New dipole relaxators in weakly reduced KTaO3:Li crystals: Dielectric spectroscopy

The temperature-frequency behaviour of ϵ′ (T, f) and tan[δ(T, f)] (13 - 300 K, 120 Hz - 8 MHz) of KTaO 3 :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 ϵ′ f (T) and a maximum in tan[δ(T)] obeying Arrhenius law with an activation energy δ ≈ 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 Fe 3+ (Fe 2+ ). After reduction, the ϵ′ 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 (Ta 3+ - V 0 ) - (Ta 3+ - V 0 ) centers, (ii) cooperative relaxations in clusters of centers which take part in i and (iii) reorientations of isolated Li + off-center ions.