Nature of low frequency dielectric permittivity behaviour in weakly and moderate concentrated KTaO3 : Li

Abstract

The low frequency complex dielectric permittivity (epsilon' and epsilon'') have been studied in the frequency-temperature region 100 Hz - 1 MHz and 5 K - 300 K in weakly (x<0.01) and moderatly (x < 0.1) concentrated K1-xLixTaO3 (KI-T). The epsilon'(T) dependencies reveal one or two well-pronounced temperature maxima depending on the Li concentration. Only one maximum in the epsilon'(T) and epsilon'' (T) behaviour is present in the weakly doped KLT, whose magnitude strongly depends on frequency. For the moderatly Li concentration an additional relaxation temperature maximum emerges with a larger potential barrier of the dipolar reorientation. In both cases the magnitudes of the permittivity maxima with temperature appear to be very large, which can not be explained only by the Li+ off-centre reorientations themselves. We present a theory, which considers two types of the Li+ related relaxors coupled to the soft TO mode that describes quantitatively the phenomena observed.