ELECTRONIC STRAIN POLARIZABILITY CONSTANTS OF THE ALKALI-HALIDES

The electronic strain polarizability constants of the alkali halides have been calculated. The cation contributions are negligibily small. Several empirical forms were used for the description of the variation of the halide ion polarizability with density. A model due to Wilson and Curtis was found to give results higher than the experimental strain polarizability constants by a nearly constant factor of 1.8 + 0.1, a fact permitting prediction of the strain polarizability constants and ρ( ∂n ∂ρ)T for the alkali halides for which experimental values are not available. A modification of the Wilson-Curtis model was found to give much better agreement, and the agreement improved monotonely with decreasing internal forces. The dispersions of the strain polarizability constants at 5893Å from the static values range from 2% for the fluorides to 7% for the iodides. Values of (∂ In ν0 ∂ Inρ)T, upon which these dispersions depend, have been obtained for all twenty alkali halides. Refinement of the modified Wilson-Curtis model must await improved van der Waals' constants. Recently proposed sets of van der Waals' constants may be rejected in favor of the classic values of Mayer. Vacuum UV spectral evidence is used to discount the contention that the cation polarizabilities in the alkali halides are greatly reduced from their free-ion values. © 1979.