An estimate of the pressure dependence of the dielectric constant in alkali halides

The determination of the pressure dependence of the static dielectric constant, ϵ S , from the Szigeti relations, demands the knowledge of the volume dependence of the Szigeti charge e *, which unfortunately is unknown. By using the classical Lorentz‐Lorenz relation and connecting the ionic polarizability to the bulk modulus a simple relation for dϵ S /d P is derived; although it gives the correct order of magnitude, the calculated values exceed the experimental ones by a factor 1.2 to 2. This difference is possibly due to the increase of e * with pressure. A simple relation for the Gruneisen constant of the transverse optical mode is also given.