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 [PbI 2 (HCOO) 2 ] and Rb 2 [PbI 2 (HCOO) 2 ], have been successfully synthesized using a moderate mixed-solvothermal method. K 2 [PbI 2 (HCOO) 2 ] and Rb 2 [PbI 2 (HCOO) 2 ] exhibit strong phase-matching SHG responses of 8 and 6.8 × 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 [PbI 2 O 4 ] 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.