Structural modification of hydroxyborates by adjusting the number of shared oxygen atoms and hydroxyl groups for further performance enhancement

In recent years, hydroxyborates with excellent properties have attracted much attention. Through dedicated efforts, three new hydroxyborates-K₂B₅O₈(OH), CsB₅O₆(OH)₄, and CsB₅O₇(OH)₂-have been successfully synthesized in a closed system. The ultraviolet (UV) cut-off edges of both K₂B₅O₈(OH) and CsB₅O₇(OH)₂ are below 200 nm, indicating their potential as candidates for deep-ultraviolet (DUV) materials. Furthermore, K₂B₅O₈(OH) exhibits nonlinear optical (NLO) activity and demonstrates significant second harmonic generation (SHG) effects, approximately 2 × KH₂PO₄ (KDP). Interestingly, although all three compounds are alkali metal borates containing five boron atoms, the calculated birefringence is 0.025 at 1064 nm for K₂B₅O₈(OH), whereas it is 0.067 and 0.070 at 1064 nm for CsB₅O₆(OH)₄ and CsB₅O₇(OH)₂, respectively, which are about three times that of K₂B₅O₈(OH). The reason for the nearly threefold difference in birefringence is analyzed from the view of the structure-property relationship. Furthermore, the effect of the number of hydroxyl groups and shared oxygen atoms on the structural dimensions, birefringence, and band gaps in all alkali and alkaline-earth metal hydroxyborates with five boron atoms has been studied and analyzed. A solid foundation for the use of hydroxyl groups to tune and design structures has been provided.