Dielectric permittivity and Fe- and Cu-doping effect in KTaO3 and K1-XLiXTaO3

The temperature-frequency behavior of the permittivity epsilon' (T,f) and tan[delta(T,f)] (5-300 K, 100 Hz-1 MHz) were comparatively studied in nominal pure KTaO3 and K1-xLixTaO3 (x = 0.001, 0.01 and 0.03) doped by Cu and Fe single crystals. In the e' (T,f) behavior doping of KTaO3 by Fe (1 wt%) and Cu (0.8 and 3 wt%) results in epsilon' increasing at lowest temperatures up to approximate to 6000 at 5K) only. However for tan [delta(T,f)] a pronounced dielectric dispersion occurs below 70 K, with tan [delta(T)] maxima positions obeying the Arrhenius law with universal values of activation energy Delta approximate to 900 - 1000 K and of high temperature relaxation time tau(0) similar to 10(-14) s. In contrast, in K1-xLixTaO3 doping by Cu and Fe results in a well pronounced low-temperature dielectric relaxation contribution in the epsilon' (T,f) behavior and dipole ordering effects. This effect appears to be very strong for Cu doping. So in K0.97Li0.03TaO3: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 K1-xLixTaO3 doped by strong soft dipole defects are discussed.