Breaking Inversion Symmetry via Vanadium Coordination Engineering in Bismuth Vanadium Selenites

ABSTRACT Three bismuth vanadium selenites, Bi(VO ₂ )(SeO ₃ ) ₂ , Bi ₂ (VO ₂ F)(SeO ₃ ) ₃ , and Bi ₂ (VO)(SeO ₃ ) ₄ , were synthesized via hydrothermal and mild solid‐state routes within a common Bi–O–Se framework. Bi(VO ₂ )(SeO ₃ ) ₂ and Bi ₂ (VO ₂ F)(SeO ₃ ) ₃ crystallize in the centrosymmetric (CS) space groups, P 2 ₁ / m and P ‐1, respectively, whereas Bi ₂ (VO)(SeO ₃ ) ₄ adopts the polar noncentrosymmetric (NCS) space group, Pc . Despite their similar structural building blocks, targeted reduction of vanadium from V ⁵⁺ to V ⁴⁺ reorganizes the coordination geometry from symmetry‐compensated VO ₆ /VO ₅ F units to directionally aligned VO ₅ polyhedra, eliminating dipole cancellation and stabilizing a polar arrangement. The CS compounds exhibit strong third‐order nonlinear responses, with third‐harmonic generation (THG) susceptibilities 4.8 and 6.4 times larger than that of α‐SiO ₂ . In contrast, the NCS phase displays pronounced SHG with an efficiency 11.6 times that of α‐SiO ₂ under 1400 nm excitation, together with a broad infrared transparency window spanning 1000–4000 nm and high laser‐induced damage tolerance. Notably, Bi ₂ (VO)(SeO ₃ ) ₄ represents the first reported V ⁴⁺ –selenite framework in which five‐coordinate vanadium units adopt a non‐antiparallel vanadyl alignment, giving rise to intrinsic noncentrosymmetry and strong nonlinear optical activity. These results demonstrate that vanadium coordination‐number reduction coupled with oxidation‐state engineering provides an effective strategy for symmetry breaking in selenite frameworks, offering a promising pathway for the design of robust materials for infrared photonic applications.