Role of iron centers in the photorefractive effect in barium titanate

Abstract

Variably valent iron impurities have been suggested as the source of photorefractive charge carriers in BaTiO3. High-purity BaTiO3 crystals were grown with transition-metal impurity levels below the 1016 cm−3 baseline estimated for photorefractivity. Iron-doped crystals were grown with concentrations of 5, 50, 250, 500, 750, and 1000 parts in 106 of iron. Changes in iron valence monitored by optical absorption in the PO2 range 1–10−4 atm were found to be consistent with a defect-chemical model indicating Fe3+ and Fe4+ to be the stable valence states in this range. Photorefractive characterization of high-purity BaTiO3 suggests that variably valent iron ions are not the dominant photorefractive species in these crystals, whereas the role of iron centers in doped crystals is complicated by large absorption losses.