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

Two new pyrophosphates nonlinear optical (NLO) materials, Rb 3 PbBi(P 2 O 7 ) 2 ( I ) and Cs 3 PbBi(P 2 O 7 ) 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 RbO 12 (CsO 11 ), RbO 10 (CsO 10 ), BiO 6 , PbO 7 (PbO 6 ) and P 2 O 7 groups, in which P 2 O 7 and PbO 7 (PbO 6 ) 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 .