Achieving strong optical nonlinearity and wide bandgap of pnictides via ionic motif–driven directed assembly of covalent groups

Noncentrosymmetric (NCS) pnictides are indispensable for nonlinear optics, ferroelectrics, magnetic Weyl electronics, etc., areas, yet their structure design remains a substantial challenge. By using asymmetric ionic unit-driven covalent groups orienting and rigidity-flexibility coupling dual strategy, we successfully design and synthesize four NCS pnictides: [Sr₄Br]₂[M^II₃Si₂₅P₄₀] (M^II = Mg, Cd) and [Ba₃Br][M^IIISi₁₀P₁₆] (M^III = Ga, In), which exhibit strong second harmonic generation effects (5.2 to 7.5 × AgGaS₂), wide bandgaps (1.81 to 1.90 electron volts), and moderate birefringence (0.030 to 0.051). An unprecedented NCS structure-inducing mechanism analysis revealed that the (Sr₄Br) and (Ba₄Br) ionic units featuring the diamond-like electrostatic force field effectively break inversion symmetry and trigger uniform arrangement of the covalent tetrahedron groups. Furthermore, the nonlinear optical (NLO) properties and birefringence can be remarkably tuned by the secondary covalent building blocks (M^II/IIIP₄ tetrahedra) with distinct bond flexibility providing a broader space for regulating the key parameters. This work might expand chemical space for exploiting high-performance pnictide NLO materials.