Field induced modification of defect complexes in magnesium-doped lithium niobate

Abstract

Dielectric constant, thermally stimulated depolarization currents (TSDC), and conductivity of undoped and 5% Mg-doped LiNbO3 single crystals between −100 °C and 200 °C have been investigated. A Debye-like dielectric relaxation with an activation energy of 135 meV is observed in the Mg-doped material, but not in undoped crystals. On heating this relaxation disappears near 140 °C and does not reappear after cooling. Anomalies observed in TSDC around this temperature are attributed to the motion of lithium vacancies, in agreement with conductivity measurements. It is proposed that in thermal equilibrium the electrons from the MgLi• donors are trapped in (4MgLi•+4VLi′) defect complexes. High-temperature poling breaks these defect complexes. The transition of the liberated electrons between the MgLi• donor centers and the NbNb forming the conduction band gives rise to the observed dielectric relaxation.