Domain wall interactions and transformation of domain pattern near the improper ferroelastic phase transition of K2Ba(NO2)4

The transformation of domain pattern near the improper ferroelastic phase transition of K2Ba(NO2 )(4) is investigated using the optical polarization method. Instability of orthogonal ferroelastic domains is shown to be due to their interaction with ultra-thin domains corresponding to the third orientation state. These domains become visible only near the phase transition, however, they may also exist at lower temperatures, probably arising from antiphase boundaries. It is demonstrated that permissible ( W -type) domain walls react forming non-permissible domain walls, which disturb the domain pattern. The atomic structure models are proposed for the domain walls of W -type to discuss their stability near the phase transition.