5. Ferroelectric Materials: History and Present Status

In spite of discovery of hysteresis characteristic in Rochelle salt, the phenomenon was recognised as anomalous dielectric response. As magnitude of spontaneous polarisation varied by changing temperature, stable ferroelectric structure was ungiven. In potassium dihydrogen phosphate (KH₂PO₄), sharp peak appeared in temperature versus dielectric constant plot. Below Curie temperature, ferroelectric hysteresis loop was observed. Ferroelectric hysteresis characteristic was also confirmed in KH₂PO₄ family: KH₂AsO₄. At that time, theoretical investigation was also started. The situation of dielectric study dramatically changed in the 1940s. Ogawa discovered that at room temperature, anomalously high dielectric constant is shown in BaTiO₃ perovskite. The relation between spontaneous polarisation and crystal structure was investigated. It was concluded that tetragonal structure is responsible for spontaneous polarisation. Hippel et al. defined that ferroelectricity is the phenomenon that high dielectric constant maximum is connected with the concerted atomic displacements. After that, ferroelectricity has been used as scientific academic term in chemistry and physics. In BaTiO₃ perovskite, ferroelectric behaviour is unstable. Because phase transition temperature is overlapped with operation temperature. In the 1990s, alternative ferroelectric, lead zirconate titanate: PbZr_xTi_1-xO₃ perovskite (PZT) was employed as ferroelectric of Ferroelectric Random Access Memory (FeRAM). However, from the viewpoints of environment and health problems, Pb-free ferroelectrics have been required. Finally, other ferroelectric perovskites and hafnium oxide are shortly introduced.