Growth, Structure And Properties Of Lithium Niobate And Lithium Tantalum Doped With Zinc

Lithium niobium and Lithium tantalum, due to excellent electrics and optical properties, have been widely applied to volume holographic memory. However, light scattering can be induced by high light intensity in lithium niobium and lithium tantalum, which results in serious holograph distortion. simultaneously, because of its high coercive field, periodic polarization wasn't achieved at room temperature in LiNbO₃ and LiTaO₃ crystals. In this dissertation, in order to improve the resistant ability to optical damage and coercive field, ZnO were doped into LiNbO₃ and LiTaO₃ .In this dissertation, Zn:LiNbO₃ and Zn:LiTaO₃ single crystals were grown by the Czochraski method with intermediate frequency heating. The optimum technological parameters were adopted. There were no macroscopic defects and good optical homogeneity in the as-grown boules. Size of as-grown boules was aboutΦ40×20mm. Vapor transport equilibration method was used to fabricate the near-stoichiometric crystal.X-ray powder diffraction experiment showed that lattice structure of the Zn:LiNbO₃ and Zn:LiTaO₃ crystals was similar to that of the congruent sample. In Zn:LiNbO₃ and Zn:LiTaO₃ crystals, results of unit cell parameters indicated that the lattice constant increased with the ZnO concentration increasing. The change mechanism of the UV-Vis absorption edge and the IR absorption peak was also discussed in the Zn:LiNbO₃ and Zn:LiTaO₃ single crystals.Analysis results showed that Zn2+ ions took priority of replacing anti-site Nb ( Nb L4+i). When the ZnO concentration reaches the threshold concentration, all of the Nb L4+i ions were completely replaced, and Zn2+ ions begin to occupy the normal Li and Nb sites simultaneously, thus self-compensation defect structure ( Zn 3N-b-3 Zn L+i) was formed. In LiTaO₃ crystals, Zn2+ ions replace anti-site Ta ( Ta L4+i). All of the Ta L4+i were completely replaced when ZnO concentration reached the threshold concentration, and Zn2+ ions had to replace the normal Li site.Curie temperature of the Zn:LiNbO₃ and Zn:LiTaO₃ crystals was obtained from DTA measurement. Curie temperature increased with ZnO concentration increasing when ZnO concentration was below the threshold concentration. However, when ZnO concentration was over the threshold concentration, the curie temperature began to decrease.Z-cut Zn:LiNbO₃ and Zn:LiTaO₃ were polished to make polarization hysteresis loop measurements. The spontaneous polarization was insensitive to the concentration of ZnO. The hysteresis loops of all samples were asymmetric due to existence of the internal field. The influence of ZnO content on the coercive field and internal field was investigated. The high coercive field and internal field in the congruent crystal were resulted from high concentration of the anti-site Nb and anti-site Ta in comparison with near-stoichiometric crystal.The photorefractive properties of the Zn:LiNbO₃ and Zn:LiTaO₃ crystals, including diffraction efficiency and photorefractive response time, were obtained by two-beam coupling experiments. With the ZnO concentration increasing, diffraction efficiency and erasure time decreased, and the photorefractive response speed increased. In near stoichiometric LiNbO₃ and LiTaO₃ crystals, diffraction efficiency and erasure time were lower than those of congruent crystals.The light scattering resistance of the Zn:LiNbO₃ and Zn:LiTaO₃ crystals was studied by the transmission facula distortion method. When the ZnO concentration was below threshold concentration, the light scattering resistance of samples showed no obvious change. In contrast, when the ZnO concentration reached threshold concentration, the light scattering resistance of samples increased sharply. The light scattering resistance was two orders of magnitude higher than that of pure LiNbO₃ and LiTaO₃ crystals. The light scattering resistance of the near-stoichiometric LiNbO₃ and LiTaO₃ crystals is better than that of congruent crystals. Analysis results indicated the photoconductivity was mainly responsible for the light scattering resistance.