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.