Abstract
In this study, transparent oxyfluoride glass-ceramics (GCs) with NaLaF4 nanocrystals (NCs) were prepared by the sol–gel method for the first time. Three different molar ratios of La(CH3COO)3/Na(CH3COO) were used to obtain the GCs, which were sintered at 450, 550 and 650 °C for 1 min. X-ray diffraction (XRD) was employed to follow the evolution of the xerogel during the heat treatments and to study crystal growth for the three temperatures. In all cases, the LaF3 crystalline phase was present, but crystallization of NaLaF4 was only promoted at 650 °C. Thermogravimetric and thermodifferential analysis (TGA-DTA) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the crystallization process. High-resolution transmission electron microscopy (HRTEM) was employed to confirm NaLaF4 crystallization and determine the size distribution. The incorporation of Nd3+ ion into NaLaF4 and LaF3 nanocrystals was confirmed by site-selective emission and excitation spectra. The Nd3+ emission intensities in both phases depend not only on the NaLaF4/LaF3 ratio but also on their emission efficiencies.
Highlights
In recent years, transparent oxyfluoride glass-ceramics have attracted considerable attention due to their widespread application in scintillators, phosphors, photovoltaics and solid-state lasers [1,2,3]
Both phases appeared for different relations of precursors; Nd3+-doped SiO2-NaLaF4 GCs with the presence of LaF3 were successfully obtained for the first time by sol–gel after heat treatment in the range 450–650 °C for 1 min
Lower temperatures did not produce significant quantities of NaLaF4, but LaF3 crystallized for all heat treatment temperatures
Summary
Transparent oxyfluoride glass-ceramics have attracted considerable attention due to their widespread application in scintillators, phosphors, photovoltaics and solid-state lasers [1,2,3]. Rare-earth (RE) fluoride crystals are excellent hosts for up- and down-conversion luminescence associated with their unique characteristics, such as their low lattice phonon energy (lower than 500 cm−1 ) [4,5]. SiO2 glass matrix, due to its high phonon energy, is compensated with the incorporation of the RE-fluoride nanocrystals. The final material combines the excellent mechanical and chemical properties of the glass with the luminescence of the crystalline phases [6]. The NaLnF4 (Ln = Y, Gd, La and Lu) series acts as a host material with excellent physical and chemical stability, as well as low phonon energy. A mechanism to improve the crystallization of NaLaF4 involves doping with rare-earth ions, which helps to stabilize the alkaline fluorides when its crystallization competes with that of other phases, such as
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