Topology-Preserving Ionic-Unit Substitution Enables Sphalerite-Like Phosphides Achieving Superior Infrared Optical Nonlinearity.

Non-centrosymmetric (NCS) inorganic phosphides have emerged as promising candidates for infrared (IR) nonlinear optical (NLO) applications; however, their rational design remains challenging because general structure-generation principles are largely absent. Here, we identify a topological relationship between sphalerite lattice and NCS A-X-M-P (A: alkaline earth metals; X: halogens; M: ds, and p block element) phosphides. This relationship holds that specific covalent clusters within the sphalerite lattice can be replaced by A4X ionic motifs while preserving the sphalerite-like topology of tetrahedral framework, and leads to a phase-evolution rule described by the general formula [A4X][MnPn+6]. Building upon this framework, we predict that alkaline-earth cations can be substituted by stereochemically active lone-pair (SCALP) cations to modulate NLO properties. Guided by these principles, eight new desired phosphides, [Ca4X][Zn1.5Si12.5P20], [Ca3.8Ge0.2X][Si11P17], [Ca3.6Sn0.4X][Si11P17], and [Ca3.6Pb0.4X][Si11P17] (X = Cl and Br) were successfully designed and synthesized via a lithium-mediated solid-state reaction. These materials exhibit strong SHG responses (1.2-2.3 × ZGP @ 2900 nm), tunable optical band gaps (1.42-2.02 eV), broad IR transparency range (up to ∼9.5 µm) and moderate birefringence (0.03-0.05 @ 2900 nm). This work establishes a predictive topological substitution principle for generating NCS phosphides and expands the chemical space of lone-pair-containing phosphides with promising infrared optical functionalities.