[K₂PbI][Ga₇Se₁₂]: A New Lead-Containing Nonlinear Optical Material with Moderate Second-Harmonic Generation Response and Large Birefringence

Nonlinear optical (NLO) materials play an important role in modern laser science and technology fields. Developing new NLO materials with comprehensive performance is highly difficult because of the mutual coupling of basic parameters (nonlinear susceptibility and laser damage threshold). Here, through combining Pb2+ ions with lone-pair electrons and GaSe4 tetrahedra and introducing alkali metal atoms or halogen atoms, a new lead-containing nonlinear optical material, namely, [K2PbI][Ga7Se12], was synthesized. It crystallizes in the orthogonal space group Imm2 (No. 44) featuring a three-dimensional framework comprising highly oriented Ga7Se16 clusters and charge-balanced K+, Pb2+, and I– ions. The band gap of the title compound is measured to be 2.0 eV. Powder second-harmonic generation (SHG) measurement suggests that the title compound displays moderate SHG intensity (0.5 × AgGaS2) in the particle size range of 25–50 μm. Theoretical calculations show that [K2PbI][Ga7Se12] exhibits a large birefringence (0.12 at 1064 nm) and a significant nonlinear coefficient (d24 = 29.14 pm/V).