**5.5.1 Yttrium silicates**

The structure analysis reveals that the hexagonal compound crystallizes in usual apatite space group P63/M with lattice parameters *a* = 9.3142 Å and *c* = 6.7010 Å. REE(1) atoms are connect‐ ed to nine oxygen atoms with the REE(1)-O bond distances ranging from 2.317 to 2.751 Å (mean 2.478 Å) and REE(2) atoms are connected to seven oxygen atoms with the RE(2)-O bond distances ranging from 2.223 to 2.690 Å (mean 2.393 Å). Oxygen anion O(4) in the apatite channel is located on the 63 axis and coordinated with three REE(2) cations arranged in a tricluster perpendicular to the c-axis. An isotropic displacement parameter was used for O(4), and H atom was assumed to ride on it. OH<sup>−</sup> anions are stacked in regular column in the apatite channel, and in locally ordered structure, their polar direction is flipped in neighboring

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

The formation of apatite-type phases of the composition of KNd6(SiO4)6O2 from the glass precursor (4K2O-Nd2O3-17SiO2) during the hydrothermal experiments (500°C and 825 bar) carried out at KOH molarities of 6 or greater was reported by HAILE et al [82]. High tempera‐ tures, high pressures and long times tended to favor the synthesis of this apatite-type phase over the K8Nd3Si12O32OH phase. In comparison with the potassium system, the concentra‐ tion of NaOH required forthe synthesis of NaNd6(SiO4)6O2 phase (system 4Na2O-M2O3-17SiO2, where M = Nd and Y) is very low. The formation of apatite-type phases in the 4Na2O-

Two series of strontium-lanthanum apatites, Sr10−xLax(PO4)6−x(SiO4)xF2 and Sr10−xLax(PO4)6−x(SiO4)xO with 0 ≤ *x* ≤ 6, were synthesized by BOUGHZALA et al [84] via the solid-

( ) ( )

( ) ( )

The raw meal was prepared via mixing SrCO3, La2O3, SiO2, SrF2 and (NH4)2HPO4 in required stoichiometric amounts (0 ≤ *x* ≤ 6). The mixture was ground in an agate mortar, pressed to pellets and calcined at the temperature of 900°C for 12 h under the flow of argon (Sr10−xLax(PO4)6−x(SiO4)xF2) and oxygen (Sr10−xLax(PO4)6−x(SiO4)xO). The product was ground and pressed again in order to improve its homogeneity. Next, thermal treatment was performed

+- + + ®

<sup>3</sup> ( 4 4 22 7 2 2 )<sup>2</sup> <sup>3</sup> 2 SrCO 2 NH HPO Sr P O 2 CO 3 H O 4 NH + ® +++ (11)

3 22 7 2 3

+- + + + ®

+ + (9)

+ + (10)

3 22 7 2 2 3

channel [81].

and

Y2O3-17SiO2 system was not observed [83].

state reaction in the temperature range of 1200 – 1400°C:

( ) ( )

( ) ( )

10 x x 4 6x x 4 2 2 2

Sr La PO SiO F 3 CO 3x / 2 H O - -

3 SrCO 6 x / 2 Sr P O SrF x SiO x La OH

10 x x 4 6x x 4 2 2 4 SrCO 6 x / 2 Sr P O x SiO x La OH Sr La PO SiO O 4 CO 3x / 2 H O - -

where *x* = 0, 1, 2, 4 and 6. Sr2P2O7 was synthesized by the following reaction at 900°C:

The formation ofthe phase with the composition (Y4Si3O12, Y4(SiO4)3 or 2Y2O3·3SiO2 [88]), which is stable between 1650 and 1950°C, was reported by TOROPOV and BONDAR [89] in the binary system of Y2O3-SiO2 and by TOPOROV and FEDOROV [90] in the ternary system of CaO-Y2O3-SiO2

(**Fig. 19**(**a**)). This was the first reported occurrence of the phase with the composition be‐ tween yttrium orthosilicate (Y2SiO5, oxyorthodsilicate, YSO, Y2O3:SiO2 = 1:1 [91]) and yttrium disilicate Y2Si2O7 (yttrium pyrosilicate, YPS, 1:3). The structure of this phase was described as the garnet type [88].

**Fig. 19.** Phase equilibrium in the system Ca2SiO4-Y4(SiO4)3 according to TOPOROV and FEDOROV [90] and Y2O3-SiO2 phase diagram [92].

Since then, authors have disagreed about the existence of such a phase because the attempts to make it starting with yttria and silica powders resulted in the formation of only Y2SiO5 and Y2Si2O7 [93]. This phase was not reported either by other studies of Y2O3-SiO2 system [92],[94], [95], which contains two compounds. Y2SiO5 andY2Si2O7 were found, with two (A and B) and five (*y*, α, β, γ and δ, also called *y*, B, C, D and E [96]) polymorphs, respectively. The first has a congruent melting, whereas the second has an incongruent one (**Fig. 19**(**b**)).

Nevertheless, the formation of oxyapatite phase of the composition of Y4.67□0.33(SiO4)3O (7:9) prepared via the oxidation of nitrogen apatite Y5(SiO4)3N was reported by other authors [7], [97]. The apatite-like phase Y4.67(SiO4)3O possesses hexagonal structure with the space group P63/M, *a* = 9.368 and *c* = 6.735 Å [78],[98]. The specific gravity of the phase is 4.39 g·cm−3 and the hardness on the Mohs scale is 5 – 7 [99].

Since the structure of YSO containing two different types of anions includes the [SiO4] 4− complex ion and an additional non-silicon-bonded oxygen ion (NBO), it could be written as Y2(SiO4)O. This compound also displays two structure types of monoclinic symmetry with different linking of O-Y4 tetrahedra. Low-temperature X1 phase and high-temperature X2 phase belong to the space groups of P21/C (Z = 2) and C2/C (Z = 8), respectively [91].

The samples of the composition of Y4(SiO4)3, and similar ones containing small amount of iron oxide, corresponding to an overall composition of Fe0.2Y4(SiO4)3O0.2, were produced by the mixed powder method and by the sol-gel route using yttrium nitrate (Y(NO3)3·5H2O), TEOS (tetraethylorthosilicate) and iron nitrate (Fe(NO3)3·9H2O) by PARMENTIER et al [7]. Nitrate was dissolved in ethanol/water mixture (volume ratio 7:3), the amount of the latter being control‐ led to give final Si concentration. Iron nitrate was added at this stage in calculated amounts corresponding to the final iron-doped apatite composition. The solution was stirred for a few hours and TEOS was added to give the appropriate silicon content and then the solution was placed in an oven at 60°C until the gelation occurred. The gel was dried at 80°C and calcined at 600°C for 1 h.

(**Fig. 19**(**a**)). This was the first reported occurrence of the phase with the composition be‐ tween yttrium orthosilicate (Y2SiO5, oxyorthodsilicate, YSO, Y2O3:SiO2 = 1:1 [91]) and yttrium disilicate Y2Si2O7 (yttrium pyrosilicate, YPS, 1:3). The structure of this phase was described as

Temperature [K]

**Fig. 19.** Phase equilibrium in the system Ca2SiO4-Y4(SiO4)3 according to TOPOROV and FEDOROV [90] and Y2O3-SiO2 phase

Since then, authors have disagreed about the existence of such a phase because the attempts to make it starting with yttria and silica powders resulted in the formation of only Y2SiO5 and Y2Si2O7 [93]. This phase was not reported either by other studies of Y2O3-SiO2 system [92],[94], [95], which contains two compounds. Y2SiO5 andY2Si2O7 were found, with two (A and B) and five (*y*, α, β, γ and δ, also called *y*, B, C, D and E [96]) polymorphs, respectively. The first has

Nevertheless, the formation of oxyapatite phase of the composition of Y4.67□0.33(SiO4)3O (7:9) prepared via the oxidation of nitrogen apatite Y5(SiO4)3N was reported by other authors [7], [97]. The apatite-like phase Y4.67(SiO4)3O possesses hexagonal structure with the space group P63/M, *a* = 9.368 and *c* = 6.735 Å [78],[98]. The specific gravity of the phase is 4.39 g·cm−3 and the

Since the structure of YSO containing two different types of anions includes the [SiO4]

complex ion and an additional non-silicon-bonded oxygen ion (NBO), it could be written as Y2(SiO4)O. This compound also displays two structure types of monoclinic symmetry with different linking of O-Y4 tetrahedra. Low-temperature X1 phase and high-temperature X2 phase

The samples of the composition of Y4(SiO4)3, and similar ones containing small amount of iron oxide, corresponding to an overall composition of Fe0.2Y4(SiO4)3O0.2, were produced by the mixed powder method and by the sol-gel route using yttrium nitrate (Y(NO3)3·5H2O), TEOS (tetraethylorthosilicate) and iron nitrate (Fe(NO3)3·9H2O) by PARMENTIER et al [7]. Nitrate was dissolved in ethanol/water mixture (volume ratio 7:3), the amount of the latter being control‐ led to give final Si concentration. Iron nitrate was added at this stage in calculated amounts corresponding to the final iron-doped apatite composition. The solution was stirred for a few

H = R

**a b**

Ca2SiO4 Y4(SiO4)3 Y2O3 SiO2 Mole

a congruent melting, whereas the second has an incongruent one (**Fig. 19**(**b**)).

belong to the space groups of P21/C (Z = 2) and C2/C (Z = 8), respectively [91].

Y4(SiO4)3

Liq

1959

+Y4(SiO4)3

αCa2SiO4(ss)+

2150 2800

αCa2SiO4(ss) + Liq.

Liquid

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

Fraction

40 60 80 100 0 20 40 60 80 100

Y2Si2O7

Cri=Tri

Liq.1+Liq.2

4−

Y2SiO5

δ=γ γ=β

Liquid

the garnet type [88].

diagram [92].

1650

hardness on the Mohs scale is 5 – 7 [99].

1850 1950

2050

Temperature [°C]

0 20

1750 1750±

αCa2SiO4(ss)

2130

20

Powders prepared by the two routes were uniaxially pressed into pellets and treated to temperatures up to 1650°C in air in a Pt crucible, or for the heat treatments at 1700°C, carbon element furnace was used, and the samples were heated in a BN-lined crucible in nitrogen atmosphere. Iron appears to have two roles depending on the temperature; it stabilizes the apatite phase at high temperatures when produced by the sol-gel route and catalyzes the decomposition of sol-gel-derived apatite at low temperatures [7].

A new phase of yttrium magnesium silicate having the apatite structure was prepared by SUWA et al [100] at 1500°C in air. Its chemical composition can vary from (Y4Mg)Si3O13 to (Y4.33Mg1.13)Si3.34O13. The hexagonal unit cell dimensions *a*0 and *c*0 of (Y4Mg)Si3O13 are 9.298 ± 0.002A and 6.635 ± 0.001A, respectively, and its axial ratio c/a is 0.714. It is optically uniaxial negative with *ε* = 1.810 ± 0.005, *ω* = 1.820 ± 0.005 and *ω* – *ε* = 0.010. The cleavages parallel and perpendicular to the c-axis were recognized. The formation of the apatite-type phase of the composition of NaY9Si6O26 in the ternary system Na2O-Y2O3-SiO2 was also reported by LEE et al [101].

**Fig. 20.** Calculated liquidus surface of the Y2O3-Al2O3-SiO2 system: three-phase equilibria with liquid phase (thick lines), liquidus surfaces for various solids (labeled area) and isothermal section (dotted lines, temperature in hundreds °C) [92].

The phase diagram of Al2O3-SiO2-La2O3 system (**Fig. 20**) can be compared with the Y2O3-Al2O3- SiO2 ternary diagram examined by BONDAR and GALAKHOV in 1964 [102]. The latter represents 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 stability of the binary compositions is substantially altered [57].

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 [57],[92].
