Crystal structure of LiNbO₃ can be described as hexagonal unit cells (Figure 1a) or rhombohedral unit cells [16,17]. In stoichiometric LiNbO₃, along c row direction, the O octahedral interstitials are filled by Li ions (one-third), Nb ions (one-third), and empty (one-third), forming –Li–Nb–▯–Li–Nb– sequence [18,19,20]. Much experimental and simulation effort have been made in the past in order to understand the defect structure in LN crystal [8]. Several defect models have been constructed—i.e., oxygen vacancy model, niobium vacancy model ([Li_1−5xNb_5x][Nb_1−4xV_4x]O₃), and lithium vacancy model ([Li_1−5xV_4xNb_x]NbO₃) [18,19,20]. Congruent LiNbO₃ crystals were grown with LiCO₃ and Nb₂O₅ as starting materials, which contain a high concentration of Nb anti-sites (Nb_Li⁴⁺) and Li vacancies (V_Li⁻) (Figure 1a(ii)) [21]. Owing to atomic radius differences between Nb and Li, it forbids Li replacement in a Nb site. Thus, the composition deviates from stoichiometric only toward the Nb-rich side [22,23]. The Li vacancy model is mostly accepted nowadays thanks to a great number of investigations, some of them very important and performed in the 1990s. This is given in detail in [8]. Since these defects are charged, further defects with counter charges are required in order to guarantee overall charge neutrality [23]. Thus, for energetic reasons, complex ionic complexes and spaced clusters are present as shown in Figure 1b [17]. However, debate still prevails on the available models on defect clusters. The understanding and control of LiNbO₃ intrinsic and extrinsic defects during crystallization and operational process is important for specific applications.

According to binary phase diagram, LiNbO₃ has a large solid solution range, which can exist and be stable on Li composition from 46.5 mol% to 50 mol% (Figure 2). The liquid–solid curve reveals a diffuse maximum at approximately 48.6% Li₂O [24]. With exceeding composition range, the secondary LiNb₃O₈ and Li₃NbO₄ phases can be created. The binary phase diagram can be determined by measuring XRD of different samples along solid lines. However, it is also needed to probe precise composition range, because LiNbO₃’s bulk properties are composition dependent [25].

LiNbO₃ polycrystalline can be grown by solid-state reaction, sol–gel, hydrothermal, vapor phase methods. The crystallization method of LiNbO₃ single crystal includes Cz, Bridgeman, high-temperature top-seeded solution growth. Cz method is the current mainstream technology for growing bulk LiNbO₃ single crystal [26,27,28]. With LiNbO₃ polycrystalline as starting materials, the Cz crystal growth is often controlled by the pulling/rotation rate and heater power [29,30]. The growth of LiNbO₃ crystal was affected by various factors together, such as the ratio of raw materials, quality of seed crystal, temperature gradient, growth parameters, etc. [24]. In reality, the Li evaporation at high temperature is hard to be eliminated, which results in the segregation of Li content inside the as-grown crystal. Congruent LiNbO₃ with good compositional uniformity can be formed with Li content can range from 47 to 50 mol%. Nearly stoichiometric LiNbO₃ composition can be achieved by more elaborate growth processes.

A slower pulling rate is helpful to obtain a crystal with less internal stress and high quality. Table 1 shows pulling rate and rotation rate [26,27,28,29,30]. Recently, 6-inch LiNbO₃ crystals have been grown with a rotation rate of 5~10 rpm, and the pulling rate of 1–2 mm/h [26]. The obtained 6-inch LiNbO₃ crystal shows good homogeneity with the absolute deviation of Curie temperature ≤1.3 °C. In addition, fast growth rate can lead to low-cost LiNbO₃ crystal, which is important for industry production. Thus, under the premise of ensuring quality, fast pulling rate is also demanded.

The performances of LiNbO₃ are most depend upon their chemical composition. Therefore, the development of the precise analysis method to detect the chemical composition (Li content) of LiNbO₃ is very important. Table 2 shows available testing methods for determine Li content of LiNbO₃, for example, X-ray diffraction (XRD), Raman spectroscopy (RS), UV–vis diffuse reflectance (DR), and differential thermal analysis (DTA) [31,32,33,34,35,36].

In Raman spectroscopy, the Li content can be calculated according to the linewidth (Γ) at 876 cm⁻¹ [37,38,39].

The Li content of LiNbO₃ can be also calculated via measuring Curie temperature where T_c is Curie temperature in °C. Curie temperature is the temperature at which LiNbO₃ tends to lose its ferroelectric properties. When use above reported characterizations, the applicability and calibration method need to be concerned. Some indirect optical and non-optical methods for the determination of the chemical composition of LN single crystals can be referred to [8].