Structural, photoluminescent properties and Judd-Ofelt analysis of Eu3+-activated CaF2 nanocubes

Eu3+-doped CaF2 nanocubes with variable europium concentration, [Eu3+] = 0, 0.6, 1.3, 1.7, 2.2 and 5.4 mol%, have been synthesized by a direct precipitation route. It has been found that, within this concentration range, the nanoparticles present the fluoride-type crystalline structure and the characteristic cubic shape of CaF2 crystals. The nanoparticle size follows a log-normal distribution with a mean value decreasing with the Eu3+ content. Rietveld refinement has been performed to calculate the lattice parameter and crystallite size. Eu3+ concentration affects both parameters giving rise to an increase in the lattice parameter and a reduction of crystallite size. The luminescent properties of Eu3+ ions in these nanostructures have been investigated under CW and pulsed excitation. A Judd-Ofelt analysis, as function of the Eu3+ content, has been performed to determine the transition probabilities, radiative lifetimes and branching ratios of the 5D0 emitting level. It was found that Ω2 and Ω4 Judd-Ofelt intensity parameters are dependent on the doping level, showing an evolution that indicates a decrease in the Eu3+ site local symmetry with increasing Eu3+ concentration. Finally, it has been observed that the characteristic luminescence decay time of the 5D0 manifold is reduced with increasing Eu3+ concentration. This effect is partially due to an increase of radiative transition probability, associated with a reduction in the local symmetry of the lanthanide ions, and also to the occurrence of concentration quenching effects.