{KTa}_{1 - x}

{Nb}_{x}

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 (

ε_{\max}

≃70 000) is observed at the temperature

T_{c}^{*}

. From room temperature down to

T_{c}^{*}

+20 K, ε obeys a Curie-Weiss law with

T_{0}

∼142.3 K. Below, ε 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 ε peak at

T_{c}^{*}

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 ε(E,T) for different dc-bias fields, we have characterized the dielectric nonlinearities in the transition range; the first nonlinear coefficient,

ε^{(2)}

, diverges in two distinct ranges, as (T-

T_{c}^{*}

)^{- n}

, with n∼9 initially and with n≃2 closer to

T_{c}^{*}

. These results suggest that diffuse phase transitions in mixed ferroelectrics are due to the ordering of polar cells modified by mutual strain interactions.

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