstalline-state polayimbilities Effect of crystalline environment on electronic polarixabilities Relationship between polarixabilities and ionic radii Photoelustic bchaviour ConclusionsRefemnces l )

A modified treatment of the interionic forces for alkali halide molecules is presented within the framework of Rittner's model. The electronic polarizabilities of ions in the molecular state are calculated using an energy level analysis. The free ion polarizabilities used in Rittner's model are replaced by the molecular state polarizabilities. The contributions arising from the van der Waals interactions and the mutual polarization of ions are recalculated. Values of the dipole moment, the binding energy, and the rotational and vibrational constants are estimated for twenty alkali halides using two alternative potential forms for the repulsion energy. The results are discussed in the light of experimental data.

An application of the Slater–Kirkwood variational method has been made to evaluate the van der Waals dipole–dipole and dipole–quadrupole energies in the four families of ionic crystals, viz., alkali halides, alkaline earth chalcogenides, alkali chalcogenides, and alkaline earth halides. The methods based on the perturbation theory adopted by previous investigators to evaluate the van der Waals potentials are critically discussed. In the present paper we have used an interpolation scheme and semiempirical formulas to estimate the dipole–dipole and the dipole–quadrupole coefficients between a number of different ions with electron configurations close to those of the rare gas atoms. An independent check of the approximate validity of the interpolation scheme has been provided by evaluating the dipole–dipole interaction coefficients between alkali metal atoms. The crystal energies of alkali halides, calculated using the new values of van der Waals potentials, agree closely with the experimental values.

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The experimental data on first and second order pressure derivatives of dielectric constants are used to make an analysis of the volume dependence of polarizabilities, effective charge parameter, and transverse-optic-mode frequencies in CaF,, SrF2, BaF,, PbF,, Sr(NO,),, Ba(NO,),, and Pb(NO,),. The short range force constant responsible for the ion-displacement polarizability is calculated with the help of Lundqvist's three-body interaction model. Reasonable values for the volume derivatives of various dielectric parameters are obtained. Values of optic-mode-Griineisen parameters and their volume derivatives are also calculated and compared with available experimental data.Die experimentellen Werte der Druckableitungen der Dielektrizitatskonstanten erster und zweiter Ordnung werden benutzt, um eine Analyse der VolumenabhLngigkeit der Polarisierbarkeiten, der effektiven Ladungsparameter und der Frequenzen der transversalen optischen Mode in CaF,, SrF,, BaF,, PbF,, Sr(NO,),, Ba(NO,), und Pb(NO,), durchzuflihren. Die kurzreichweitige Kraftkonstante, die fur die Ionenverschiebnngspolarisation verantwortlich ist, wird mit Hilfe des Lundqvist-Drei-Korperwechselwirkungsmodells berechnet. Verniinftige Werte fur die Volumenableitungen der versehiedenen dielektrischen Parameter werden erhalten. Die Werte der Griineisenparameter der optischen Mode und ihre Volumenableitungen werden cbenfalls berechnet und mit vorhandenen experimentellen Werten verglichen.