An excellent lead oxyiodide with a strong second-harmonic generation response and a large birefringence induced by the oriented arrangement of highly distorted [PbO4I2] polyhedra

Nonlinear optical (NLO) crystals are an important component in modern lasers. Nevertheless, it is a great challenge to rationally synthesize NLO crystals with strong second-harmonic generation (SHG) responses and large birefringence. Herein, a cationic substitution strategy has been successfully developed to transform SHG-inert centrosymmetric Cs2I2[Pb(OOCCH2COO)] into SHG-active K2I[PbI(OOCCH2COO)] with a NCS polar structure. K2I[PbI(OOCCH2COO)] features highly distorted [PbO4I2] polyhedra that are aligned in an oriented arrangement, resulting in the effective superposition of their microscopic second-order nonlinear susceptibilities. Remarkably, K2I[PbI(OOCCH2COO)] not only exhibits the strongest phase-matching SHG response of 6.3 x KDP to date among malonates, but also possesses a large birefringence (Delta n(obv.) = 0.218@546 nm) and good thermal stability. As elucidated through theoretical calculations its strong SHG response and large birefringence mainly originate from the highly distorted [PbO4I2] polyhedron. This paper presents a new approach for the development of NLO crystals with prominent performance.