Ferroelastic domain structures and phase transitions in barium sodium niobate

Ferroelastic domains in barium sodium niobate (Ba₂NaNb₅O₁₅) have been studied by X-ray Lang and white-beam synchrotron-radiation topography. In Lang topographs contrast was observed from domain walls and the domains themselves, the latter being due to anomalous dispersion. Most ferroelastic domain boundaries lay on (110) or (10) planes. The contrast from the ferroelastic domain walls suggests that displacements exist within the walls, probably due to walls zigzagging between {110} planes. As the temperature was raised, significant changes occurred in the domain configuration, and a single-domain state was formed at 559 K, well below the orthorhombic-to-tetragonal phase transition at 573 K. No evidence for an incommensurate phase or an associated domain structure could be found in the samples studied.