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 NaCaBe2B2O6F, KCa-Be2B2O6F, and KSrBe2B2O6F 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 NaCaBe2B2O6F, the following substitution of cations directed to two centrosymmetric compounds KCa-Be2B2O6F and KSrBe2B2O6F. In all of their structures, the a-b plane is the infinite lattice layer (Be3B3O6F3)(infinity), made up of BO3 and BeO3F anionic groups, and for the first time, it was found that the adjacent layers are connected with fluorine bridge atoms to form (Be6B6O12F3)(infinity) 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 NaCaBe2B2O6F 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 NaCaBe2B2O6F has promising applications in the UV spectrum region.