Electrodecoration of tilt grain boundaries in potassium bromide bicrystals

Abstract

Continuous application of large steady electric fields (10⁵ V/m) causes silver ions to move with high mobility (10⁻¹⁰ m²/Vs at 200°C) into tilt grain boundaries where they form macroscopic colloids. This behaviour, which varies with temperature, applied field, and segregation of residual impurity, is a particular case of a general inhomogeneous ion transport, called injection transport, in which cations from an electrode are injected into a substructure, adjacent to precipitates, that is selectively eroded by fields above a certain threshold value. The irregular shape of electro-diffusion tracer profiles is caused by injection transport. A detailed mechanism of injection transport involves excitation by the field, followed by exciton decay within the anion sublattice that leads to halogen atom ejections and lattice decomposition near interfaces of precipitates of residual impurity. F-centre products of the decay assist in colloid formation. A re-examination of experiments that claim to demonstrate the existence of dislocation pipe diffusion indicates that they are more likely to be straightforward examples of injection transport. With this change of interpretation, a consistent pattern is imposed on the variety of results that have hitherto emerged from experiments on cation grain boundary diffusion.