Postgrown Li-rich vapor transport equilibration-induced Mg diffusion within MgO:LiNbO₃ crystal

A number of congruent LiNbO₃ crystals homogeneously doped with 5 mol% Mg in growth melt were subjected to Li-rich vapor transport equilibration (VTE) treatments at 1100 °C for different durations. Secondary ion mass spectrometry study shows that the VTE induces the Mg diffusion within the crystal and an inhomogeneous Mg depth profile. The surface Mg concentration, determined from measured ordinary refractive index, shows a strong VTE duration dependence. Neutron activation analysis shows that the amount of MgO diffusing out of the crystal is ignorable, allowing to conclude that the Mg ions counter diffuse to the crystal surface at the early stage of VTE and then come back toward equilibrium as the Li concentration comes to equilibrium. The VTE-induced Li₂O content increase in crystal was determined by the gravimetric method. The crystalline phase, crystal composition, and site occupation of Mg and Li are discussed.