Structure-property relations in new fluorophosphate glasses singly-and co-doped with Er3+ and Yb3+

Rare earth (RE3+)-doped fluorophosphate glasses are among the most promising candidates for high-efficiency laser generation in the near-infrared spectral region. By proper choice of composition, these materials can combine the advantages of fluorides (low phonon energies, low refractive indices, extensive optical window, low hygroscopicity) and of oxides (high chemical and mechanical stability and high dopant solubility), resulting in enhancement of the RE3+ emissive properties. In this work, we present the synthesis and structural/spectroscopic investigation of new glasses with composition 25BaF(2)25SrF(2)(30-x)Al(PO3)(3)xAlF(3)(20-z)YF3:zREF(3), where x = 20 or 15, RE = Er3+ and/or Yb3+, z = 0.25 -5.0 mol%. Results indicate considerable improvement of the emissive properties of both ions when compared to phosphate or even other fluorophosphate host compositions. Long excited state lifetimes (tau = 10 ms for the Er3+ level 4I(13/2), and tau = 1.3 ms for the Yb3+ level F-2(5/2)) imply high fluorescence quantum efficiencies eta (up to 85% for both ions). Structural characterization by Raman and multinuclear solid state NMR spectroscopies indicate that the metaphosphate-type chain structure of the Al(PO3)(3) vitreous framework is partially depolymerized and dominated by Q((0)) and Q((1)) units crosslinked by six-coordinate Al species. As revealed by Al-27[P-31] rotational echo double resonance (REDOR) NMR results the average local aluminum environment of the x = 20 sample comprises 1.6 phosphate and 4.4 fluoride species. These results indicate a clear bonding preference between aluminum and phosphorus, which is consistent with the desired dominance of fluoride species in the local environment of the rare earth and alkaline earth atoms in these glasses. (C) 2015 Elsevier B.V. All rights reserved. (AU)