Specific features of growth and structure of LiNbO$_{3}$ : Zn crystals near the ZnO concentration threshold of 6.76 mol %

The crystallization conditions and Raman spectra of LiNbO$_{3}$ : Zn crystals (0.02–8.91 mol % ZnO in the melt) have been investigated. It has been established that the most favorable conditions for growing optically and compositionally homogeneous heavily doped LiNbO$_{3}$ : Zn crystals, which are characterized by a low photorefractive effect, are implemented in the ZnO concentration range of $\sim$4.0–6.76 mol % in the melt. Since the distribution coefficient $K_{\operatorname{eff}}$ decreases significantly with an increase in the ZnO concentration in the melt, one can obtain LiNbO$_{3}$ : Zn crystals with significantly different defect structures but identical zinc concentrations. A change in the zinc concentration in crystals has been shown to induce a stepwise change in the sequence order of the main (Li and Nb) and doping (Zn) cations and vacancies and stepwise anisotropic expansion of the oxygen octahedra along the polar axis. The number of kinks in the concentration behavior of the spectral-line widths (five kinks for the lines with frequencies of 630 ($A_1$(TO)) and 876 cm$^{-1}$ ($A_1$(LO))) significantly exceeds the number of thresholds (two) known from the literature.