Aging and other related phenomena in potassium niobo-tantalate crystals studied through the frequency dependence of their dielectric constant

We report on dielectric measurements in the frequency range from 1 to 1000 kHz in two disordered ferroelectric KTa1-yNbyO3 (KTN) crystals (y congruent to 0.02). We have studied the frequency dependence of the dielectric const ant (real and imaginary parts) during several thermal evolutions: isotherma l aging, rejuvenation by cooling or heating after a temperature plateau, me mory. In every case the frequency dependence is well described by a power l aw. The temperature and plateau duration dependences of the exponents have been studied. We give a description of our results within a model where the variations of the dielectric constant are attributed to the motion (growth and reconformation ) of ferroelectric domain walls. In particular we are a ble to explain the identity of the exponents of the frequency power law dep endence of memory and isothermal aging.