S = 1/2 Chain in BiVO₃F: Spin Dimers versus Photoanodic Properties

BiVO₃F was prepared, characterized, and identified as a unique example of bismuth vanadyl oxyhalide with paramagnetic V⁴⁺ centers. Its crystal structure shows 1D magnetic units with rare alternation of edge-sharing O-O and F-F μ₂ bridges along the octahedral chains. Structural pairing across the O₂ edges induces antiferromagnetic spin dimers (S = 0) with J/K_b ≈ 300 K, ∼15 times greater than the exchange across the F₂ bridges, within a non-ordered magnetic ground state. Despite multiple compositional, structural, and electronic analogies with the BiVO₄ scheelite compound, one of the most promising photoanodes for solar water splitting, the photoactivity of BiVO₃F is relatively modest, partially due to this electronic pairing benefitting fast electron-hole recombination. Similar to monoclinic VO₂, the V⁴⁺ spin dimerization deters the singlet → triplet electronic photoexcitation, but results in potential carrier lifetime benefits. The reduction of the bandgap from an E_g of ∼2.4 eV to ∼1.7 eV after incorporation of d¹ cations in BiVO₄ makes BiVO₃F an inspiring compound for local modifications toward an enhanced photoactive material. The direct d → d transition provides a significant enhancement of the visible light capture range and opens a prospective route for the chemical design of performant photoanodes with a mixed anionic sublattice.