Strategy for a Rational Design of Deep-Ultraviolet Nonlinear Optical Materials from Zeolites

Deep-ultraviolet (DUV) nonlinear optical (NLO) materials play a crucial role in cutting-edge laser technology. In order to solve the serious layered growth tendency of the sole commercial DUV NLO crystal KBe₂BO₃F₂ (KBBF), developing alternative systems of compounds with bulk crystal habits has become an urgent task for practical applications. Herein, a novel strategy was developed by applying non-centrosymmetric (NCS) cancrinite (CAN)-type zincophosphates {Na₆(OH)₂(H₂O)₂}Cs₂[ZnPO₄]₆ with bulk-crystal habits as the prototype to design new DUV NLO crystals. Two new anhydrous alkali zincophosphates, namely, {(Li_{6 -x}Na_xO)A₂}[(ZnPO₄)₆] (A = Cs, Rb; x = 2-3) crystallizing in the NCS hexagonal space group P6₃ (no. 173) with a CAN-type framework, were successfully synthesized via a modified fluoro-solvo-hydrothermal method by applying triethylamine (TEA) and concentrated NaF solution as a co-solvent. Interestingly, the rigidity of the NCS CAN-type framework acting as the host ensures the non-centrosymmetry of the resulting new compounds. Meanwhile, the replacement of water molecules by guest cationic species in the channels or cages can greatly improve the thermal stability of the resultant crystal and tune its NLO properties. The synergetic effect of the host framework and the guest species makes the two compounds transparent down to the DUV region (<200 nm) and exhibit SHG effects. Therefore, the proposed rational design strategy of applying the known zeotype NCS frameworks as prototypes together with the modified fluoro-solvo-hydrothermal method opens a great avenue for highly effectively exploring new DUV NLO materials.