**1. Introduction**

Lithium niobate (LiNbO3, or LN) crystal is one of the most prominent materials for applications in many practical fields, such as optical modulators [1], holographic storage [2], waveguides [3,4], resonators [5], integrated optics devices and three-dimensional (3D) displays, resulting from its superior and diverse physical performance [6,7]. Since the first successful growth by Czochralski method in 1965 [8], crystal growth, photorefractive properties, and theoretical simulations have been studied in depth, and substantial research progress has been reported for LN crystals [9–14]; for example, Ø6" pure LN crystals with high homogeneity has been reported recently [15].

Normally, LN is a non-stoichiometric compound, and the (Li)/(Nb) ratio of congruent composition is 48.38/51.62 [16,17]. According to the broadly accepted Li-vacancy model, the congruent composition induces a large concentration of intrinsic defects that exist in LN, which mainly are Li vacancies (VLi<sup>−</sup>) and anti-site Nb5<sup>+</sup> (NbLi<sup>4</sup><sup>+</sup>). Small polarons (an electron trapped at NbLi<sup>4</sup>+) together with bipolarons (a pair of electrons trapped at adjacent NbLi<sup>4</sup><sup>+</sup> and NbNb<sup>5</sup>+) play the role of laser-induced optical damage (also named photorefraction) centers in LN [10]. The serious disadvantage of laser-induced optical damage in LN limits its usability in nonlinear optical applications [18]. Doping optical damage resistant additives into LN crystal is an effective approach to suppress the optical damage; especially, Zhong et al. first reported that the laser-induced optical damage could be suppressed by doping MgO with high concentration, exceeding its threshold (about 5 mol%) [19]. The mechanism is when the concentration of MgO exceeded its threshold, Mg2<sup>+</sup> repelled anti-site Nb5<sup>+</sup> to the site of normal-Nb. Thereby, the formation of small polarons and bipolarons, which serve as an optical damage center, are suppressed remarkably [20]. This discovery impelled magnesium doped LN (LN:Mg) crystals to play a significant role in nonlinear optics and have achieved industrial growth. Consequently, a summarized result of researches on optical grade heavily Mg-doped LN crystals has revealed that most of them

have been grown by the Czochralski method with the diameter of Ø1"–2", and the homogeneity has not been satisfactory [21–25]. Furthermore, only one crystal can be grown in one furnace each time by the Czochralski method, which indicates low growth efficiency.

Bridgman method is one of the main methods for industry crystal growth, such as optical crystals [26], piezoelectric crystals [27,28], ferroelectric crystals [29], and semiconductor crystals [30,31], for its many advantages, especially multiple crystals can be grown in one furnace at the same time, which means high production efficiency. However, there are few reports about the preparation of lithium niobate by the Bridgman method [32,33], especially growing large optical grade heavily Mg-doped LN crystals with high homogeneity is still difficult. Since the lithium niobate crystal belongs to the trigonal crystal system, the crystal is easy to crack. According to previous reports, the growth of crystals becomes more difficult as the concentration of magnesium is increased because the high concentration of MgO in the melt cause (Li)/(Nb) ratio extremely deviate from the congruent composition [34,35]. Besides, the segregation coefficient of MgO deviated from one could induce inhomogeneity along the growth direction [36]. So, the growth process of the Bridgman method for LN:Mg crystals should be optimized.

In this study, we used the Bridgman method to grow Ø2" LN crystals with different concentrations of magnesium ions. In order to obtain Ø2" heavily Mg-doped LN with high homogeneity, systematically optimized scheme, including polycrystalline powers preparation, thermal field design, and growth technologies of the Bridgman method, was demonstrated in this work. The homogeneity of LN: Mg crystals was also checked.
