Multisite formation in gadolinium doped SrF2 nanoparticles

Gadolinium doped SrF2 nanoparticles have been synthesized by the precipitation method and characterized by site-selective optical spectroscopy and electron paramagnetic resonance (EPR) spectroscopy methods at cryogenic temperatures. The type of Gd3+ centres present in SrF2 is strongly dependent on the heat treatment parameters - the chosen temperature and gas composition during the annealing. Gd3+ ions in SrF2 annealed at temperatures up to 400 °C are mainly incorporated into sites with cubic symmetry. Higher annealing temperatures enable charge compensators to locate in a closer vicinity of the Gd3+ ion lowering the site symmetry. Oxygen compensated Gd3+ centres form after heating in air atmosphere, whereas for samples heated at an identical temperature in inert atmosphere interstitial fluorine ions serve as charge compensators. A comparison with previous studies of transparent glass ceramics containing SrF2:Gd3+ shows similar patterns in gadolinium centre formation with the precipitated nanoparticles annealed in inert atmosphere here. The obtained results give a fundamental insight in trivalent defect formation mechanisms in nanostructured SrF2 systems.