Deep-Ultraviolet Nonlinear-Optical Crystals LiBePO4 and BeP2O6 by Ionic Potential Modulation towards Uniform Arrangement of PO4 Groups

Uniform arrangement of functional groups is a key factor of improving nonlinear properties in nonlinear-optical (NLO) materials, but currently there is no feasible and guiding strategy to modulate the uniform arrangement. Herein, we first apply ionic potential concept to deep-ultraviolet (DUV, λ < 200 nm) NLO phosphates for uniform arrangement of PO4 tetrahedral functional groups. Adopting Cs4LiBe4P7O24 with non-uniform arrangement of PO4 as structural model, by removing low ionic potential Cs+ and Li+ successively, two DUV NLO crystals LiBePO4 and BeP2O6 were synthesized. LiBePO4 features [Be3P3O18] six-membered ring constructed by alternate connection of BeO4 and PO4, when BeP2O6 exhibits two kinds of [PO3]∞ helical chains bridged by BeO4. Remarkably, arrangement of the PO4 in LiBePO4 and BeP2O6 successfully realizes uniform evolvement. As a result, LiBePO4 exhibits enhanced second-harmonic-generation (SHG) effect up to 4.3 × Cs4LiBe4P7O24, while BeP2O6 shows even more enhanced SHG effect, reaching 7.0 × Cs4LiBe4P7O24 (2.1 × KDP). Moreover, BeP2O6 exhibits short DUV absorption edge below 175 nm and the shortest SHG phase-matching output wavelength down to 211 nm. Also, the universality of the new ionic potential modulation strategy is supported through searching known NLO materials containing alkali/alkaline-earth metal cations.