**10.3.6. FAP-anorthite-diopside glass-ceramics**

There is a considerable interest in oxyfluoride glasses and glass-ceramics for laser amplifiers and up-conversion processors. Fluoride-containing crystals have low phonon energies and apatite crystals, in particular, are good host phases to adsorb rare-earth ions. Fluoridecontaining glass-ceramics also often crystallize on a nanoscale, which is an added advantage, since optically transparent materials are required for the applications such as fiber amplifi‐ ers. The evidence of the nanoscale crystallization in an FAP-anorthite-diopside-based glassceramics was found by HILL et al [74].

#### **10.3.7. Apatite-wollastonite ceramics**

Wollastonite-hydroxyapatite ceramics was successfully prepared by a novel method, corre‐ sponding to the thermal treatment of a silicone embedding micro- and nanosized fillers in air. CaCO3 nanosized particles, providing CaO upon the decomposition, acted as "active" filler, whereas different commercially available or synthesized hydroxyapatite particles were used as "passive" filler. The homogeneous distribution of CaO, at a quasi-molecular level, fa‐ vored the reaction with silica derived from the polymer, at only 900°C, preventing extensive decomposition of hydroxyapatite. Open-celled porous ceramics suitable for scaffolds for bonetissue engineering applications were easily prepared from the filler-containing silicone resin mixed with sacrificial PMMA microbeads as the templates [75].

#### **10.3.8. Oxyapatite glass-ceramics**

The crystallization of oxyapatite of the composition NaY9(SiO4)6O2 (P63/M, *a* = 9.334 Å and *c* = 6.759 Å, *c*:*a* = 1:0.7241, *V* = 509.97 Å, **Fig. 7**) from three different glasses from the SiO2-B2O3- Al2O3-Y2O3-CaO-Na2O-K2O-F glass-ceramics system with different F and B2O3 content after the

**Fig. 7.** Crystal structure of oxyapatite NaY9(SiO4)6O2 (perspective view along the *c*-axis) where the tetrahedra repre‐ sents [SiO4] 4− structural units [76].

heat treatment was observed by VAN´T HOEN et al [76]. The formation of oxyapatite proceeds according to the mechanism of controlled surface nucleation and crystallization. Therefore, applying suitable chemical composition and heat treatment, it is possible to produce glassceramics containing oxyapatite crystals. This type of crystal exhibits similar crystal structure as that of fluorapatite. Therefore, there is an isotype relationship between those two phases: the differences in the crystal structures are found in the structural units, with fluorapatite containing [PO4] 3− and [F]<sup>−</sup> , while oxyapatite containing [SiO4] 4− and [O]2−. Because of its specific optical properties (high opacity), this new glass-ceramic material may be used as layering material for dental restoration [76].
