Rare-earth doped fluoride phosphate glasses: structural foundations of their luminescence properties

We report a detailed structural investigation of a series of fluoride-phosphate glasses with different phosphate/fluoride ratios in the system xSr(PO₃)₂-(100 - x)[AlF₃-CaF₂-SrF₂-MgF₂] with x = 5, 10, 20, 40. Raman and multinuclear solid NMR spectroscopies confirm that the polyphosphate network structure is successively transformed to a structure dominated by Al-O-P linkages with increasing AlF₃ content. Average numbers of Al-O-P linkages have been quantified by ²⁷Al/³¹P NMR double-resonance techniques. The majority of the fluoride species are found in an alkaline earth metal/aluminum rich environment. The local environments for rare-earth ions have been characterized by EPR spectroscopy of Yb³⁺ ion spin probes and by photoluminescence experiments on Eu³⁺ dopant ions, including the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transition intensity ratio, the normalized phonon sideband intensities in the excitation spectra, and the lifetime of the ⁵D₀ excited state. The results indicate clear correlations between these parameters as a function of composition, and confirm that even at the highest fluoride levels, there is still some residual rare-earth phosphate coordination.