NEW DIPOLE RELAXATORS IN WEAKLY REDUCED KTaO3:Li CRYSTALS: DIELECTRIC SPECTROSCOPY

The temperature-frequency behaviour of epsilon'(T, f) and tan[delta(T, f)] (13 - 300 K, 120 Hz - 8 MHz) of KTaO3:Li (0.1 wt%) single crystals before and after a weak reduction was studied. Below 70 K before reduction, the samples reveal a shoulder in epsilon'(f)(T) and a maximum in tan[delta(T)] obeying Arrhenius law with an activation energy Delta approximate to 1000 K. This is associated with an ordering in clusters of paired reorienting dipolar defects, composed of off-center Li+ (the leading defect) and another polar impurity, or of a small number of 'pure' clusters of Li+ ions and(or) reorientations of the hole polaron (O- - center), localized near an impurity ion Fe3+(Fe2+). After reduction, the epsilon'(f)(T) shoulder changes into a maximum, and four new low-temperature Arrhenius-like relaxations emerge. The new relaxations are associated with: (i) charge transfer relaxation in paired oxygen vacancies V-0 - V-0 (with or without trapped electrons) or paired (Ta3+ - V-0) - (Ta3+ - V-0) centers, (ii) cooperative relaxations in clusters of centers which take part in i and (iii) reorientations of isolated Li+ off-center ions.