**10.3.4. Apatite-wollastonite glass-ceramics**

**Fig. 6.** Ternary phase diagram of Na2O-Al2O3-P2O5 system [51],[58].

466 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

**10.3.3. Lithium vanado-phosphate glasses**

7

state devices [63].

Iron phosphate glass is a versatile matrix for the immobilization of various radioactive elements found in high-level nuclear waste (HLW). Among various compositions of iron phosphate glass, the one with 40 mol.% Fe2O3-60 mol.% P2O5 was found to be chemically durable. It also has the ability to accommodate large amounts of certain nuclear wastes, especially those that are not well suited for borosilicate glasses. Better chemical durability of iron phosphate glass is attributed to the presence of more hydration-resistant Fe-O-P bonds

Lithium vanado-phosphate (LiVP) glasses have been largely studied due to their potential application as cathode materials as a result of mixed electronic-ionic conductivity character.7 Furthermore, lithium and vanadium structural rearrangements in the glass matrix could modify the transport properties of the systems. Interestingly, the modifier ions depolymer‐ ize the glass network, creating useful channels that enhance the ionic conductivity, but they can also break some V4+/O/V5+ linkages that are essential to the electronic conductivity because they are supposed to be the preferential path for small polaron hopping. The population of

Fast ion conducting (FIC) phosphate glasses have become very important due to a wide range of applications in solid-

compared to P-O-P bonds available in other phosphate glasses [59],[60],[61].

**10.3.2. Iron phosphate glasses**

Wollastonite (CaSiO3) is white glassy silicate mineral that occurs as masses or tabular crystals of metamorphosed limestone. A silica chain GC that contained crystalline apatite and wollastonite (AW) was introduced in MgO-CaO-SiO2-P2O5 glassy matrix and it showed excellent bioactivity, biocompatibility, machinability and adequate mechanical properties such as Young's modulus (117 GPa), compressive strength (1080 MPa) and bending strength (215 MPa) [52],[65],[66],[67].

Wollastonite-2M and pseudowollastonite (low- and high-temperature forms of wollastonite, respectively) are the most common calcium silicate biomaterials proposed for bone tissue regeneration [68]. The major drawback of the CaSiO3 bioceramics is their relatively fast dissolution rate that could reduce their mechanical strength. In addition, the pH of surround‐ ing medium significantly increases, which could affect the osseointegration of the substitute material within the natural bone. The problem can potentially be solved by the development of multiphase materials containing highly dissolvable phases such as wollastonite, on one hand, and stable phases such as HAP, on the other hand [69].

From the bioactive ceramics, the A/W glass-ceramics show high bioactivity and high mechan‐ ical strength [56]. The A/W glass-ceramics composed of apatite and wollastonite crystalline phases in a glassy matrix was developed by KOKUBO [70]. This bioceramics is highly bioac‐ tive and also mechanically strong in comparison with other glasses and glass-ceramics because of wollastonite and apatite crystals' presence.

The A/W glass-ceramics is used in some medical applications, either in powder form as a bone filler or as a bulk material. These materials are currently manufactured by the powder processing methods, providing uniform crystallization of apatite and wollastonite phases in the glassy matrix, as the crystallization of parent glass in a bulk form leads to the appear‐ ance of large cracks [69],[71]. The glass-ceramics exposed to the SBF releases predominantly Ca and Si ions, due to the dissolution of amorphous phases and wollastonite-2M, leading to the formation of an apatite-like layer on the surface of the material [69].

The addition of ZnO increased the chemical durability of A-W glass-ceramics, resulting in a decrease in the rate of apatite formation in simulated body fluid. On the other hand, the release of zinc from the glass-ceramics increased with increasing ZnO content. The addition of ZnO may provide bioactive CaO-SiO2-P2O5-CaF2 glass-ceramics with the capacity for appropriate biodegradation as well as the enhancement of bone formation [56]. The effect of MgO was investigated by MA et al [72]. As the MgO content increased, the glass crystallization temper‐ ature increased and the crystallization of the glass-ceramics was changed from the bulk crystallization to the surface crystallization. The addition of MgO slowed down the rate of dissolution and retarded the formation of apatite layer.
