Non-linear dielectric properties of KTa1-xNbxO3

We report measurements of the non-linear dielectric constant in the mixed crystal KTa_1-xNb_xO₃ with x=15.7% and 1.2%. For both concentrations the dielectric peak is observed to decrease with increasing field and to shift towards higher temperature. The dispersion ε´(w), is also suppressed by the field. On the framework of Landau theory, we fit ε´(E) with an expansion in a power series of the applied dc field E. ε⁽²) is seen to exhibit two distinct temperature dependencies. At high temperatures, it scales as (T-T_c)^-γ where γ is found to be in fair agreement with Landau theory for low concentrations. At lower temperatures it then crosses over to a much weaker dependence. This crossover is interpreted as a transition from a weakly correlated (dynamic) regime to a strongly correlated (quasistatic) regime of polar clusters. The non-analyticity due to these polar clusters is discussed