The effect of annealing temperature on the site occupancy and the persistent luminescence of Mn2+-doped magnesium germanate

Mn2+-doped magnesium germanate (MGO:Mn2+) nanoparticles exhibit persistent luminescence in the nearinfrared (NIR) region. The origin of the luminescence is from the Mn2+ d-d transition when Mn2+ occupies an octahedral site in MGO that was previously occupied by Mg2+. MGO has two stable stoichiometric forms: MgGeO3 and Mg2GeO4. The octahedra surrounding the Mg2+ ions in these two structures have different extents of distortion. Previous studies have found that under high temperatures, MgGeO3 could turn into Mg2GeO4. However, when Mg2+ is partially substituted with Mn2+, whether the structural transition follows the same trend as the undoped MGO is unclear. In this work, MGO:Mn2+ with mixed MgGeO3 and Mg2GeO4 is produced by thermal annealing at different temperatures. The photoluminescence intensity and persistent luminescence lifetime are found highly dependent on the annealing temperature and the crystallinities of the MGO host. The site where Mn2+ occupies in MGO with mixed stoichiometries is evaluated using X-ray absorption spectroscopy combined with structure simulation. The distribution of the Mn2+ at different Mg2+ sites can be quantitatively identified. This is further related to the observed optical properties, and the mechanism of luminescence enhancement of high-temperature-treated MGO:Mn2+ is rationalized.