Dielectric constant and ordering effects of pure and doped KTaO3 quantum paraelectric; Cu-codoping enhancement effect

epsilon'(T, f) and tan[delta(T, f)] spectra (10-300 K, 120 Hz-1 MHz) of pur e KTaO3 (KTO), KTO:Li (0.1-3%), KTO:Fe 1%, KTO:Cu 0.83% below 70 K reveal a Debye-like universal Arrhenius relaxation with Delta approximate to 80-85 meV, tau(0) similar to 5 x 10(-14) s. In KTO:Li + Fe and KTO:Li + Cu this c ontribution increases, confirming dipole-pair ordering effects with the lea ding role of Li+ in pair relaxation (V. Trepakov er al., J. Phys.: Condens. Matter 7, 3785 (1995)). A dramatic Cu-codoping dielectric enhancement effe ct in KTO: Li + Cu was observed. In KTO:Li 3% + Cu 0.2% in region 50-60 K e psilon'(T) reveals a maximum of similar to 10(5), accompanied by temperatur e hysteresis and dielectric loops. Ferroelectric-type phase transition with a critical dynamic central peak, induced by Cu2+(Ta)V-O centers as a slowe st relaxator or oscillator in KTO:Li, is considered as an epsilon' enhancem ent cause. A possibility of incommensurate phase induced by Cu2+-V-O center s and cluster-induced cooperative behavior is discussed too.