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

ABSTRACT 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 A 4 X ionic motifs while preserving the sphalerite‐like topology of tetrahedral framework, and leads to a phase‐evolution rule described by the general formula [A 4 X][M n P n+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, [Ca 4 X][Zn 1.5 Si 12.5 P 20 ], [Ca 3.8 Ge 0.2 X][Si 11 P 17 ], [Ca 3.6 Sn 0.4 X][Si 11 P 17 ], and [Ca 3.6 Pb 0.4 X][Si 11 P 17 ] (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.