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Abstract: We have measured OH absorption spectra of a 0.47-mm-thick Z-cut MgO (5mol% in melt):LiNbO3 crystal subjected to post-growth Li-poor vapor transport equilibration (VTE) treatments at 1100°C for different durations ranging from zero to 395h. These spectra allow the evolution of OH absorption characteristics with prolonged VTE to be followed. After 2h of VTE process an additional absorption appears at 3483cm−1. A transition regime that the original 3536cm−1 and new 3483cm−1 absorptions simultaneously appear exists within the VTE duration range of 2–16h. In this regime, the 3536cm−1 absorption becomes weak gradually and eventually disappears around 16h while the 3483cm−1 absorption increases remarkably with the prolonged VTE. The presence of transition regime gives a hint that the 3483cm−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 Li2O content reduction and the measurement of the surface Li2O content as a function of the VTE duration. A quantitative analysis and discussion shows that the model is well supported by the experimental results. [Copyright &y& Elsevier]