“…However, the terminal oxygen atoms with dangling bonds are not conducive to DUV transparency, which is one of the most essential concerns for UV/DUV materials. For example, the dangling bonds of β-Rb 2 Al 2 B 2 O 7 , BaAlBO 3 F 2 , and CsAlB 3 O 6 F are removed by introducing [AlO 4 ] and [AlO m F n ] ( m + n = 4, 5, 6) units, and all of them possess DUV cutoff edges. − Additionally, a part of terminal O atoms in the [BO 4 ] unit can be swapped by large electronegative F atoms to generate [BO 4– x F x ] ( x = 1, 2, 3, 4) units resulting in high-performing fluorooxoborates with DUV absorption edges, − such as MB 4 O 6 F (M = NH 4 , Na, Rb, Cs), − MB 5 O 7 F 3 (M = Mg, Ca, Sr), − Ba 3 Mg 3 (BO 3 ) 3 F 3, M 2 B 6 O 9 F 2 (M = Li, Na, NH 4 , K), ,− PbB 5 O 8 F, etc. Especially, the dangling bonds of terminal O atoms of [BO 3 ] and [BO 4 ] in [BO 3– x (OH) x ] ( x = 1, 2, 3) and [BO 4– x (OH) x ] ( x = 1, 2, 3, 4) groups can be eliminated by introducing the [OH] unit − and also possible enhance the energy gap, hyperpolarizability, and polarizability anisotropy. − It is worth noting that many hydroxyl borates are excellent DUV optical crystals, such as AEB 8 O 15 H 4 (AE = Ca, Sr), Ba 2 [B 4 O 7 (OH) 2 ], Li 2 CsB 7 O 10 (OH) 4 , β -A 2 [B 5 O 8 (OH)] 2 [B(OH) 3 ]·H 2 O (A = Ca, Ba), Zn 2 BO 3 (OH), Ba 2 B 13 O 19 (OH) 5 ·5H 2 O, , etc.…”