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.
The structural phase transformation of CsC1 crystals under the effect of high temperature is investigated using an interionic potential model which takes account of repulsive interactions up to second neighbours and of van der Waals (vdW) interactions between dipole-dipole and dipolequadrupole. It is found that the transition temperature a t which the CsCl crystal transforms from B, (CsCI) to B, (NaCl) structure can be predicted correctly only if larger values of vdW potentials are used in calculations. The same potential parameters are used to predict the high pressure equation of state for CsCl, CsBr, and CsI crystals. The use of overlap integrals in the Born-Mayer potential form yields good agreement between theory and experiment for the high pressure compression data.Mit einem Interionenpotentialmodell, das abstoDende Wechselwirkung bis zu zweiten Nachbarn und die Van der Waals (vdW)-Dipol-Dipol-und Dipol-Quadrupol-Wechselwirkung einschlieh, wird die strukturelle Phasentransformation von CsC1-Kristallen unter dem EinfluB hoher Temperatur untersucht. Es wird gefunden, daB die obergangstemperatur, bei der der CsC1-Kristall von der B, (CsC1)-in die B, (NaC1)-Struktur ubergeht, sich korrekt nur dann voraussagen laDt, wenn groDere Werte der vdW-Potentiale in den Berechnungen benutzt werden. Dieselben Potentialparameter werden benutzt, urn die Hochdruckzustandsgleichung fur CsC1-, CsBr-und CsJ-Kristalle vorherzusagen. Die Benutzung von Uberlappungsintegralen in dem Born-Mayer-Potentialausdruck liefert gute Ubereinstimmung zwischen Theorie und Experiment fiir die Hochdruckkompressionsdaten.
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