Spin-forbidden near-infrared luminescence from a F ₃ ⁺ colour centre generated upon annealing in mechanochemically prepared nanocrystalline BaLiF ₃

We report the properties of a unique colour centre in mechanochemically synthesized inverse perovskite BaLiF₃ submicron crystals that are luminescent at ∼765 nm. The spin-forbidden luminescence with a lifetime of 5 ms is attributed to a F₃⁺ (F-centre aggregate) in the fluoride octahedra, with three fluoride anion vacancies (3F⁺) filled with two electrons (2e^-). The Zeeman splitting of the electronic origin and its temperature dependence indicate that the transition is from a singlet excited state to a triplet ground state. The F₃⁺ emission occurred after annealing (≥500 °C) the mechanochemically prepared pure BaLiF₃ nanocrystals and is characterized by a structured emission with a relatively narrow zero-phonon line. A reduction of photoluminescence intensity of the F₃⁺ band upon increasing X-ray dose was observed. Importantly, it is observed that the F₃⁺ luminescence is stable in the dark but bleaches upon exposure to natural sunlight. Our results point to the potential for a new colour centre-based nano-laser in the near-infrared region. Additionally, our experiments also indicate that BaLiF₃:_{}F₃⁺ has some potential for data storage, and X-ray imaging and dosimetry.