Domain Patterns and Phase Transitions in Ferroelectric Crystal KNbO₃

We have studied the domain configurations and structures of KNbO₃ crystal in orthorhombic and tetragonal phases by polarizing microscopy. The phase transition processes among orthorhombic, tetragonal and cubic phases and thermal hysteresis during phase transitions are observed directly. Domain-pattern changes and the stable domain patterns during the phase transitions are discussed.