Deep-ultraviolet nonlinear-optical crystals LiBePO4 and BeP2O6 synthesized by ionic potential modulation towards uniform arrangement of PO4 groups†

The uniform arrangement of functional groups is a key factor in 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 the ionic potential concept to deep-ultraviolet (DUV, λ < 200 nm) NLO phosphates for a uniform arrangement of PO₄ tetrahedral functional groups. Adopting Cs₄LiBe₄P₇O₂₄ with a non-uniform arrangement of PO₄ as a structural model, by removing low ionic potential Cs⁺ and Li⁺ successively, two DUV NLO crystals LiBePO₄ and BeP₂O₆ were synthesized. LiBePO₄ features a [Be₃P₃O₁₈] six-membered ring constructed by alternate connection of BeO₄ and PO₄, while BeP₂O₆ exhibits two kinds of [PO₃]_∞ helical chains bridged by BeO₄. Remarkably, the arrangement of the PO₄ in LiBePO₄ and BeP₂O₆ exhibits uniform evolution. As a result, LiBePO₄ exhibits an enhanced second-harmonic-generation (SHG) effect up to 4.3 times that of Cs₄LiBe₄P₇O₂₄, while BeP₂O₆ shows an even more enhanced SHG effect, reaching 7.0 that of Cs₄LiBe₄P₇O₂₄ (2.1 × KDP). Moreover, BeP₂O₆ exhibits a short DUV absorption edge below 175 nm and the shortest SHG phase-matching output wavelength down to 211 nm. The universality of the new ionic potential modulation strategy is supported through analyzing known NLO materials containing alkali/alkaline-earth metal cations.