Dielectric Response in Microscopically Heterogeneous Dielectrics: Example of KTaO_3:Nb

摘要

We showed that the striking dielectric properties of KTN 1.8 can be understood and described on the basis of assumption that in highly polarizable quantum paraelectrics even a small concentration of dipolar impurity centres can form nano-scale polar regions (clusters). The main addition to the dielectric permittivity at high temperatures originates from these local heterogeneities. At high temperatures, when the correlation radius is larger than the heterogeneity radius, the heterogeneities provide a Curie-Weiss law and an average over the bulk Curie temperature governs the temperature dependence of the total dielectric permittivity. When approaching the Curie temperature the correlation radius increases but it saturates when reaching the heterogeneity size. Together with quantum effects this leads to the appearance of an intermediate state, in which the dielectric permittivity is saturated. The orientation of the cluster dipole moments as well as local random fields in an external field results in the appearance of an additional contribution to the dielectric permittivity in the low temperature ferroelectric phase as it has been observed for KTN 1.8.