Two new pyrophosphates nonlinear optical (NLO) materials, Rb3PbBi(P2O7)2 (I) and Cs3PbBi(P2O7)2 (II), were successfully designed and synthesized. Both compounds exhibit large NLO effects and birefringences. Material I presents the scarce case of possessing the coexistence of large birefringence (0.031 at 1064 nm and 0.037 at 532 nm) and second harmonic generation (SHG) response (2.8× potassium dihydrogen phosphate (KDP)) in ultraviolet NLO phosphates and its SHG is the largest in the phase‐matching (PM) pyrophosphates. Both I and II have three‐dimensional (3D) crystal structures composed of corner‐shared RbO12 (CsO11), RbO10 (CsO10), BiO6, PbO7 (PbO6) and P2O7 groups, in which P2O7 and PbO7 (PbO6) units form an alveolate [PbPO]∞ skeleton frame. Theoretical calculations reveal that the P−O, Bi−O and Pb−O units are mainly responsible for the moderate birefringence and large SHG efficiency of I.

Exploring novel photoelectric functional materials via chemical substitution-oriented design is an effective strategy, which can be expanded to the discovery of high-performance UV nonlinear optical (NLO) materials. Two new NLO pyrophosphates, Rb3BaBi­(P2O7)2 (I) and Cs3BaBi­(P2O7)2 (II), are rationally developed by a cation substitution technique based on A3PbBi­(P2O7)2 (A = Rb and Cs), of which I inherits the large second-harmonic generation (SHG) response (exptl 2.5 × KDP; calcd 2.9 × KDP) and moderate birefringence (0.025@1064 nm) accompanied by a broadened UV transparent region. Compounds I and II are isomeric and exhibit different Ba–P–O frameworks. Especially, I possesses a large SHG effect benefiting from the favorable pentagonal-net topological structure. Detailed theory calculations elucidate the origin of the linear and nonlinear optical properties of the compounds. The insights obtained from the atomic-level module adjustment involving lone-pair-active optical anisotropy are useful for designing more efficient UV NLO materials.

Two new pyrophosphates nonlinear optical (NLO) materials, Rb3PbBi(P2O7)2 (I) and Cs3PbBi(P2O7)2 (II), were successfully designed and synthesized. Both compounds exhibit large NLO effects and birefringences. Material I presents the scarce case of possessing the coexistence of large birefringence (0.031 at 1064 nm and 0.037 at 532 nm) and second harmonic generation (SHG) response (2.8× potassium dihydrogen phosphate (KDP)) in ultraviolet NLO phosphates and its SHG is the largest in the phase‐matching (PM) pyrophosphates. Both I and II have three‐dimensional (3D) crystal structures composed of corner‐shared RbO12 (CsO11), RbO10 (CsO10), BiO6, PbO7 (PbO6) and P2O7 groups, in which P2O7 and PbO7 (PbO6) units form an alveolate [PbPO]∞ skeleton frame. Theoretical calculations reveal that the P−O, Bi−O and Pb−O units are mainly responsible for the moderate birefringence and large SHG efficiency of I.

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