High resolution electron microscopic study of the hexagonal bronze potassium niobium tungstate (PNT)

High-resolution electron microscopic studies are reported on the nature of the complex domain patterns which occur in the room temperature phase of the hexagonal bronze KWNb2O9. Two-dimensional lattice images reveal the domain wall configurations, with walls having virtually zero thickness, containing steps (kinks) of atomic dimensions. Wall rearrangements, following kink propagation under the influence of the electron beam, are also described. Symmetrical zone axis images, for extremely thin edges of the specimens, reveal directly the nature of the (W, Nb) atom displacements responsible for the hexagonal component of the superstructures occurring throughout five phases, which exhibit a series of structural phase transitions, including ferroelectric phases, in the temperature range 270-560°C.