Designing Visible-Light Photoactive Thioapatites Using Thiovanadate Groups: The Ba₅(VO_4-αS_α)₃X (X = F, Cl, I) Phases

The new thioapatite Ba₅(VO_4-αS_α)₃X (X = F, Cl, I) series of compounds was prepared and characterized. Compared to known apatite phases built from unconnected vanadate VO₄ groups separated by Ba²⁺ cations delimiting halide-filled channels, their crystal structure is built from mixed anion thiovanadate VO_4-αS_α, where V⁵⁺ is surrounded by both O and S, therefore exhibiting a triple anion lattice. Here, the strategy consisting in incorporating a chalcogenide anion aims at raising the valence band to bring the band gap to the visible range in order to reach photoactive materials under visible light. Both the halide anion nature and the S/O ratio impact the materials' photoconductivity. While the photocurrent response is comparable to that found in the recently investigated apatite phase Pb₅(VO₄)₃I, a short carrier lifetime is detected as well as a shift of the activity toward the visible light. This apatite series combining thiovanadate and halide-filled channels opens new perspectives in the extended field of apatites and their applications.