Optical microscopic study of the NH4Cl phase transition with observations of slip bands, heterophase and domain structure

Slip bands parallel to { 100} planes have been observed almost universally in NH4Cl single crystals. They are described and characterized and it is shown that they induce inhomogeneous internal stresses which play an important role in the nucleation process at the order-disorder phase transformation. The heterophase structure at the phase coexistence consists of slabs parallel to { 111 } planes, in agreement with theoretical predictions concerning the shape of inclusions in a matrix of cubic symmetry. The phase transformation involves large internal stresses which result in plastic deformations. Pictures of the domain structure in the ordered phase using the electro-optical effect are presented for the first time : they show that domain boundaries are also {111} planes. Theoretical arguments are given to explain this orientation.