Molecular Engineering Design to Resolve the Layering Habit and Polymorphism Problems in Deep UV NLO Crystals: New Structures in MM′Be₂B₂O₆F (M═Na, M′═Ca; M═ K, M′═Ca, Sr)

A novel series of alkali and alkaline earth metal combined fluorine beryllium borates NaCaBe 2 B 2 O 6 F, KCaBe 2 B 2 O 6 F, and KSrBe 2 B 2 O 6 F were successfully synthesized through molecular engineering design and grown in crystals by spontaneous nucleation technique from self-flux systems. The idea, introduction of relatively small alkali and alkaline earth metal cations and the fluorine anion, successfully resulted in the novel UV NLO crystal NaCaBe 2 B 2 O 6 F, the following substitution of cations directed to two centrosymmetric compounds KCaBe 2 B 2 O 6 F and KSrBe 2 B 2 O 6 F. In all of their structures, the a – b plane is the infinite lattice layer (Be 3 B 3 O 6 F 3 ) ∞ made up of BO 3 and BeO 3 F anionic groups, and for the first time, it was found that the adjacent layers are connected with fluorine bridge atoms to form (Be 6 B 6 O 12 F 3 ) ∞ double layers, instead of oxygen bridge atoms usually occurred in other oxides. This structural characteristic is greatly beneficial to improve the layering-growth habit and eliminate polymorphism of a crystal. Optical measurements on the nonlinear optical crystal of NaCaBe 2 B 2 O 6 F reveal that this crystal is phase-matchable and its short-wavelength absorption edge is down to deep UV (below 190 nm). Theoretical calculations on electronic structure were carried out to explain the experimental results. Our preliminary results indicate that NaCaBe 2 B 2 O 6 F has promising applications in the UV spectrum region.