**5.5.3. N-apatite**

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

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

Sr-Ba (Binary series III.)

0.50

0.75

(Ca*x*Sr*y*Ba*z*)Y4[SiO4]3O, Sr

0.25

0.75

Ca, Sr and Ba, is shown in **Fig. 27**.

where *x*+*y*+*z* = 1

1.00

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

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 =

1.00

0.00

0.75

Field of ternary

0.50

solid solutions

Ba Ca Ba-Ca (Binary series II.)

0.00 0.25 0.50 0.75

Ca-Sr (Binary series I.)

0.25

0.00

1.00

The main secondary phases in Jänecke prism17 [117] for Si3N4-Al2O3-SiO2-Y2O3-YN-AlN system18 are shown in **Fig. 28**. The formal exchange of oxygen by nitrogen leads to the compounds of N-apatite (Y10(SiO4)6N2, H-phase19), N-melilite20 [118] (Y2Si3O3N4, M-phase), N-wollastonite (YSiON2, K-phase) and N-woehlerite (Y4Si2O7N2, J-phase). The latter one forms a complete solid solution with Y4Al2O9 (YAM) of the composition of Y4Si2−xAlxO7+xN2−x (Jss-phase) [119],[120], [121],[122],[123],[124].

**Fig. 28.** Representation of the Si3N4-Al2O3-SiO2-Y2O3-YN-AlN phase equilibrium.

Y10(SiO4)6N2 (N-apatite, H-phase, (Y,Si,□)10[Si(O,N)4]6(O,N,□)<sup>2</sup> [125], silicon-yttrium oxyni‐ tride) was first identified by RAE et al [126] as a compound with the compositional mixture of 10Y2O3·9SiO2·Si3N4 that was stable up to 1750°C. There were other suggested composi‐ tions, such as Y10Si7O23N4 [127]. Later work by GAUCKLER et al [128] established N-apatite as a stoichiometric compound with the formula unit of Y10[SiO4]6N2 and the apatite structure (space group P63/M). The lattice constants of the hexagonal cell were reported to be *a* = 9.638

<sup>17</sup> Jänecke prism is used to visualize the phase relationships among α-sialon, β-sialon and other phases in the M-Si-Al-O-N system. α- and β-sialons are isostructural with α- and β-Si3N4, respectively. The substitution of Al-O for Si-N in β-Si3N4 yields β-sialon with general formula: Si6−xAlzOzN8−z (0 < *z* < 4.2). The structure is built up by Si and Al tetrahedra coordinated with oxygen and nitrogen. The unit cell contains two Si3N4 units. α-sialons are solid solutions based on the α-Si3N4 structure, with the general formula: Mp+xSi12−(m+n)Al(m+n)OnN16−n, where **M** is metal ion such as Li, Ca, Ba, Y and RE with a valence of **p+** and index **m** = **px** [117].

<sup>18</sup> Yttria is often used additive to improve the sintering behavior of Si3N4 [124].

<sup>19</sup> In dependence on the system composition, the general composition of H-phase (N-apatite) can be written as (M,REE)10(SiO4)6N2. The specification of cations then leads to the names such as Mg-Nd-N-apatite [123].

<sup>20</sup> The melilite-type structure (tetragonal mineral melilite ((Ca,Na)2(Al,Mg,Fe2+)(Si,Al)2O7)) is sorosilicate from the group of melilite, first described (Capo di Bove, near Rome, Italy) in 1976 and named from the Greek words *meli* "honey" and *lithos* "stone"). Y2Si3O3N4 was described by FANG et al [118]. N atoms fully occupy the bridging site (2*c*) and O atoms fully occupy the terminal site (4*e*) with 2 O and 6 N atoms at the bridging 8*f* site. The preferential distribution of O and N atoms at the 8*f* site results in two different local coordinations of Y and three different types of Si atoms.

Å and *c* = 6.355 Å. The electronic structure and bonding of the complex ceramic crystal Y10(SiO4)6N2 was studied by CHING et al [121]. This crystal is an insulator with direct band gap of 1.3 eV. It has some unique properties related to one-dimensional chain structure in the *c*direction and planar N-Y bonding in the *x*-*y* plane.

The ternary phase diagrams of the Si3N4-Y2O3-SiO2 [123] and Si3N4-La2O3-SiO2 systems [129] are shown in **Fig. 29**(**a**) and (**b**). The apatite phase is able to form various solid solutions that may influence the development of strength in silicon nitride densified by yttria [130].

**Fig. 29.** Phase relationships in the systems Si3N4-SiO2-Y2O3 [123] (a) and Si3N4-SiO2-La2O3 at 1700°C and 1550°C (dashed lines) [129] (b).

The hexagonal lanthanum N-apatite phase of the composition of La5(SiO4)3N (isostructural with apatite) can be prepared from the mixture of La2O3 and Si powder sintered at tempera‐ tures in the range from 900 to 1200°C under the flow of nitrogen. The melting temperature of this phase was determined to be ~1600°C. It was observed that continuous heating and addition of Pd into the reaction mixture favored the formation of La5(SiO4)3N. Prolonged heating of this compound yields La4.67(SiO4)3O [129],[131],[132],[133],[134],[135]. The absorption bands observed in infrared spectrum of lanthanum oxynitrides are introduced in **Table 4**.



**Table 4.** The absorption bands observed in infrared spectrum of lanthanum oxynitrides [131].

Å and *c* = 6.355 Å. The electronic structure and bonding of the complex ceramic crystal Y10(SiO4)6N2 was studied by CHING et al [121]. This crystal is an insulator with direct band gap of 1.3 eV. It has some unique properties related to one-dimensional chain structure in the *c*-

The ternary phase diagrams of the Si3N4-Y2O3-SiO2 [123] and Si3N4-La2O3-SiO2 systems [129] are shown in **Fig. 29**(**a**) and (**b**). The apatite phase is able to form various solid solutions that

**Fig. 29.** Phase relationships in the systems Si3N4-SiO2-Y2O3 [123] (a) and Si3N4-SiO2-La2O3 at 1700°C and 1550°C (dashed

The hexagonal lanthanum N-apatite phase of the composition of La5(SiO4)3N (isostructural with apatite) can be prepared from the mixture of La2O3 and Si powder sintered at tempera‐ tures in the range from 900 to 1200°C under the flow of nitrogen. The melting temperature of this phase was determined to be ~1600°C. It was observed that continuous heating and addition of Pd into the reaction mixture favored the formation of La5(SiO4)3N. Prolonged heating of this compound yields La4.67(SiO4)3O [129],[131],[132],[133],[134],[135]. The absorption bands

observed in infrared spectrum of lanthanum oxynitrides are introduced in **Table 4**.

**Wavenumber [cm−1] Mode Frequency [cm−1] [cm−1]** *δ* (Si-O) (A1) 730 *ν* (Si-N) Si-O2 800 SiO2 *δ* (Si-O) (B2) 840 Si-N Si-O-Si (A1) 872 SiO4 (*ν*1) SiO4 (*ν*4) 909 *ν* (Si-N) vs 376 – 385 *δ* (Si-N) (sh) 930 Si-O-Nx SiO4 (*ν*4) 940 *ν* Si-N vs

432 *δ* (Si-N) (sh) 960 448 Si-N s or O-Si-O bend. 905

may influence the development of strength in silicon nitride densified by yttria [130].

direction and planar N-Y bonding in the *x*-*y* plane.

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

lines) [129] (b).

MITOMO et al [129] used the mixture of Si3N4 and La2O3 powder heated in a 10 mm diameter carbon die using a hot-pressing apparatus. In some compositions and at temperatures above 1600°C, liquid phases were squeezed out of the die by applied pressure resulting in a change in the overall composition. The compacted powder was therefore heated at the pressure of 20 MPa up to 1400°C and the temperature was then raised to 1700°C without the pressure. The specimen was kept at 1700°C for 1 h and then allowed to cool.

The sintering temperature of Si3N4 with La2O3 additions is 1700 to 1800°C. Heating of pow‐ der mixture of various Si3N4/L2O3 ratios at 1700°C results in the formation of 2Si3N4·La2O3, La5(SiO4)3N21 and β-Si3N4 or a glass. The reactions occurring during heating were determined as follows [129]:

$$\begin{aligned} \text{Si}\_3\text{N}\_4 + \text{La}\_2\text{O}\_3 &\xrightarrow{1200-1250 \text{°C}} \text{Si}\_3\text{N}\_4 + \left( \text{La}\_4\text{Si}\_2\text{O}\_7\text{N}\_2 + \text{LaSiO}\_2\text{N} \right) \\ &\xrightarrow{1400-1500 \text{°C}} \text{LaSiO}\_2\text{N} + \text{Si}\_3\text{N}\_4 \\ &\xrightarrow{1650-1750 \text{°C}} 2\text{Si}\_3\text{N}\_4 \cdot \text{La}\_2\text{O}\_3 + \text{liquid} \\ &\xrightarrow{200^\circ \text{C}} 2\text{Si}\_3\text{N}\_4 \cdot \text{La}\_2\text{O}\_3 + \text{La}\_5(\text{SiO}\_4)\_3\text{N} + \text{glass} \end{aligned} \tag{14}$$

The results of SAKAI et al [136] indicate that N-apatite and N-diopside containing grain boundary phase may improve the oxidation resistance of silicon nitride. Since the oxidation of Si3N4 leads to the formation of protective SiO2 layer on the surface:

$$\text{Si}\_3\text{N}\_4 + \text{3 O}\_2 \rightarrow \text{3 SiO}\_2 + \text{2 N}\_2\tag{15}$$

<sup>21</sup> The presence of La5(SiO4)3N is inevitable in the production of high-density materials by liquid-phase sintering; therefore, the amount of La5(SiO4)3N and glassy phase must be minimized to obtain materials with good high-temperature strengths [129].

the silicon nitride shows excellent oxidation resistance. Formed SiO2 then reacts with the grain boundary constituents to form silicates:

$$\text{NiO}\_2 + \text{MeO}\_x \rightarrow \text{MeSiO}\_{2\text{ }\text{x}} \tag{16}$$

but in the case of MgO, the formed layer did not act as protection [136]. Y4.67(SiO4)3O apatite (britholite phase2 ) is formed as the oxidation product of silicon yttrium oxynitride (H-phase) in the temperature range from 700 to 1400°C [137],[138].
