A comparative study of the electrical properties of reduced and unreduced LiTaO3 crystals

Abstract

We have carried out a comparative study of the electrical properties of lithium tantalate (LiTaO₃) crystals in a wide temperature range (300–900 K) before and after reductive treatment in H₂O vapor and subsequent oxidative annealing. The results demonstrate that, in the temperature range of Li⁺ ion conduction (550–900 K), the activation enthalpy for ionic conduction in the reduced lithium tantalate crystal is H_a = 1.37 eV, which slightly exceeds that in the initial state of the crystal (1.34 eV). In the temperature range 390–450 K, the σ(T) data for the unannealed crystal are well represented by the Arrhenius law in the presence of two carrier types, with activation energies E₁ = 1.03 eV and E₂ = 0.29 eV, characteristic of proton and electron hopping conduction, respectively. After reductive annealing, the activation energy for conduction is ~0.65 eV, characteristic of the activation energy for bipolaron conduction. After subsequent oxidative annealing of the reduced crystals in dry air, the activation energy is ~1.2 eV. It seems likely that the presence of oxygen vacancies in the reduced LiTaO₃ crystal stimulates hydrogen release from the crystal during oxidative annealing.