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

摘要

A detailed structural investigation of a series of fluoride phosphate laser glasses with nominal composition 25BaF(2)-25SrF(2)-(30 - x)Al(PO3)(3)-xAlF(3)-(20-z)YF3:zREF(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 Al-27/P-31 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 (S = 1/2), revealing composition-dependent echo-detected lineshapes and strong hyperfme coupling with F-19 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 D-5(0) -> F-7(2)/D-5(0) -> F-7(1) transitions intensity ratio, the normalized phonon sideband intensities in the excitation spectra, and excited state D-5(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.