Evolution characteristic of OH absorption in Mg (5 mol% in melt):LiNbO ₃ crystal with post-grown Li-poor vapor transport equilibration

We have measured OH absorption spectra of a 0.47-mm-thick Z-cut MgO (5 mol% in melt):LiNbO ₃ crystal subjected to post-growth Li-poor vapor transport equilibration (VTE) treatments at 1100°C for different durations ranging from zero to 395 h. These spectra allow the evolution of OH absorption characteristics with prolonged VTE to be followed. After 2 h of VTE process an additional absorption appears at 3483 cm ^{- 1}. A transition regime that the original 3536 cm ^{- 1} and new 3483 cm ^{- 1} absorptions simultaneously appear exists within the VTE duration range of 2-16 h. In this regime, the 3536 cm ^{- 1} absorption becomes weak gradually and eventually disappears around 16 h while the 3483 cm ^{- 1} absorption increases remarkably with the prolonged VTE. The presence of transition regime gives a hint that the 3483 cm ^{- 1} absorption is due to the VTE-induced formation of a new center. Based upon the spectral features, we have suggested a simple three-layer (two on surface and one in the center of crystal plate) model to describe the depth profile of the photorefractive damage MgO concentration threshold in the crystal in the transition regime. A Li out-diffusion theory is suggested and correlated with the model. To support the Li out-diffusion theory, some additional experiments have been done. These include the depth profile characterization of VTE-induced Li ₂O content reduction and the measurement of the surface Li ₂O content as a function of the VTE duration. A quantitative analysis and discussion shows that the model is well supported by the experimental results.