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

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 Mg_Li^• donors are trapped in (4Mg_Li^•+4V_Li^′) defect complexes. High-temperature poling breaks these defect complexes. The transition of the liberated electrons between the Mg_Li^• donor centers and the Nb_Nb forming the conduction band gives rise to the observed dielectric relaxation