**Xiaodong Yan, Tian Tian \*, Menghui Wang, Hui Shen, Ding Zhou, Yan Zhang and Jiayue Xu \***

Institute of Crystal Growth, School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China; 176081108@mail.sit.edu.cn (X.Y.); wmh\_fzs@163.com (M.W.); hshen@sit.edu.cn (H.S.); dzhou@sit.edu.cn (D.Z.); yanzhang@sit.edu.cn (Y.Z.)

**\*** Correspondence: tiant@sit.edu.cn (T.T.); xujiayue@sit.edu.cn (J.X.); Tel.: +86-021-608-731-17 (T.T.); +86-021-608-734-89 (J.X.); Fax: +86-021-608-731-17 (T.T.); +86-021-608-734-89 (J.X.)

Received: 31 December 2019; Accepted: 25 January 2020; Published: 29 January 2020

**Abstract:** A series of LiNbO3 crystals doped with various MgO concentrations (0, 3%, and 5 mol%) was simultaneously grown in one furnace by the modified vertical Bridgman method. The wet chemistry method was used to prepare the polycrystalline powders, and the growth conditions were optimized. The full width at half maximum of high-resolution X-ray rocking curves for (001) reflection of 5 mol% Mg doped lithium niobate (LN) crystal was about 8", which meant it possessed high crystalline quality. The OH<sup>−</sup> absorption spectra shifted to 3534.7 cm<sup>−</sup>1, and the UV absorption edge violet shift indicated that 5 mol% MgO successfully doped in LN and exceeded the threshold. The extraordinary refractive index gradient of 5 mol% Mg doped LN crystal was as small as 2.5 <sup>×</sup> <sup>10</sup>−5/cm, which exhibited high optical homogeneity.

**Keywords:** lithium niobate; doping magnesium; Bridgman method; high homogeneity
