Crystal structure of LiNbO
3 can be described as hexagonal unit cells (
Figure 1a) or rhombohedral unit cells
[1][2][16,17]. In stoichiometric LiNbO
3, 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
[3][4][5][18,19,20]. Much experimental and simulation effort have been made in the past in order to understand the defect structure in LN crystal
[6][8]. Several defect models have been constructed—i.e., oxygen vacancy model, niobium vacancy model ([Li
1−5xNb
5x][Nb
1−4xV
4x]O
3), and lithium vacancy model ([Li
1−5xV
4xNb
x]NbO
3)
[3][4][5][18,19,20]. Congruent LiNbO
3 crystals were grown with LiCO
3 and Nb
2O
5 as starting materials, which contain a high concentration of Nb anti-sites (Nb
Li4+) and Li vacancies (V
Li−) (
Figure 1a(ii))
[7][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
[8][9][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
[6][8]. Since these defects are charged, further defects with counter charges are required in order to guarantee overall charge neutrality
[9][23]. Thus, for energetic reasons, complex ionic complexes and spaced clusters are present as shown in
Figure 1b
[2][17]. However, debate still prevails on the available models on defect clusters. The understanding and control of LiNbO
3 intrinsic and extrinsic defects during crystallization and operational process is important for specific applications.