On the question of the calculation of migration volumes in ionic crystals

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Abstract
The migration volume vm consists of two contributions vm s= - T(∂sm/∂P) |T and vh m = (∂hm/∂P)|T where ∂m, hm denote the migration entropy and enthalpy. By analysing the recent experimental data of Andeen, Hayden and Fontanella (1980) on SrF2 + Er+3, obtained by dielectric-loss techniques under various pressures, the term vs m is found to be a considerable contribution t o vm. At 330 K the ratio |vs m|vh m is 23%. Further the compressibility ∼m of the migration volume is found to exceed the bulk compressibility K by one order of magnitude. The application of a recent macroscopic model proposed by Varotsos and Alexopoulos (1978) not only explains that ∼m is appreciably larger than but it also leads t o a value of vs m/vh m appreciably different than zero. The common disagreement between the ‘static lattice’ calculations of vm and experiments lies in the fact that the contribution cannot be neglected.