**5. Conclusions**

In summary, the LN:Mo,Zn crystals were grown with different concentrations and measured about photorefractive properties. Through the experiments, we find that its photorefractive properties are similar to those of LN:Mo,Mg crystals, but different from those of LN:Mo,Zr crystals, and the doping of zinc can shorten the response time and improve the photorefractive sensitivity of the LN:Mo,Zn crystal, especially 0.65 s and 4.35 cm/J at 442nm for the LN:Mo,Zn7.2 crystal. It is indicated that the valence state of ions has a significant effect on the photorefractive properties of crystals. The valence states of Mo ions in crystal was identified by the XPS results, and the MoNb<sup>+</sup> and MoLi<sup>3</sup>+/4<sup>+</sup> defects were served as the photorefractive center for fast photorefraction. The experimental results show that the LN:Mo,Zn7.2 crystal can be used as another candidate material for holographic storage.

**Author Contributions:** Conceptualization, L.X., H.L. and Y.K.; Funding acquisition, D.Z., T.T. and Y.K.; Investigation, L.X.; Methodology, L.X., D.Z., S.L., S.C., and L.Z.; Project administration, Y.K. and J.X.; Resources, L.X., S.L., S.C. and J.X.; Writing (original draft), L.X.; Writing (review and editing), L.X., H.L., D.Z., S.S., X.W., L.Z., and Y.K.

**Funding:** This research was partially supported by the National Natural Science Foundation of China (11674179, 61705116, and 61605116) and the Program for Changjiang Scholars and Innovative Research Team in University with grant [IRT\_13R29], National Basic Research Program of China (2013CB328706), and International Science & Technology Cooperation Program of China (2013DFG52660).

**Conflicts of Interest:** The authors declare no conflicts of interest.
