Transmission electron microscope studies of phase transitions in single crystals and ceramics of ferroelectric Pb(Sc_1/2Ta_1/2)O₃

An account is given of transmission electron microscope investigations of the phase transitions in single crystals and ceramics of the complex perovskite structured ferroelectric `relaxor' compound Pb(Sc<sub>1/2</sub>Ta<sub>1/2</sub>)O<sub>3</sub>. The crystal symmetries pertaining to both the non-polar paraelectric (PE) and polar ferroelectric (FE) states have been studied by the technique of convergent-beam electron diffraction. A new phase transition has been discovered in the temperature range for which the FE and PE states coexist. The new phase transition is interpreted as the creation of a modulated antiferroelectric state, and this is viewed as marking a departure from relaxor behaviour towards more `normal' ferroelectric behaviour.