**5.5.2 AM and AEE-yttrium orthosilicate oxyapatites**

Alkaline metals (AM) and alkaline-earth element oxyapatites (oxybritholites) are described in this chapter. Phosphate minerals of the apatite supergroup possess strong affinity for strontium [106]. The apatite-type phase of the composition of Sr2Y8Si6O26 (Sr2Y8(SiO4)6O2) was prepared by ZUEV et al [107] via the two-stage calcination of mixture of SrCO3, Y2O3, Eu2O3 and SiO2 in air in order to investigate the spectral characteristics of Sr2Y8(SiO4)6O2:Eu polycrystals. The structure of apatite phase is shown in **Fig. 21**. The sol-gel synthesis and the characteriza‐ tion of Sr2Y8(1−x)Eu8xSi6O26 solid solution doped by Eu, where *x* = 0.01 – 0.4, was described by KARPOV and ZUEV [108]. The formation of calcium analogue (Ca2Y8(SiO4)6O2, calcium-yttriumsilicate oxyapatite) was observed during the crystallization of SiO2-Al2O3-CaO-Na2O-K2O-F-Y2O3 glass ceramics [109] and during the degradation of advanced environmental barrier coatings [110]. The phase Ca4Y6(SiO4)6O2 was often prepared in order to investigate its luminescent properties [111],[112].

**Fig. 21.** The structure of Sr2Y8(SiO4)6O2 [107].

**Fig. 22.** The structure of NaY9(SiO4)6O2 oxyapatite (the perspective view along the c-axis) [114].

the only other example of REE2O3-Al2O3-SiO2 phase diagram found so far in the literature. The authors identified the liquidus surface of the whole ternary field, but they failed to elucidate the subsolidus phase relationships among different binary compounds. Due to the much smaller ionic size of Y3+ ion with respect to La3+ ion (1.18 and 1.015 Å for the eightfold coordination, respectively [103]) and lower bond-valence parameter (2.019 and 2.172 Å [104]),

The β-alumina-like phase LaAl11O18 is no longer stable, while the garnet-like phase Y3Al5O18 and Y4Al2O9 monoclinic compound exist. The lacunar apatite-like phase Y14Si9O39 reported by WILLS et al [105] does not appear in the Y2O3-Al2O3-SiO2 ternary diagram; however, a com‐ pound with similar Y/Si atomic ratio, namely Y4Si3O12, also reported by WILLS et al [105], does. Since the formation of Y4Si3O12 phase was not confirmed, it may be stabilized by impurities

Alkaline metals (AM) and alkaline-earth element oxyapatites (oxybritholites) are described in this chapter. Phosphate minerals of the apatite supergroup possess strong affinity for strontium [106]. The apatite-type phase of the composition of Sr2Y8Si6O26 (Sr2Y8(SiO4)6O2) was prepared by ZUEV et al [107] via the two-stage calcination of mixture of SrCO3, Y2O3, Eu2O3 and SiO2 in air in order to investigate the spectral characteristics of Sr2Y8(SiO4)6O2:Eu polycrystals. The structure of apatite phase is shown in **Fig. 21**. The sol-gel synthesis and the characteriza‐ tion of Sr2Y8(1−x)Eu8xSi6O26 solid solution doped by Eu, where *x* = 0.01 – 0.4, was described by KARPOV and ZUEV [108]. The formation of calcium analogue (Ca2Y8(SiO4)6O2, calcium-yttriumsilicate oxyapatite) was observed during the crystallization of SiO2-Al2O3-CaO-Na2O-K2O-F-Y2O3 glass ceramics [109] and during the degradation of advanced environmental barrier coatings [110]. The phase Ca4Y6(SiO4)6O2 was often prepared in order to investigate its

SiO4

4f –Sr/Y

6h –Y 2a –O2– 6h 2a

the stability of the binary compositions is substantially altered [57].

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

**5.5.2 AM and AEE-yttrium orthosilicate oxyapatites**

luminescent properties [111],[112].

**Fig. 21.** The structure of Sr2Y8(SiO4)6O2 [107].

[57],[92].

The precipitation of NaY9(SiO4)6O2 apatite-type compound (sodium nonayttrium hexa‐ kis(silicate) dioxide) in the SiO2-B2O3-Al2O3-Y2O3-CaO-Na2O-K2O-F glass-ceramics system (**Section 10.3.8**) was described by VAN'T HOEN et al [113]. The hexagonal structure of NaY9(SiO4)6O2 oxyapatite (**Fig. 22**) was resolved by GUNAWARDANE et al [114]. The phase crystallizes in P63/M space group with the cell parameters *a* = 9.334, *c* = 6.759 Å, *c*:*a* = 0.7241, *V* = 509.97 Å and *Z* = 1).

Lithium yttrium orthosilicate (LiY9(SiO4)6O2, lithium nonayttrium hexakis(silicate) dioxide) crystallizes in centrosymmetric space group P63/M. The structure closely resembles those of fluorine apatite. There are two different crystallographic sites for Y3+ ion, which are coordi‐ nated by seven and nine O atoms. One-fourth of the nine-coordinated site is occupied by Li atoms, thus maintaining the charge balance. Si atom occupies the tetrahedral site [115].

The preparation, the properties and the effect of sintering additives of hexagonal (P63/M) strontium-yttrate-silicate oxyapatite (oxybritholite2) with the composition of SrY4(SiO4)3O as the main product of sinter-crystallization process, in which the non-equilibrium melt was formed in the temperature interval from 1300 to 1550°C in the SrO-Y2O3-SiO2 system, was described by PTÁČEK et al [116]. The formation of non-equilibrium melt is facilitated by borate fluxes, alkaline fluxes and talc. The apparent activation energy and the frequency factor of the sinter-crystallization process were determined to be 1525 kJ mol−1 and 1.04·1045 s-1, respective‐ ly. The material shows low value of linear thermal expansion coefficient of (1.1 ± 0.1)·10−6°C−1 in the temperature range from 25 to 850°C.

The course of synthesis can be expressed by the following reaction formula [116]:

$$\text{AEEEO}\_3 + 2\text{ Y}\_2\text{O}\_3 + 3\text{ SiO}\_2 \rightarrow \text{AEE}\text{ Y}\_4\text{[SiO}\_4\text{]}\_3\text{O} + \text{CO}\_2\text{(g)}\tag{12}$$

This **reaction 12** is too general to describe formed intermediates (SrSiO3, Sr2SiO4, SrY2O4, …16) and the process of sinter-crystallization of apatite:

<sup>16</sup> Detailed description of formed intermediates can be found in work [116].

$$\begin{aligned} \text{AEEEO}\_3 + 2 \text{ Y}\_2\text{O}\_3 + 3 \text{ SiO}\_2 &\rightarrow \text{intermendiates} \rightarrow \\ \text{non} - \text{equilibrium melt} &\rightarrow \text{AEE} \text{ Y}\_4 \text{[SiO}\_4\}\_3\text{O} + \text{CO}\_2 \text{(g)} \end{aligned} \tag{13}$$

Since the formation of SrY4(SiO4)3O proceeds thorough non-equilibrium melt phase, the effect of sintering additives such as borate fluxes, fluorides and carbonates of alkaline metals as well as talc was investigated. Sintering additives facilitate the formation of melt phase and increase the length of sinter-crystallization interval. The expansion after the thermal decomposition of strontium carbonate is reduced as well. Calcinate, treated to the temperature lower than the temperatures of sinter-crystallization interval, has hydraulic activity. Therefore, it can be applied in special composite cements as an activator for latent hydraulic and pozzolanic materials.

**Fig. 23.** SEM picture and WDX analysis of hexagonal crystal of CaY4(SiO4)3O apatite phase.


**Table 3.** The influence of sintering additive on the behavior during thermal treatment [116].

After the process of sinter-crystallization, the reactivity of glassy phase with water drops. A significant benefit of talc is the fact that the glassy phase surrounding the crystals of apatite phase becomes resistant against the influence of water with this sintering additive. Further‐ more, magnesium is not being incorporated into the structure of apatite phase during the crystallization of SrY4(SiO4)3O from non-equilibrium melt. The influence of sintering addi‐ tives on the behavior during the thermal treatment is summarized in **Table 3** [116].

[ ] ( )

44 2 3


3 23 2

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

**Fig. 23.** SEM picture and WDX analysis of hexagonal crystal of CaY4(SiO4)3O apatite phase.

**Effect on sinter-crystallization process Decreasing intensity or temperature of effect →**

**Table 3.** The influence of sintering additive on the behavior during thermal treatment [116].

Expansion before sinter-crystallization Pure >> NaF >> Talc ≈ Li2CO3 ≈ Li2B4O7 >> Na2CO3 > LiBO2 > K2CO3 Firing shrinkage (sample treated to 1600°C) LiBO2 > Li2B4O7 ≈ Pure > NaF > K2CO3 ≈ Na2CO3 > Talc > Li2CO3 Initial temperature of sinter-crystallization Pure ≈ Na2CO3 ≈ Talc > Li2B4O7 > K2CO3 ≈ NaF > LiBO2 > Li2CO3 Maximum rate of sinter-crystallization Li2CO3 > Pure > Li2B4O7 ≈ Talc ≈ NaF ≈ K2CO3 ≈ Na2CO3 > LiBO<sup>2</sup> Length of interval of sinter-crystallization LiBO2 >> Li2B4O7 ≈ NaF > Li2CO3 > pure > Talc > Na2CO3 > K2CO3

materials.

AEECO 2 Y O 3 SiO int ermediates

non equilibrium melt AEE Y SiO O CO g + +® ®

Since the formation of SrY4(SiO4)3O proceeds thorough non-equilibrium melt phase, the effect of sintering additives such as borate fluxes, fluorides and carbonates of alkaline metals as well as talc was investigated. Sintering additives facilitate the formation of melt phase and increase the length of sinter-crystallization interval. The expansion after the thermal decomposition of strontium carbonate is reduced as well. Calcinate, treated to the temperature lower than the temperatures of sinter-crystallization interval, has hydraulic activity. Therefore, it can be applied in special composite cements as an activator for latent hydraulic and pozzolanic

The important feature of this compound is the formation of colored center after the exposi‐ tion to X-ray radiation (**Fig. 24**); hence, the prepared material is an important candidate for optical applications, sensors and dosimeters.

**Fig. 24.** Semitransparent disc of SrY4(SiO3)4O sintered specimen (a) and formation of colored center on X-ray irradiated area (Cu(Ka), 40 kV and 30 mA) [116].

On the other hand, this reaction also indicates that the synthesis of individual apatite ana‐ logues (AEEY4(SiO4)3O, where AEE = Ca, Sr and Ba) and their solid solutions proceeds via similar ical intermediates formed in the temperature range, which is affected by the thermal stability of AEE carbonates that increases in the order: CaCO3, SrCO3 and BaCO3.

**Fig. 25.** SEM picture of hexagonal crystal of CaY4[SiO4]3O apatite phase.

While the synthesis of CaY4(SiO4)3O leads to well-developed hexagonal crystals (**Fig. 25**), the attempts for the preparation of BaY4(SiO4)3O phase were not successful. This synthesis leads to well-developed crystals of yttrium orthosilicate (Y2SiO5) surrounded by BaO-Y2O3-SiO2 glassy phase (**Fig. 26**).

**Fig. 26.** SEM of Y2SiO5 crystal surrounded by barium-containing glassy phase.

The investigation of this system leads to the conclusion that non-limited Ca2+ ↔ Sr2+ substitu‐ tion can be performed in the binary system of (Ca-Sr)Y4[SiO4]3O. On the contrary, the BaY4[SiO4]3O analogue of CaY4[SiO4]3O and SrY4[SiO4]3O apatite cannot be prepared; there‐ fore, the extent of Ca2+ ↔ Ba2+ and Sr2+ ↔ Ba2+ substitutions is limited to 28 ± 4 and 38 ± 4%, respectively. The field of ternary solid solutions in the AEEY4[SiO4]3O system, where AEE = Ca, Sr and Ba, is shown in **Fig. 27**.

**Fig. 27.** The miscibility in the AEE Y4(SiO4)3O system.
