The morphological evolution of initially planar phase boundaries between ionic crystals due to ion fluxes which are driven by an externally applied electric field is investigated. In this particular study, the boundaries under investigation are interfaces between KC1 and AgCl. It is shown that the moving boundary is morphologically unstable if the flux of silver ions is directed towards the KCl‐crystal, whereas it is stable if the flux of potassium ions is driven towards AgCl. This behaviour can be explained by the rules which derive from a formal stability analysis. The geometrical evolution of the moving phase boundary in its later stages is mainly governed by the elastic properties of the contiguous phases.

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In binary or quasi‐binary systems the superposition of chemical diffusion and field‐driven migration may lead to unstable diffusion fronts. This means that an initially one‐dimensional transport geometry may become three (two)‐dimensional. This concept is tested with the solid solution AgCl‐NaCl above T=200°C, which is above the critical point of the miscibility gap. Morphological instabilities of the concentration profile in the inter‐diffusion zone are found if an external electric field is applied. This field must exceed a critical value and the AgCl‐side must be anodic in order to induce an instability of the diffusion front. Numerical solutions of the transport equations and the given boundary conditions corroborate the experimental results.