New lead-iodide formates with a strong second-harmonic generation response and suitable birefringence obtained by the substitution strategy

Nonlinear optical (NLO) crystals featuring a strong second-harmonic generation (SHG) response and suitable birefringence to achieve phase-matching are in urgent demand in industrial and commercial applications. Based on the substitution strategy, two new NLO lead-iodide formates, K-2[PbI2(HCOO)(2)] and Rb-2[PbI2(HCOO)(2)], have been successfully synthesized using a moderate mixed-solvothermal method. K-2[PbI2(HCOO)(2)] and Rb-2[PbI2(HCOO)(2)] exhibit strong phase-matching SHG responses of 8 and 6.8 x KDP, respectively, a suitable birefringence and transparent window covering most of the visible light and mid-IR region. Crystal structures and theoretical calculations unveil that the origins of the strong SHG response and suitable birefringence can be credibly attributed to the oriented arrangement of the highly distorted [PbI2O4] hexa-coordinated polyhedra, which are consistent in their local dipole moments, as well. This research provides a new strategy to explore high-performance NLO crystals.