The Role of Phase Boundaries in Heterogeneous Solid State Reactions (III). An Experimental Study with Ag2S

The α/β‐Ag2S‐phase diagram at the transition temperature has been determined experimentally. α‐Ag2S single crystals were grown and transformed in β‐Ag2S. Using Ag/AgJ‐probes along the crystal, the local partial Gibbs energy of Ag in Ag2S was determined as a function of time during the unidirectional transformation of α‐Ag2S into β‐Ag2S and vice versa. Additionally, polarisation measurements were performed at the α/β‐Ag2S boundary. From these measurements it is concluded that the transport of point defects from the boundary into the bulk phases is essentially rate controlling and that the exchange fluxes across the interface are high enough not to become rate determining. The concentration of point defects in front of the moving boundary and eventually in the bulk periodically supersaturates, which leads to a precipitation of these defects in form of metallic silver.