A Homoatomic Heteroleptic Coordination Strategy for Exceptional Infrared Linear and Nonlinear Optical Properties in ANb ₂ PS ₄ (S ₂ ) ₃ (A = K,Rb,Cs)

The disulfide dianion [S₂]^2-, as an important structural motif in sulfides, can incorporate isolated S^2- to form homoatomic heteroleptic polyhedra [MS_<i>x</i>(S₂)_<i>y</i>]. However, the sulfides that contain [MS_<i>x</i>(S₂)_<i>y</i>] units have been rarely reported and have not been systematically investigated as infrared (IR) nonlinear optical (NLO) materials. It is verified in this work that the homoatomic heteroleptic coordination strategy enhances local structural asymmetry, leading to improved optical anisotropy and NLO performance. Three compounds, namely, ANb₂PS₁₀(A = K,Rb,Cs), were identified and prepared. They exhibit a large phase-matchable second-harmonic generation (SHG) response, with KNb₂PS₁₀, RbNb₂PS₁₀, and CsNb₂PS₁₀ showing the effects of 1.5, 2.2, and 1.8 (×AgGaS₂@2090 nm), respectively, and large birefringence (Δ<i>n</i> = 0.39@546 nm) with KNb₂PS₁₀ as a representative. The first-principles calculations indicate that [Nb₂S₄(S₂)₄]^8- units play a dominant role in inducing strong SHG responses and large birefringence. By validating the homoatomic heteroleptic coordination strategy, this work opens avenues for the design of advanced optical materials.