Dielectric permittivity and Fe-and Cu-doping effect in KTaO₃ AND K_1-xLi_xTaO₃

Abstract

The temperature-frequency behavior of the permittivity ϵ' (T, f) and tan[δ(T, f)] (5—300 K, 100 Hz-1 MHz) were comparatively studied in nominal pure KTaO₃ and K_1-xLi_xTaO₃ (x = 0.001, 0.01 and 0.03) doped by Cu and Fe single crystals. In the ϵ' (T, f) behavior doping of KTaO₃ by Fe (1 wt%) and Cu (0.8 and 3 wt%) results in ϵ' increasing at lowest temperatures up to ≈ 6000 at 5K) only. However for tan [δ(T, f)] a pronounced dielectric dispersion occurs below 70 K, with tan [δ(T)] maxima positions obeying the Arrhenius law with universal values of activation energy Δ ≈ 900—1000 K and of high temperature relaxation time τ₀ ∽ 10⁻¹⁴ s. In contrast, in K_1-xLi_xTaO₃ doping by Cu and Fe results in a well pronounced low-temperature dielectric relaxation contribution in the ϵ' (T, f) behavior and dipole ordering effects. This effect appears to be very strong for Cu doping. So in K_0.97Li_0.03TaO₃:Cu (0.2 wt%) appearance of a low temperature phase transition is suggested. The possible mechanisms of polarization relaxation, dipole ordering and ferroelectric-type phase transition with a critical dynamic central peak in K_1-xLi_xTaO₃ doped by strong soft dipole defects are discussed.