Fano resonance of Li-doped KTa1−xNbxO3 single crystals studied by Raman scattering

Abstract

The enhancement of functionality of perovskite ferroelectrics by local structure is one of current interests. By the Li-doping to KTa1−xNbxO3 (KTN), the large piezoelectric and electro-optic effects were reported. In order to give new insights into the mechanism of doping, the microscopic origin of the Fano resonance induced by the local structure was investigated in 5%Li-doped KTN single crystals by Raman scattering. The coupling between the continuum states and the transverse optical phonon near 196 cm−1 (Slater mode) caused a Fano resonance. In the vicinity of the cubic-tetragonal phase transition temperature, TC-T = 31 °C, the almost disappearance of the Fano resonance and the remarkable change of the central peak (CP) intensity were observed upon heating. The local symmetry of the polar nanoregions (PNRs), which was responsible for the symmetry breaking in the cubic phase, was determined to E(x, y) symmetry by the angular dependence of Raman scattering. The electric field induced the significant change in the intensity of both CP and Fano resonance. From these experimental results, it is concluded that the origin of the Fano resonance in Li-doped KTN crystals is the coupling between polarization fluctuations of PNRs and the Slater mode, both belong to the E(x, y) symmetry.

Electric field induced critical points and field gradient by trapped electrons in Li-doped KTa1-xNbxO3 single crystals

“…Recently, Rahaman et al reported the breaking of local symmetry and the microscopic origin of the CP in the cubic phase of a high quality KLTN/5/27 crystal by the observation of E(x,y) symmetry of PNRs with the rhombohedral R3m symmetry using Raman scattering. 15 It is important to note that the intensity of the LA mode is relatively weak and it exhibits asymmetric nature (Fig. 1).…”

“…11,12 The existence of PNRs in KTN and Li-doped KTN was reported by various measurements. [13][14][15][16][17][18][19] In the cubic phase, the PNRs are characterized by two temperatures. First, the Burns temperature, T B , at which the dynamic PNRs begin to appear.…”

“…20 Second, the intermediate temperature, T*, at which dynamic-to-static transition of PNRs starts. [13][14][15][16] Due to the giant electro-mechanical response in the vicinity of the critical end point (CEP) 21 the study of the temperaturefield phase diagram is the current topics in advanced materials such as ferroelectric materials. Aftabuzzaman and Kojima also reported that the CEP is the driving mechanism for the polarization rotation and the related giant electromechanical response of relaxor ferroelectrics.…”

Role of polar nanoregions with weak random fields in Pb-based perovskite ferroelectrics

“…Therefore, to identify the local structural symmetry of PNRs, the angular dependence of Raman scattering (one of ILS) combined with Raman tensor analysis can be a very powerful technique, because it measures the scattering intensity with respect to the polarization direction of an incident light. Recently, the local symmetry of CPs and some other Raman modes in a paraelectric cubic phase related to rhombohedral PNRs were identified by the angular dependence of Raman scattering in Li-doped KTN18 and PMN19 single crystals. The angular dependences of Raman spectra of PT and PMN-33PT were also measured in the tetragonal phase to identify the symmetry of the modes2021.…”

“…To observe the variation of the diagonal and off-diagonal elements in the Raman tensor in the vicinity of a phase transition point, it is necessary to measure both the polarized (VV) and depolarized (VH) scattering separately18. So far, the low-frequency ILS studies on a phase transition have been performed by using VV or unpolarized light scattering14232425262728.…”