An X-ray Topographic Study of the Paraelectric-to-Ferroelectric Phase Transformation in Nearly Perfect Single Crystals of Ammonium Sulfate

The basic defect structure of solution-grown single crystals of ammonium sulfate and changes to it as it undergoes a paraelectric to ferroelectric phase transition at 223 K are investigated using conventional Lang and white-synchrotron-radiation X-ray topography. Topographs of thick (ca 1 mm) plates reveal the existence of lattice strain prior to transformation. The shape of the phase boundary is revealed from studies of thinner plates (ca 0.3 mm). These show sharply defined zigzag-shaped boundaries between the paraelectric and the ferroelectric phases in plates cut parallel to (010) and (001). Cracks and dislocation loops (Burgers vector [001]) generated by plastic deformation are found to originate from the apices of the zigzag boundaries. No distinct phase boundaries were observed in (100) plates. The preferred (energetically favoured) orientations (i.e. the zigzag segments) of the phase boundaries in (010) and (001) as well as their blurred appearance in (100) plates are explained by the specific properties of the strain tensor associated with the transition. Ferroelectric domains have also been detected by the faint dynamical X-ray contrast of their boundaries (180° walls).