CaM ₃ Si ₄ P ₁₁ (M = Al, Ga): [P ₂ ] Motifs‐Modified Quasi‐T ₃ Supertetrahedra Drive Superior Infrared Nonlinear Optical Performances

Inorganic pnictides hold great potential as infrared (IR) nonlinear optical (NLO) materials, but decoupling the bandgap, NLO performance, and birefringence trade‐off remains a significant challenge. Here, we propose a tetrahedra‐modification strategy based on [P 2 ] linear motifs to design high‐performance IR NLO materials. Using a phosphorus‑supersaturation synthesis method, two new phosphide IR NLO crystals, CaM 3 Si 4 P 11 (M = Al, Ga) featuring quasi‐T 3 M 3 Si 4 P 11 [P 2 ] 6 supertetrahedra were successfully designed and synthesized. They achieve an excellent balance of key properties including wide bandgaps (2.37, 2.41 eV), strong NLO effects (3.0, 3.1 × AgGaS 2 @2900 nm), high LDTs (5.3, 4.8 × AgGaS 2 ) and moderate birefringence (0.090, 0.115@546 nm), underscoring their significant potential as promising IR NLO materials. Structure–property analysis reveals that the bandgap, NLO effects and birefringence arise from the synergistic effect between the tetrahedral groups and the [P 2 ] motifs, with the latter exhibiting highly polarizable π‐interactions.