Controllable Synthesis of a Strong Ultraviolet Nonlinear Optical Crystal by Tailoring the Lead Oxychloride Polyhedron

The exploration of ultraviolet (UV) nonlinear optical (NLO) crystals owning a strong second-harmonic generation (SHG) response has consistently been a difficult task. Herein, we have controllably tailored the lead-oxychloride polyhedron by tuning the amount of KCl under mixed-solvothermal conditions to afford a new strong UV NLO crystal, K₃[Pb₂Cl₅(HCOO)₂]·H₂O. Interestingly, benefiting from the oriented arrangement of the large [Pb₆Cl₂₄O₁₂] rings, the microscopic second-order susceptibilities of the distorted [PbCl₅O₂] polyhedrons are effectively superimposed. Remarkably, K₃[Pb₂Cl₅(HCOO)₂]·H₂O owns a unique 3D polar framework, a strong phase-matchable SHG response that is 3.1 times as strong as KDP, a suitable birefringence of 0.108@1064 nm, a broad high transparent window extending to 340 nm in the UV region, and feasible crystal growth. Dipole moment and theoretical calculations elucidate that the strong SHG response originates from the distorted [PbCl₅O₂] polyhedron. This study will shed light on the development of subsequent UV NLO crystals.