Domains and domain walls in order-disorder ferroelastics

We have studied the domain structure in a number of crystals undergoing proper or improper ferroelastic phase transitions of the displacive or order-disorder type. In all cases a regular pattern of domain walls was observed near proper ferroelastic phase transitions, whereas needle or lense shaped domains appeared in the improper ferroelastic phases of the order-disorder type. The observations are explained by the different magnitudes of the attractive force between domain wall junctions.