Electric current induced compositional variation in LiNbO₃ fiber crystal grown by a micro-pulling down method

The electric-field modified partitioning of Li ion species was studied for the growth of LiNbO₃ fiber crystals via a micro-pulling down (μ-PD) method. DC power was applied between the platinum seed and the outlet of a capillary pipe attached to the bottom of the crucible so as to generate a constant electric current. The electric current, passing through the molten zone, the solid-liquid interface and the crystal, induced an electric field near the interface which influences the transport and segregation of ionic species in the melt. The compositional change in terms of mol% Li₂O was examined by measuring the Curie temperature of crystals grown from melts with various compositions. The melt with Li₂O concentration larger than the congruent-melting composition (k₀^{Li2 O} < 1) decreased its Li₂O content near the interface by an electric current flowing from the crystal to the melt, while it did not change by the inverse current. In contrast, the melt with Li₂O concentration smaller than the congruent composition (k₀^{Li2 O} > 1) increased its Li₂O content by the electric current from the melt to the crystal, while it did not change by the inverse current. The above variation was peculiar to growth via a μ-PD method under an electric field induced by an imposed electric current, which confirmed that the major ionic species driven by the electric field was the Li ion species resulting in a change in the bulk Li₂O concentration both in the melt near the interface and in the grown crystal.