[14p-421-1~12] 9.1 誘電材料・誘電体

▲ [14p-421-3] Electric Field Induced Elastic Anomaly in Uniaxial Relaxor Ferroelectric Ca_xBa_1-xNb₂O₆ Single Crystals Studied by Broadband Brillouin Scattering Spectroscopy

〇(D)Aftabuzzaman Md¹、Helal Md Al¹、Dec Jan²、Kleemann Wolfgang³、Kojima Seiji¹ (1.Univ. Tsukuba、2.Univ. Silesia、3.Univ. Duisburg-Essen)

キーワード：Brillouin scattering, uniaxial relaxor, nanodomain, macrodomain

The relaxor ferroelectric (REFs) Ca_xBa_1−xNb₂O₆ (x = 0.30, CBN30) single crystals with uniaxial tetragonal tungsten bronze (TTB) structure were investigated under the external electric field (E) along a [001] direction using the broadband Brillouin scattering spectroscopy. On field heating (FH) under 1.0 kV/cm, the increase of longitudinal acoustic velocity (V_LA) was observed around 160 ºC due to the alignment of nanodomains along the field direction. Consequently, the Curie temperature (T_C) shift towards the higher temperature at which a sharp minimum of V_LA was observed. Under 0.5 kV/cm, both the observed alignment of nanodomains and the shift of T_C were lower than those of 1.0 kV/cm. Since, the E suppresses the random fields (RFs) which are responsible for the diffusive phase transition, a decrease of the diffuseness and enhancement of the long-range ferroelectric order were observed by increasing the field strength. Therefore, under the higher E, the alignment of nanodomains into macrodomains is enhanced and the phase transition becomes sharper. Below T_C, upon the application of the E, a coexisting state of nano- and macrodomains was observed at around 3.0 kV/cm. The macrodomain state is induced by the E corresponding to the high-frequency LA (LA_H) mode, whereas the nanodomain state is due to the pinning of domain walls by the RFs corresponding to the low-frequency LA (LA_L) mode. This coexisting state persists up to 13 kV/cm due to incomplete switching of the nanodomains into the macrodomains or single-domain state. Near T_C, the effect of RFs increases which try to stabilize the nanodomains by pinning of domain walls, consequently suppress the formation of the macrodomains. In addition, the piezoelectiric coupling of the local strain with local polarization of PNRs becomes stronger in the vicinity of T_C and therefore, the frequency of the LA_H and LA_L modes decrease towards T_C.