Results of LiNbO3:Zn researches often diverge because sharp changes in physicochemical characteristics of the system melt-crystal that occur at doping are usually not taken into consideration. When a series of doped crystals is grown Zn is usually added to the melt with step 1.5-2 mol%. This obstructs detection of exact threshold concentration. We have grown LiNbO3:Zn crystals doped by 4.0 divided by 9.0 mol% ZnO with step similar to 1 mol%. Around the dopant, threshold concentration step was narrowed to similar to 0.1 mol%. We have researched physicochemical properties of the system melt-crystal and structure evolution by Raman spectroscopy and full-profile analysis of XRD. We have detected exact threshold concentration for LiNbO3:Zn and concluded that crystals doped by prethreshold concentrations of ZnO are the best for most applications because of the higher optical homogeneity.