**4. Conclusions**

LN: Mg single crystals doped with different MgO concentrations were grown successfully in one furnace at the same time by the Bridgman method. Wet chemistry method was employed for LN: Mg polycrystalline powders preparation to enhance doping homogeneity of MgO and avoid Li2O volatilization. The critical growth conditions included a small vertical temperature gradient, a low growth rate, adjusting the descending speed at different growth stages to keep a micro convex solid-melt interface. X-ray rocking curves of (001) reflection showed that the LN: Mg5 crystal had a high crystallinity. Compared with the congruent LN crystal, the OH− absorption peaks and the ultraviolet absorption edge exhibited that the doping concentration of MgO exceeded the threshold. Moreover, LN: Mg5 crystal had high optical homogeneity for the extraordinary refractive index gradient that was as small as 2.5 <sup>×</sup> <sup>10</sup>−5/cm.

**Author Contributions:** T.T. conceived and designed the experiments. T.T., X.Y. and M.W. performed the experiments, analyzed the data. H.S., D.Z., and Y.Z. contributed the measurements. J.X. contributed useful and deep discussions. T.T. and X.Y. wrote the manuscript. All authors read and approved the final version of the manuscript to be submitted.

**Funding:** This work was partially supported by the National Natural Science Foundation of China (61605116, 51972208, and 51972213), Jiangsu Planned Projects for Postdoctoral Research Funds (1501131C).

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

## **References**


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