Breaking Inversion Symmetry via Vanadium Coordination Engineering in Bismuth Vanadium Selenites

초록

Three bismuth vanadium selenites, Bi(VO2)(SeO3)2, Bi2(VO2F)(SeO3)3, and Bi2(VO)(SeO3)4, were synthesized via hydrothermal and mild solid-state routes within a common Bi-O-Se framework. Bi(VO2)(SeO3)2 and Bi2(VO2F)(SeO3)3 crystallize in the centrosymmetric (CS) space groups, P21/m and P-1, respectively, whereas Bi2(VO)(SeO3)4 adopts the polar noncentrosymmetric (NCS) space group, Pc. Despite their similar structural building blocks, targeted reduction of vanadium from V5+ to V4+ reorganizes the coordination geometry from symmetry-compensated VO6/VO5F units to directionally aligned VO5 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 alpha-SiO2. In contrast, the NCS phase displays pronounced SHG with an efficiency 11.6 times that of alpha-SiO2 under 1400 nm excitation, together with a broad infrared transparency window spanning 1000-4000 nm and high laser-induced damage tolerance. Notably, Bi2(VO)(SeO3)4 represents the first reported V4+-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.

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