Raman Scattering Study on the Phase Transition Dynamics of Ferroelectric Oxides

Abstract

Ferroelectric phase transitions are conventionally divided into two types: an order-disorder and a displacive-type.[1] In the former one, which is frequently seen in hydrogen-bond-type ferroelectrics such as KH2PO4 (KDP), local dipole moments μs are randomly distributed be‐ tween opposite directions in the paraelectric phase, leading to zero macroscopic net polariza‐ tion P (= Σμ). A spontaneous polarization in the ferroelectric phase is then driven by their ordering through the ferroelectric phase transition. In the later type, in contrast, there are no dipole moments in the paraelectric phase. The spontaneous polarization in the ferroelectric phase stems from relative polar displacement between cationic and anionic sublattices. It is in‐ duced by freezing of a so-called ferroelectric “soft mode”, which is known as a strongly anhar‐ monic optical phonon mode at Γ-point in the Brillouin zone (BZ). The displacive-type transition is often found in the ferroelectric oxides as represented by perovskite-type com‐ pounds such as PbTiO3. Since the ferroelectric oxides have been widely applied for the elec‐ tronic devices, such as actuators, sensors, and memories, due to their chemical stability, the large spontaneous polarization, and the relatively high transition temperature, a better under‐ standing of the soft mode behavior would be important from viewpoints of both application and fundamental sciences. Spectroscopic techniques employing light scattering, infrared ab‐ sorption, and neutron scattering have been generally utilized for investigating the dynamics of the soft mode. Among them, Raman scattering has an advantage especially in a low-fre‐ quency region, which is significant to resolve the critical dynamics of the soft mode near the transition temperature. In the present review, we will discuss the soft mode behavior at the ferroelectric phase transition by referring the Raman scattering studies on CdTiO3 and 18Osubstituted SrTiO3. Furthermore, a microscopic origin of the soft mode is figured out from a viewpoint of local chemical bonds with the help of first-principles calculations.