Hyper-Raman scattering from the LO modes of perovskite ferroelectric relaxors

Abstract

Hyper-Raman scattering is used to study the temperature dependence of the longitudinal optic (LO) modes of three prototypical ferroelectric relaxors in a broad temperature range from 20 to 800 K. The three LO bands observed in all spectra of the three materials are linked to the three transverse optic (TO) modes in cubic relaxors where LOi is linked to TOi, with i = 1 to 3. The Last, Slater, and Axe eigenvector pictures of the TO modes are consistent with our observations on LO1, LO2, and LO3, respectively. Within this framework, the splitting of LO2 would mostly be linked to a structural disorder on the B site while the strength of an anomaly of LO1 near the freezing temperature Tf relies on the ability of the material to develop a long-range ordering. Moreover, the more pronounced the splitting, the more important the structural disorder and the lower the value of the dielectric constant.