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

We report the properties of a unique colour centre in mechanochemically synthesized inverse perovskite BaLiF submicron crystals that are luminescent at ∼765 nm. The spin-forbidden luminescence with a lifetime of 5 ms is attributed to a F (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 F emission occurred after annealing (≥500 °C) the mechanochemically prepared pure BaLiF nanocrystals and is characterized by a structured emission with a relatively narrow zero-phonon line. A reduction of photoluminescence intensity of the F band upon increasing X-ray dose was observed. Importantly, it is observed that the F 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 BaLiF:F has some potential for data storage, and X-ray imaging and dosimetry.