Optical properties of Er3+ doped NaLuF4 transparent glass-ceramics produced by spark plasma sintering

Transparent oxyfluoride glass-ceramics (GCs) containing NaLuF4 nanocrystals doped with Er3+ have been prepared by combination of glass melt-quenching and spark plasma sintering (SPS) of glass powders. The parameters, such as temperature, pressure, particle size and holding times were optimized to obtain the densification of the powdered samples together with the desired transparency. The major advantage of this technique is the shorter times (few minutes) compared with those used for the preparation of these GCs by conventional heat treatments (more than 10 h). SPS presents an important inconvenient, which is the carbon contamination coming from graphite punch, this has been improved using Pt foils. The structural study indicates the presence of nanocrystals corresponding to the crystalline phase α-NaLuF4, with a crystal size between 26 and 31 nm. The optical characterization, which includes near infrared emission and infrared-to-visible up-conversion, confirms the incorporation of Er3+ ions in the nanocrystals. The longer lifetimes of the 4I11/2 and 4I13/2 levels together with the enhancement of the upconverted green and red emissions (as well as the weak blue emission) in the SPS-GCs samples, if compared with the precursor glass, indicates the presence of Er3+ ions in a crystalline environment. Possible energy conversion mechanisms for the green and red emissions based on the temporal behavior of the upconverted luminescence are discussed.