Structure–Property Relations in Fluorophosphate Glasses: An Integrated Spectroscopic Strategy

A detailed structural investigation of a series of fluoride phosphate glasses with nominal compositions 25BaF 2 -25SrF 2 –(30– x )Al(PO 3 ) 3 – x AlF 3 –(20– z )ScF 3: z REF 3 with x = 25, 20, and 15, RE = Yb and Eu, and 0 ≤ z ≤ 1.0, and of a Sc-free set of glasses with compositions w [80(Ba/Sr)F 2 –20AlF 3 ]–(1– w )[80Ba(PO 3 ) 2 –20Al(PO 3 ) 3 ] ( w = 25, 50, 75), doped with 0.2 mol % Yb 3+ or Eu 3+, has been conducted. As indicated by Raman scattering and solid state NMR, the network structure is dominated by aluminum–oxygen-phosphorus linkages, which can be quantified by means of 27 Al/ 31 P NMR double resonance techniques. The ligand environment of the rare-earth ions is studied by (1) 45 Sc NMR of the diamagnetic mimic Sc 3+, (2) pulsed X-band EPR spectroscopy of Yb 3+ spin probes, and (3) excitation and emission spectroscopy of Eu 3+ dopants. The rare-earth ions are found in a mixed environment of fluoride and phosphate ions, which changes systematically as a function of glass composition. In the Sc-containing glasses the quantitative makeup of this ligand environment has been determined by 45 Sc{ 19 F} and 45 Sc{ 31 P} rotational echo double resonance (REDOR). Comparison of the P- to F-ligand ratio with the batch composition indicates that the Sc 3+ ions show a clear preference for phosphate over fluoride ion ligation. These REDOR results were correlated with Yb 3+ EPR data, the intensity ratio of Eu 3+ transitions 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1, and the lifetime values of the Eu 3+ emitting level 5 D 0 . As a result, it was possible to obtain a global interpretation in terms of the associated quantitative ligand distribution (fluoride versus phosphate) in the first coordination sphere of the rare earth ions. The calibration of EPR and luminescence spectra on the basis of such solid state NMR data defines a new spectroscopic strategy for characterizing the rare-earth local environments in promising laser glasses.