Two Pyrophosphates with Large Birefringences and Second‐Harmonic Responses as Ultraviolet Nonlinear Optical 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 .