Influence of post-grown Li-rich and Li-poor vapor transport equilibration on composition, OH⁻ absorption and optical-damage threshold of Mg (5 mol%) : LiNbO₃ crystals

Abstract

Li-rich (Li-poor) vapor transport equilibration (VTE) treatments on a number of Z-cut 0.47 mm thick congruent MgO (5 mol% in melt) : LiNbO₃ crystals were carried out at 1100°C over different durations ranging in 1–172 (40–395) h. Neutron activation analysis shows that neither Li-rich nor Li-poor VTE-induced Mg and Nb loss from the crystal occurred. The Li₂O content in the crystal was measured as a function of VTE duration by the gravimetric method. The Li-rich/Li-poor VTE effects on OH⁻ absorption were studied in comparison with the as-grown crystal. The study shows that the Li-rich VTE results in OH⁻ absorption band annihilation. After further oxidation treatment the band reemerges and peaks at the same wavenumber as that of the as-grown crystal (∼3535.6 cm⁻¹), showing that the MgO concentration in the Li-rich VTE crystal is still above the optical-damage threshold. The Li-poor VTE causes OH⁻ band shift to 3486.3–3491.6 cm⁻¹, indicating that the MgO concentration in all Li-poor VTE crystals is all below the optical-damage threshold. Further successive Li-rich VTE and oxidation treatments on the Li-poor VTE-treated crystal lead the band to shift back to 3535.6 cm⁻¹, showing that the post Li-rich VTE brought the Li-poor VTE-treated crystal above the optical-damage threshold again. It is found that the peaking position, band width, peaking absorption and band area of the absorption at ∼3486 cm⁻¹ all increase monotonously with the decrease of the Li₂O content arising from prolonged Li-poor VTE, and quantitative relationships to the Li₂O content are established for the latter two parameters. The VTE effects on the OH⁻ absorption are conducted with the VTE-induced OH⁻ content alteration and charge redistribution.