AVP2S7 (A = K, Rb): Dual-OptimizedInfrared Nonlinear Optical Thiophosphates via Heterovalent-Substitution-DrivenSymmetry Breaking and Reinforced Interlayer Interactions

Transforming centrosymmetric (CS) materials into noncentrosymmetric (NCS) analogues through rational structural modification is an effective pathway for designing nonlinear optical (NLO) materials. Herein, we demonstrate a heterovalent-substitution-driven symmetry-breaking transformation from CS V2P2S6 (C2/m) to NCS AVP2S7 (A = K, Rb) (C2). The introduction of alkali metal cations reconstructs the staggered checkerboard array of [VS6] octahedra into alternately arranged [AS8] bicapped trigonal prisms and [VS6] octahedra, thereby eliminating the inversion center. Simultaneously, the weak van der Waals interactions between the {[V2P2S6]}∞ neutral layers are replaced by the {[VP2S7]−}∞ polyanionic layers with strong electrostatic ionic bonds of alkali metal cations, significantly enhancing the structural stability. AVP2S7 exhibit a second harmonic generation response of 0.5 × AgGaS2 together with enhanced laser-induced damage thresholds. Theoretical calculations reveal that the NLO activity primarily originates from the cooperative alignment of [P2S7] dimers. This study establishes a heterovalent substitution strategy that simultaneously achieves symmetry breaking and structural stabilization, providing a rational route for developing new infrared NLO materials.