Novel transparent oxyfluoride nano-glass-ceramics containing undoped and 0.5 Eu (mol%) doped NaGdF nanocrystals (NCs), were obtained by sol-gel for the first time. X-ray diffraction (XRD) confirms the precipitation of α- and β-NaGdF NCs with a size ranging between 5 and 24 nm. When the Na:Gd ratio, temperature and heating time are fixed to 0.95:1, 550–600 °C and 1–8 h, respectively, only the β-NaGdF phase precipitates. These results are confirmed by high-resolution transmission electron microscopy (HRTEM). The decomposition of trifluoroacetic acid (TFA) and the formation of fluoride lattice bonds were studied by Fourier transform infrared spectroscopy (FTIR). Thus, the sol-gel route permits NaGdF glass-ceramics to be obtained through the control of Na:Gd ratio, temperature and heating time. The incorporation of dopant ions in the NaGdF NCs was confirmed by photoluminescence measurements which show narrow and well-resolved Eu emission and excitation spectra in GCs. Moreover, the low asymmetry ratio values between the electric dipole (D → F) to the magnetic dipole (D → F) transitions indicate that Eu ions are incorporated in the NaGdF NCs. Furthermore, efficient Gd→ Eu energy transfer results in an enhancement of the reddish Eu emissions, further supported by fluorescence decay lifetimes.

This work was supported by MINECO under Projects N° MAT2013-48246-C2-1-P/2-P and MAT2017-87035-C2-1-P/-2-P (AEI/FEDER, UE),and Basque Country University PPG17/07 and GIU17/014. This paperis a part of dissemination activities of the project FunGlass. This projecthas received funding from the European Union's Horizon 2020 researchand innovation programme under grant agreement No 739566