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

Abstract

Two new pyrophosphates nonlinear optical (NLO) materials, Rb₃ PbBi(P₂ O₇ )₂ (I) and Cs₃ PbBi(P₂ O₇ )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 P₂ O₇ groups, in which P₂ O₇ 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.