Novel sol-gel SiO2-NaGdF4 transparent nano-glass-ceramics

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.