Swift heavy ion and UV irradiation of natural cryolite (Na₃AlF₆)

The successful application of fluorides in dosimetry and optics has encouraged study of the optical properties of doped cryolite and cryolite-structure materials. However, natural cryolite (Na₃AlF₆) has never been described in terms of its defect structure, crucial for optical applications. To address this knowledge gap, we irradiate cryolite with 238U ions and use excitation lifetime photoluminescence to constrain the defect structures formed. Untreated, high-energy ion and UV-irradiated samples are compared in terms of their emission spectra and luminescence lifetimes. Particle irradiation introduces an orange colour to originally black, purple and white cryolite. Untreated cryolite shows blue emissions at ∼2.86 and ∼2.60 eV (435 and 475 nm) and a green emission at 2.27 eV (545 nm). After uranium ion irradiation, the green luminescence intensity is enhanced compared to blue emissions. Subsequent ultraviolet irradiation causes ∼57 % reduction in the dark blue emission intensity and 40–70 % increase in green. All luminescence lifetimes are prolonged by particle irradiation, but stay in the nanosecond range. We infer that a vacancy-related defect is produced by particle and UV irradiation and responsible for the green luminescence. Another defect, presumably electronic, is removed by particle irradiation and UV. The ∼2.27 eV green luminescence centre is interpreted as an F-centre localised on a fluorine vacancy. The blue luminescence centres are interpreted as electron-hole pairs, localised at the AlF6 octahedron normal lattice sites for ∼2.86 eV and octahedral defect sites for ∼2.60 eV. The experiment provides insight into natural defect formation processes in cryolite from the Ivittuut deposit in Southern Greenland.