Nonlinear Dielectric Susceptibilities of Sr0.61Ba0.39Nb2O6 Relaxor Single Crystals

Abstract

Linear and nonlinear dielectric susceptibilities of Sr0.61Ba0.39Nb2O6 (SBN61) single crystals were measured along the polar c-axis. Both the linear and higher-order nonlinear dielectric responses showed significant frequency dispersion near the transition temperature (Tc), which signified the relaxor behavior in this system. The temperature dependence of the dielectric nonlinearity a3=χ3/χ14 was obtained, which showed a dip slightly above Tc. This fact seems to reflect the growth of polar nanoregions into long-range polar domains at around and below Tc. The second-harmonic nonlinear dielectric susceptibility showed significant values above Tc in the paraelectric regime, where the point group is 4/mmm with an inversion symmetry. We attributed this observation to the existence of polar clusters with broken inversion symmetry, which may be the remnants of long-range order above Tc due to the quenched random fields. This result is consistent with the suggestions made based on the linear birefringence, the strain and the second-harmonic generation measurements.