Dielectric and ac conductivity studies on undoped and MgO-doped near-stoichiometric lithium tantalate crystals

Abstract

Dielectric and ac impedance studies have been carried out on undoped and Mg (0.5 and 1 mol%) doped near-stoichiometric lithium tantalate (SLT) single crystals. No particularly significant qualitative change is observed in the nature of the ferroelectric phase transition on doping by Mg. However, an anomaly is observed in the ac impedance data: the ac conductivity is found to be low for 0.5 mol% doping of Mg, compared to 1.0 mol% doping. Also, the activation energy, calculated from the Arrehenius equation, is highest for 0.5 mol% doping, which suggests that hopping of Li⁺ ions becomes more hindered in 0.5 Mg SLT. The Nyquist plot shows a single semicircle above 100 Hz. The electrical behavior of lithium tantalate can be modeled as a resistance parallel to a constant-phase element. The calculated impedance from this equivalent circuit is also found to be high for the 0.5 mol % doping. This feature is explained in terms of a Li-vacancy model.