Dielectric nonlinearity and spontaneous polarization of KTa1-xNbxO3 in the diffuse transition range.

Abstract

${\mathrm{KTa}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Nb}}_{\mathit{x}}$${\mathrm{O}}_{3}$ (KTN) in an intermediate-concentration range (x=15.7%) has been studied using dielectric-constant and spontaneous-polarization measurements. A large peak in the dielectric constant (${\mathrm{\ensuremath{\epsilon}}}_{\mathrm{max}}$\ensuremath{\simeq}70 000) is observed at the temperature ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$. From room temperature down to ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$+20 K, \ensuremath{\epsilon} obeys a Curie-Weiss law with ${\mathit{T}}_{0}$\ensuremath{\sim}142.3 K. Below, \ensuremath{\epsilon} deviates from this law and P-E hysteresis loops are observed. These results indicate that the phase transition exhibits a diffuse character which is related to the appearance of microscopic polarized regions or cells. The \ensuremath{\epsilon} peak at ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$ exhibits a cusp shape and becomes flattened upon application of low dc-bias fields. A corresponding dielectric loss peak is also observed, the position of which reveals the relaxational character of the transition. Through measurements of \ensuremath{\epsilon}(E,T) for different dc-bias fields, we have characterized the dielectric nonlinearities in the transition range; the first nonlinear coefficient, ${\mathrm{\ensuremath{\epsilon}}}^{(2)}$, diverges in two distinct ranges, as (T-${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$${)}^{\mathrm{\ensuremath{-}}\mathit{n}}$, with n\ensuremath{\sim}9 initially and with n\ensuremath{\simeq}2 closer to ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$. These results suggest that diffuse phase transitions in mixed ferroelectrics are due to the ordering of polar cells modified by mutual strain interactions.