Photoelastic constants and the Clausius-Mossotti field in the alkali halides

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Abstract
From the observed values of a quantity S, representing a certain combination of the dielectric and the photoelastic constants, Leigh and Szigeti (1967 and 1968) concluded that in valency crystals the internal field effects are far outside the range predicted by either the Clausius-Mossotti or the Drude field. Although even for the alkali halides there is a large discrepancy between the measured values of the photoelastic constants and those calculated from the Clausius-Mossotti principles, it is shown in the present paper that these discrepancies are compatible with the generally held view that for alkali halides the electrostatic field acting on the electronic polarization is, in a good approximation, the Clausius-Mossotti field. In particular, the criteria which Leigh and Szigeti used for the valency crystals also lead to this conclusion.