“Cation Activation”: An Effective Strategy for the Enhancement of Birefringence

Abstract For traditional birefringent materials, the anion groups play the leading role in birefringence, while the alkali‐metal and alkaline‐earth‐metal cations are nearly inactive. This work proposes a strategy to activate the functions of the cation groups and uses the hydroxyborate systems for illustration. The experimental verification is carried out in the same anionic frame by replacing the traditional metal cations with the birefringence‐active units including planar groups of protonation [C(NH 2 ) 3 ], [C 3 N 2 H 5 ], and [CN 4 H 7 ]. The structures of four new hydroxyborates, α ‐Rb 2 [B 4 O 5 (OH) 4 ]·H 2 O, β ‐Rb 2 [B 4 O 5 (OH) 4 ]·H 2 O, [C(NH 2 ) 3 ] 2 [B 4 O 5 (OH) 4 ]·H 2 O, and (CN 4 H 7 )B 5 O 6 (OH) 4 are obtained for the first time, and [C(NH 2 ) 3 ] 2 [B 4 O 5 (OH) 4 ]·H 2 O exhibits a markedly enlarged birefringence, which is about 2.75 times that of α ‐Rb 2 [B 4 O 5 (OH) 4 ]·H 2 O theoretically because the activated cations appear. This strategy breaks the dominant status of anions in the design of UV/deep ultraviolet materials with large birefringence and has significant implications for the rational design and discovery of materials with large birefringence in the future.