We show here conclusively that the internal field originates from nonstoichiometric point defects in LiNbO3 crystals. The switching fields required for 180° domain reversal in congruent crystals [C=Li2O/(Li2O+Nb2O5)=0.484] are ∼4–5 times larger than the switching fields for nearstoichiometric crystals (C=0.498). An internal field of ∼2.5 kV/mm observed in congruent crystals disappears in stoichiometric crystals. The concentration of hydrogen incorporated during crystal growth has no effect on the switching or internal fields. The measured spontaneous polarization, Ps=80±5 μC/cm2 is relatively insensitive to the crystal nonstoichiometry and the hydrogen content.
We grew LiTaO3 single crystals with a composition close to stoichiometry by using a double crucible Czochralski method. The switching field required for 180° ferroelectric domain reversal and the internal fields originating from nonstoichiometric point defects were compared for the stoichiometric and conventional commercially available crystals. The switching fields for the domain reversal in the stoichiometric crystal with a Curie temperature of 685 °C was 1.7 kV/mm. This is about one thirteenth of the switching field required for the conventional LiTaO3 crystals with a Curie temperature near 600 °C. The internal field in the stoichiometric crystal drastically decreased to 0.1 kV/mm.
Recent studies have shown that lithium nonstoichiometry has a tremendous influence on domain reversal characteristics in ferroelectric lithium tantalate. This work presents a systematic study of the domain reversal characteristics such as threshold coercive fields for domain reversal, domain stabilization times, ''backswitching'' phenomena, domain switching and wall pinning times, and sideways wall mobility in near-stoichiometric LiTaO 3 with Li/͑LiϩTa͒ϳ0.498. These properties are contrasted with those of congruent LiTaO 3 ͓Li/͑LiϩTa͒ϳ0.485͔. A qualitative model is proposed based on nonstoichiometric dipolar defects to explain the dependence of threshold coercive field on defect density, and on repeated field cycling, the origin of domain backswitching, and domain stabilization times.
Green-induced infrared absorption ͑GRIIRA͒ was investigated by a photothermal technique for undoped and Mg-doped LiNbO 3 crystals that have different Li/Nb ratios. Threshold effect on GRIIRA was found, threshold MgO concentrations being the same for GRIIRA and photorefraction. We suggest that GRIIRA is associated with the formation of the small polaron that is located on Nb antisite defect. The remarkable decrease of GRIIRA in Mg:LiNbO 3 can then be attributed to the elimination of this intrinsic defect, Nb in Li, following the incorporation of Mg on Li sites. For nonlinear optical applications, LiNbO 3 doped with MgO at concentrations over threshold has a combined advantage of having almost no GRIIRA and photorefraction.
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