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

KTa{sub 1{minus}{ital x}}Nb{sub {ital x}}O{sub 3} (KTN) in an intermediate-concentration range ({ital x}=15.7%) has been studied using dielectric-constant and spontaneous-polarization measurements. A large peak in the dielectric constant ({epsilon}{sub max}{congruent}70 000) is observed at the temperature {ital T}{sub {ital c}}{sup *}. From room temperature down to {ital T}{sub {ital c}}{sup *}+20 K, {epsilon} obeys a Curie-Weiss law with {ital T}{sub 0}{similar to}142.3 K. Below, {epsilon} deviates from this law and {ital P}-{ital 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 {epsilon} peak at {ital T}{sub {ital c}}{sup *} 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 {epsilon}({ital E},{ital T}) for different dc-bias fields, we have characterized the dielectric nonlinearities in the transition range; the first nonlinear coefficient, {epsilon}{sup (2)}, diverges in two distinct ranges, as ({ital T}{minus}{ital T}{sub {ital c}}{sup *}){sup {minus}{ital n}}, with {ital n}{similar to}9 initially and with {ital n}{congruent}2 closer to {ital T}{sub {ital c}}{sup *}. These results suggestmore » that diffuse phase transitions in mixed ferroelectrics are due to the ordering of polar cells modified by mutual strain interactions.« less