Transition to the Ferroelectric State in Barium Titanate

Abstract

The phase transition in BaTi${\mathrm{O}}_3$ at the Curie point was investigated on single crystals with optical and electrical techniques. Besides raising the cubic-tetragonal transition about 15°C in the usual way with an electric field along the cube-edge direction, an orthorhombic phase was induced above the Curie point by a field along a face-diagonal direction. Birefringence, polarization, and dielectric constant were measured above and below the Curie point as functions of field strength and field direction. The data fit a unified description using the free energy equation of Devonshire. An atomic model for the induced orthorhombic phase is proposed with Ti ions displaced in alternate cube-edge directions. The shift of the ferroelectric transition with electric field was observed optically and compared with predictions derived from the free-energy curves. Two types of transition were observed: In the slow transition the new phase nucleates at the edges of the crystal and grows by domain-wall motion; in the fast transition, which occurs only if the electric field is changed faster than about 1 kv/cm sec, the entire crystal switches in a uniform, continuous motion in 1 to 2 μsec.