Influence Of Li-rich Vapor Transport Equilibration On Surface Refractive Index And Mg Concentration Of Mgo (5 Mol%)-doped Congruent Linbo₃ Crystal

Many important physical properties of lithium niobate crystal(LiNbO₃,LN) strongly depend on the composition and lattice defects inside the crystal. Vapor transport equilibration (VTE) is a practical and effective method to alter the Li-composition in a congruent LN crystal. Up to date, a number of papers involving the preparation of Li-rich or Li-poor lithium niobate using vapor transport equilibration technique have been reported. Taking it as the main technique of this paper, micro-impact and characters of VTE-induced LN were discussed.Physical appearance of MgO:LN was fully researched from the growth of crystal, crystalline phase, and technology for composition description.Then, a refractive index model for MgO:LN was discussed. From proposing a defect structure model, the effects on refractive index was discussed when Nb on Nb site, Nb on Li site, Mg on Li site, plasmons and phonons respectively. After that, a generalized Sellmeier equation which describes the refractive indices of LN and Mg:LN as a function of composition, wavelength, and temperature. The equation gives an accurate description of the refractive indices in the composition range from 0 to 9 mol% Mg, in the wavelength range from 400 to at least 1200nm and for the temperatures between -50 and at least 250℃. The maximum error in the calculated refractive indices is about 0.002.On that basis, Li-rich VTE effects on surfaces refractive index of congruent MgO(5mol%):LiNbO₃ crystals were investigated. The results show that the influence of the VTE on the surface ordinary index no is substantial in both the variation tendency against VTE duration and the index change amplitude. The surface extraordinary index ne shows VTE effects similar to no but with smaller alteration amplitude.SIMS study shows that the diffusion of MgO to the crystal surface has taken place in the early stage of VTE and resulted in an inhomogeneous depth profile and varied surface Mg concentration depending on VTE duration. NAA results show that the Mg ion hardly evaporates away from the crystal. As the VTE is prolonged, the Li concentration tends to equilibrium, part of Mg ions at crystal surface diffuse back into the bulk of the crystal toward equilibrium, resulting in the decrease of surface Mg concentration and hence improved homogeneity of the Mg concentration. After the 140h VTE procedure the Mg concentration at any depth position is homogeneous within the spatial range of at least 10 m. Li-rich VTE results in reduction of defect number and increase of composition in the crystal, and did not modify the crystalline phase.At last, it was suggested how Mg ions occupy the sites and the crystalline phase in Mg-doped LN.