Tailoring local coordination structure of the Er³⁺ ions for tuning the up-conversion multicolor luminescence

The regulation of the local structure around Er³⁺ ions is an important channel for adjusting the characteristic of up-conversion luminescence. In this paper, the cubic-phased Er³⁺:CaF₂ crystals with different Er³⁺ doping concentrations were fabricated with temperature gradient technique (TGT) method and the effect of the local coordination structure of the Er³⁺ ions on its luminescence performance was investigated. The local coordination structure of Er³⁺ ions was simulated by density functional theory. The computational results show that clusters evolve from low order to high order with the increase of Er³⁺ ion doping concentration. In this evolution process, the local structure transforms from cubic structure to the co-existence of cubic and lower symmetric square anti-prism structures. Meanwhile, the distance between Er³⁺ ions in the cluster decreased first and then increased slightly, and in dimers and trimers this distance reached the minimum. Under 980 nm excitation, with the increase of Er³⁺ ion concentration, the intensity ratios of the red and green emissions of Er³⁺:CaF₂ first increased from 0.61 to 42.03 and then decreased to 12.11. The corresponding up-conversion luminescence gamut was adjusted from monochrome green to red to red-yellow. This work provides a new thread for realizing upconversion multicolor luminescence by regulating the clusters of rare earth ions.