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/Kb ≈ 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 Eg 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.