The population and activity of oxygen in the diffusion boundary layer within a congruent LiNbO₃ melt

The population and activity of oxygen in the diffusion boundary layer (DBL) of a congruent lithium niobate (c-LN) melt were ascertained by measuring the crystallization electromotive force (c-EMF) resulting from the segregation of ionic species during c-LN crystal growth. A non-zero c-EMF was observed during c-LN growth and its magnitude varied with the growth rate; the resulting crystals were always composed of c-LN irrespective of the c-EMF magnitude. The positive EMF potential in the crystal and negative potential in the melt were attributed to the presence of a positive charge in the crystal and a negative charge in the DBL. The negative EMF in the DBL indicates that the ratio between the metals (Li and Nb) and oxygen was not stoichiometric in this region, although this ratio was stoichiometric in the bulk melt. In addition, the oxygen concentration was high relative to the concentration of metals, indicating that oxygen can be regarded as an independent component. Thermodynamic analyses demonstrated that the equilibrium partition coefficient, k₀, for oxygen in this system is unity, such that the concentration of oxygen in the melt will be equivalent to that in the crystal. Thus, the oxygen activity will be unity throughout the melt, including the DBL, and will equal that in the LN crystal that is grown, while low Li and Nb concentrations will be present in the DBL with the congruent ratio.