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

Abstract

A detailed structural investigation of a series of fluoride phosphate laser glasses with nominal composition 25BaF 2 –25SrF 2 –(30 – x )Al(PO 3 ) 3 – x AlF 3 –(20 – z )YF 3: z REF 3 with x = 25, 20, 15 and 10, RE = Yb and Eu, and 0 ≤ z ≤ 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 27 Al/ 31 P 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 Yb 3+ spin probe ( S = 1 / 2 ), revealing composition-dependent echo-detected lineshapes and strong hyperfine coupling with 19 F nuclei in hyperfine sublevel correlation (HYSCORE) spectra consistent with the formation of Yb 3+ –F bonds. In addition, photoluminescence spectra of Eu 3+ -doped samples reveal that the 5 D 0 → 7 F 2 / 5 D 0 → 7 F 1 transitions intensity ratio, the normalized phonon sideband intensities in the excitation spectra, and excited state 5 D 0 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 ( x = 25), the data suggest residual rare-earth–phosphate coordination.