Network Structure and Rare-Earth Ion Local Environments in Fluoride Phosphate Photonic Glasses Studied by Solid-State NMR and Electron Paramagnetic Resonance Spectroscopies

A detailed structural investigation of a series of fluoride phosphate laser glasses with nominal composition 25BaF₂–25SrF₂–(30 – <i>x</i>)­Al­(PO₃)₃–<i>x</i>AlF₃–(20 – <i>z</i>)­YF₃:<i>z</i>REF₃ with <i>x</i> = 25, 20, 15 and 10, RE = Yb and Eu, and 0 ≤ <i>z</i> ≤ 1.0 has been conducted using Raman, solid-state nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopies. The network structure is dominated by the preferred formation of aluminum-to-phosphorus linkages, which have been quantified by means of ²⁷Al/³¹P NMR double-resonance techniques. The fluoride ions are found in mixed Al/Y/Ba/Sr environments accommodating the luminescent dopant species as well. The local environments of the rare-earth species have been studied by pulsed EPR spectroscopy of the Yb³⁺ spin probe (<i>S</i> = ¹/₂), revealing composition-dependent echo-detected lineshapes and strong hyperfine coupling with ¹⁹F nuclei in hyperfine sublevel correlation (HYSCORE) spectra consistent with the formation of Yb³⁺–F bonds. In addition, photoluminescence spectra of Eu³⁺-doped samples reveal that the ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ transitions intensity ratio, the normalized phonon sideband intensities in the excitation spectra, and excited state ⁵D₀ lifetime values are systematically dependent on fluoride content. Altogether, these results indicate that the rare-earth ions are found in a mixed fluoride/phosphate environment, to which the fluoride ions make the dominant contribution. Nevertheless, even at the highest fluoride levels (<i>x</i> = 25), the data suggest residual rare-earth–phosphate coordination