KCs2Pb2(HCOO)2Cl5: A Lead Formate with Strong Second-Harmonic-Generation Response Obtained by an Anionic Substitution

Nonlinear optical (NLO) materials are playing an increasingly vital role in laser science and technology. Herein, we report a new lead formate, KCs2Pb2(HCOO)2Cl5, successfully synthesized by using mild mix-solvothermal methods. By the anionic substitution of a [CH3COO] group with a [HCOO] group, the dipole moment of the distorted [PbCl4O2] polyhedron is increased. In contrast, polarization from the anionic group and the volume of the unit cell are reduced. Hence, KCs2Pb2(HCOO)2Cl5 possesses a strong second-harmonic-generation (SHG) response of 4.2 times that of KH2PO4 (KDP), a transparent window covering 0.43–1.87 μm, a large band gap of 3.52 eV, and moderate birefringence (0.11@1064 nm). Systematic theoretical calculations illustrate the origin of the linear and nonlinear optical properties of KCs2Pb2(HCOO)2Cl5. This research provides a viable strategy for the design and synthesis of new NLO crystals.