Ferroelectric phase transition of Li-doped KTa1-xNbxO3 single crystals with weak random fields: Inelastic light scattering study

Abstract

The broadband gigahertz dynamics of a ferroelectric phase transition of the 2.5%Li-doped KTa1-xNbxO3 crystals with x = 0.38 (KLTN/2.5/38) was investigated by inelastic light scattering. Its phase transition was found to be slightly diffused type due to weak random fields (RFs) caused by disorder in B-site of perovskite structure. The microscopic origin of the central peak (CP) and the local symmetry of the polar nanoregions (PNRs) in the cubic phase were studied by Raman scattering. In Brillouin scattering, the remarkable elastic anomaly of the longitudinal acoustic (LA) mode was observed near paraelectric cubic to ferroelectric tetragonal phase transition temperature, TC-T = 46 °C, which is attributed to the coupling between fluctuating local polarization in PNRs and the LA mode. The intermediate temperature, T* ∼ 97 °C, at which a dynamic to static transition of PNRs occurs, was determined from the significant increase of damping and intensity of the LA mode and the CP upon cooling. In the vicinity of the TC-T, a slightly stretched slowing down of the relaxation time, τCP, a feature of order-disorder nature of the ferroelectric phase transition with weak RFs, was clearly observed.