Structural and luminescence characterization of europium-doped niobium germanate glasses and glass-ceramics: Novel insights from 93Nb solid-state NMR spectroscopy

Abstract

A thorough understanding of network former roles in optical glasses and glass-ceramics, as well as structural environments of luminescent rare-earth atoms, is paramount to assessing their suitability and effectiveness as components for fully-optical integrated devices. The present contribution sheds light on the overall poorly understood structural role of Nb2O5 in oxide glasses and offers a rationale for the luminescence changes concomitant to the heat-treatment induced glass-to-crystal transition in niobium germanate glasses. To this end, results from 93Nb nuclear magnetic resonance (NMR), Raman, and Eu3+ ion based luminescence spectroscopies as well as 3D micromanufacturing are reported for glasses and glass-ceramics in system (89.9-x)GeO2-xNb2O5-10K2O-0.1Eu2O3. Thanks to very-fast 93Nb magic-angle spinning (MAS) and a novel excitation scheme for Triple-quantum (TQ)MAS solid-state NMR spectroscopy distinct Nb(OGe)6-y(ONb)y species were identified for the first time: at low Nb concentrations structural units with rather low y-values are formed, reaching down to isolated Nb(OGe)6 units – next to units bearing non-bridging oxygen atoms; at intermediate Nb contents and beyond, structural units with increasingly higher y-values up to Nb(ONb)6 are found, resembling the local environments found in crystalline Nb2O5. After the glass-to-crystal transition the Nb local environment maintains a high degree of disorder while some residual glass remains. Moreover, photoluminescence spectroscopy of the europium probes suggests the rare-earth ions are preferentially incorporated into high y-value Nb(OGe)6-y(ONb)y polyhedra based on spectral features such as the phonon sideband, the 5D0 → 7F2/7F1 ratio, and split 4f bands. Finally, femtosecond laser microfabrication of waveguides was successful in the presented europium-doped niobium germanate glasses, making them promising candidates for further studies as photonic devices.