# Class-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications
## 5 Transparent Glass-Ceramics
### OxofluorideGCs  
For the last ten years, the GlaSS group at Ceramics and Glass Institute (CSIC) focused attention on the preparation of transparent glass-ceramics with low-phonon fluoride crystals, such as LaF\({}_{3}\), NaGdF\({}_{4}\), NaLuF\({}_{4}\), KLaF\({}_{4}\), etc., with rare-earth ions as dopants for optical applications [29; 30; 62; 63; 64; 65; 66; 67; 68]. The main emphasis was on maximizing the crystalline fraction, maximum incorporation of the RE dopant, and further enhancement in luminescence profile. More efforts were developed towards understanding the mechanisms of nucleation and crystallization of fluoride nanocrystals. All these materials were prepared by homogeneous nucleation by conventional melt quenching and then heat treatment processes at temperatures slightly higher than T\({}_{\text{g}}\). LaF\({}_{3}\) crystals smaller than 20 nm diameter were precipitated from the base glass and formed within phase-separated droplets. The glass transformed into nano-glass-ceramics by heating at 645 \({}^{\circ}\)C for 20 h [61]. Oxofluoride GCs doped with 0.1 and 0.5 Pr and co-doped with 0.1-0.5 Pr-Yb and 0.5-1 Pr-Yb GCs treated at 620 and 660 \({}^{\circ}\)C were perfectly transparent, owing to the formation of nanosized crystals of LaF\({}_{3}\). With the increasing concentration of dopants, the nuclei density increased, but the nuclei size became smaller [63]. Similarly, other dopants were studied to obtain LaF\({}_{3}\)-based glass-ceramics, using thermal treatments in the range 1-5 h at 620 \({}^{\circ}\)C [64; 65]. Transparent oxyfluoride GCs containing NaGdF\({}_{4}\) nanocrystals doped with 0.1 Pr\({}^{3+}\) and 0.5 Pr\({}^{3+}\) and co-doped with 0.5 Pr\({}^{3+}\)-2 Yb\({}^{3+}\) ions (mol%) were also obtained [66]. The X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) confirmed the precipitation of NaGdF\({}_{4}\). The kinetics at 550 \({}^{\circ}\)C as a function of the dwelling time resulted in an increase of the nanocrystal size up to 12.5 nm and remained constant after 80 h. In addition, XRD of doped and co-doped GC heat-treated at 550 \({}^{\circ}\)C during 80 h showed that the nanocrystal size increased from 13 up to 30 nm with the dopant concentration. Transparent NaLuF\({}_{4}\) glass-ceramics doped with 0.5 mol% Er\({}^{3+}\) and 0.5 Er\({}^{3+}\)-xYb\({}^{3+}\) (x = 2, 4 mol%) were prepared by the melting-quenching method, and, after suitable thermal treatment at 600 \({}^{\circ}\)C for 20 h, cubic NaLuF\({}_{4}\) nanocrystals ranging from 10 to 50 nm (depending on dopant concentration) were obtained. Up-conversion green emission related to \({}^{2}\)H\({}_{11/2}\), \({}^{4}\)S\({}_{3/2}\rightarrow^{4}\)I\({}_{15/2}\) transition, and red emission related to \({}^{4}\)F\({}_{9/2}\rightarrow^{4}\)I\({}_{15/2}\) transition were observed in all GCs [67]. Cubic (\(\alpha\)-phase) and hexagonal (\(\beta\)-phase) KLAF\({}_{4}\) glass-ceramics were also obtained doped with four different Nd\({}^{3+}\) concentrations (0.1-2 mol%). These samples were prepared by heat treatment at 590 \({}^{\circ}\)C from 1 to 150 h and at 660 \({}^{\circ}\)C from 1 to 192 h. The phase evaluation depended on the Nd\({}^{3+}\) dopant concentration. In the case of 0.1 mol% of the Nd\({}^{3+}\)-doped sample, only after heat treatment at 660 \({}^{\circ}\)C for times longer than 144 h, \(\beta\)-KLAF\({}_{4}\) nanocrystals were formed. In the case of 0.5 mol% of Nd\({}^{3+}\)-doped sample, \(\beta\) -KLAF\({}_{4}\) nanocrystals were formed only for times longer than 15 h, and the authors concluded that hexagonal nanocrystals needed longer growth times [68]. On the other hand, the sol-gel method was also explored. Sol-gel oxyfluoride LaF\({}_{3}\) glass-ceramics were obtained with very fast treatments (1 min) at 550 \({}^{\circ}\)C temperature. It was proposed that the crystallization process of LaF\({}_{3}\) in sol-gel materials was not diffusion-controlled nucleation and growing process, but a chemical reaction followed by the fast precipitation of crystals [76].