HREM study of ferroelectric materials

Abstract

High-resolution imaging of ferroelectric materials poses unusual problems, since specimens tend to be mechanically unstable in the electron beam due to electrostatic interactions. It is often difficult to correct specimen induced astigmatism. Despite this situation significant new information has been obtained concerning the atomic structure and configurations of lattice defects, domain walls and commensurate/incommensurate as well as normal crystallographic structural phase transitions. Our studies have concerned a wide range of ferroelectric materials, such as the perovskite-type structures of KNbO3 BaTiO3 lithium niobate type LiTaO3 (Li1-x, Ag)TaO3 , proton-exchanged and doped lithium niobate (LiNbO3 : Fe,Ti,H) , the hexagonal bronze potassium niobium tungstate PNT, the essentially tetragonal tungsten bronze framework structures of Ba2NaNb5O15 (BNN) and (Sr1-x, Ba x) Nb2O6 (SBN) and the microstructure of lead zirconium titanate (PZT). Some interesting results concerning direct determination of surface polarity of ruby, sapphire and haematite will also be reviewed. The deliberate use of incident beam tilting techniques, in conjunction with dark-field imaging, proved most beneficial for revealing weak symmetry elements in superlattice structures.