**5.4 Other apatite-type REE silicates**

axis. Six Tb(1) comprise a sixfold channel parallel to the c-axis. It is worth to note that the channel is considered to play an extremely important role in oxide ion conductivity [67].

**Fig. 13.** Unit cell of Tb5Si2BO13 (a) and coordination environment of kinds of Tb cations (b) [67].

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

The structure and optical properties of noncentrosymmetric borate RbSr4(BO3)3 (RSBO) was described by XIA and LI [68]. RSBO can be viewed as a derivative of the apatite-like structure. Based on the anionic group approximation, the optical properties of the compound are compared to those of the structure-related apatite-like compounds with the formula "A5(TOn)3X". When the structures of all apatite-like crystals are presented in orthorhombic unit cell, the arrangements of planar anionic BO3 groups are all similar to one-third of the BO3 groups aligned perfectly parallel at corner- and face-centered locations, whereas the othertwo-

Europium borate fluoride, Eu5(BO3)3F with an apatite-type structure, was synthesized by KAZMIERCZAK and HÖPPE [69] as single-phase crystalline powder starting from europium oxide, europium fluoride and boron oxide at 1370 K. Eu5(BO3)3F crystallizes in the space group PNMA.

**(1)**

**(c)**

*c* = 7.30 A

Sr

**Fig. 14.** The crystal structure of Sr10[(PO4)5.5(BO4)0.5](BO2) (1): (a) the projection along [001] showing the channels formed by Sr3 (gray triangles) and the positions of the XO4 tetrahedra (gray; Z = 11/12 P + 1/12B) and (b) the side view with

and F- ions within the Sr channels of Sr10(PO4)5.5(BO4)0.5(BO2) (c(1)) and Sr10(PO4)6F2 (c(2)) [70].

Sr Sr Sr

O B

O

O

Sr

O B

groups and the coordinating trigonal antiprism formed by Sr3. The comparison of the arrangement

Sr Sr

4.73 A

2.57 A

Sr Sr [001]

Sr

Sr Sr

**(2)** *<sup>P</sup>*63*Im*

F

F

F

Sr Sr <sup>F</sup>

Sr Sr Sr

*c* = 7.28 A

Sr

3.64 A

3.64 A

thirds of BO3 groups are distributed differently.

Sr1

Sr2

[BO2] –

c

emphasized [BO2]

of [BO2] − a

a

−

**(b)**

Sr3 [BO2] –

*<sup>P</sup>*<sup>3</sup> **(a)** –

ZO4

Hexagonal apatite-type phase of the composition of Pr9K(SiO4)6O2 (space group P63/M, *a* = 9.6466 Å and *c* = 1136 Å, *V* = 573.28 Å3 , *ρ*calc = 5.48 g·cm−3 and *Z* = 1) was synthesized by WERNER AND KUBEL [75] in a potassium fluoride flux. Potassium fills one (4*f*) of two metal positions present in the structure (**Fig. 15**) with the occupancy factor of 25%. The remaining positions of this site (Pr2/K2) are occupied by praseodymium.

<sup>15</sup> The Kagomé lattice (**d**) is one of the most interesting lattices in 2D, especially in materials in which the Kagome lattice is built from magnetic ions. Each of its vertices touches a triangle, hexagon, triangle and hexagon (the planes of cornersharing equilateral triangles). The vertices correspond to the edges of the hexagonal (honeycomb) lattice (**c**), which in turn is the dual of triangular lattice (can be derived from triangular lattice by periodical removal of ¼ sites) (**b**). Since it has the same coordination number (*z* = 4), the Kagomé lattice is also related to the square lattice (**a**) [72], [73]. Numerous Kagomé compounds built from stacked Kagomé layers were found in Alunite (Jarosite, KFe3+3(SO4)2(OH)6, (**e**)) family of minerals [74].

**Fig. 15.** The perspective view of Pr9K(SiO4)6O2 along the c-axis: regular planar coordination of the channel oxygen O(4) and the SiO4 tetrahedra are marked in gray, and the unit cell is outlined [75].

Oxygen from the silicate groups forms a coordination polyhedron (ninefold) in the shape of a distorted threefold capped trigonal prism. These face sharing [(Pr2/K2)O9]-polyhedra build up chains, which are interconnected via the SiO4 groups. The resulting channel framework accommodates sevenfold oxygen-coordinated praseodymium (Pr1), attached to the inside of the tubes that are aligned parallel to the c-axis. Oxide ions O4, located on the longitudinal axis of the channels, exhibit anomalously high atomic displacement parameters along the cdirection [75].

Single crystals of apatite-type Nd9.33(SiO4)6O2, Pr9.33(SiO4)6O2 and Sm9.33(SiO4)6O2 were descri‐ bed in **Section 4.2.2**. The structure of samarium orthosilicate oxyapatite (Sm5(SiO4)3O, **Fig. 16**) was resolved by MORGAN et al [76]. The phase crystallizes in hexagonal system with the space group P63/M and the cell parameters: *a* = 9.4959 Å, *c* = 7.0361 Å, *c*:*a* = 0.7410 and *V* = 549.46 Å3 . The preparation and the structure of single crystal of strontium tetrapraseodymium tris(silicate) oxide (SrPr4(SiO4)3O), which was grown by the self-flux method using SrCl2, was described by SAKAKURA et al [77].

**Fig. 16.** The structure of Sm5(SiO4)3O viewed along the c-axis [76].

The M(2) sites are almost exclusively occupied by praseodymium. The complete series of apatite-like compounds REE9.33□0.67[SiO4]6O2, LiREE9[SiO4]6O2 and NaREE9[SiO4]6O2 were synthesized by FELSCHE [78] with REE: La → Lu. Apatite-type neodymium silicates doped with various cations at the Si site, Nd10Si5BO27−δ (B = Mg, Al, Fe, Si), were synthesized by XIANG et al [79] via the solid-state reaction.

The crystal growth and the structure of three new neodymium-containing silicates, Na0.50Nd4.50(SiO4)3O, Na0.63Nd4.37(SiO4)3O0.74F0.26 and Na4.74Nd4.26(O0.52F0.48)[SiO4]4, prepared using the eutectic mixture of KF/NaF were investigated by LATSHAW et al [80]. Na0.50Nd4.50(SiO4)3O and Na0.63Nd4.37(SiO4)3O0.74F0.26 adopt the apatite structure and crystallize in hexagonal space group P63/M, while Na4.74Nd4.26(O0.52F0.48)[SiO4]4 crystallizes in tetragonal space group I-4 and exhibits rare-earth mixing on the sodium site. The unit cell parameters of the crystals are:

**1.** Na0.50Nd4.50(SiO4)3O: *a* = 9.5400 Å and *c* = 7.033 Å;

**Fig. 15.** The perspective view of Pr9K(SiO4)6O2 along the c-axis: regular planar coordination of the channel oxygen O(4)

Oxygen from the silicate groups forms a coordination polyhedron (ninefold) in the shape of a distorted threefold capped trigonal prism. These face sharing [(Pr2/K2)O9]-polyhedra build up chains, which are interconnected via the SiO4 groups. The resulting channel framework accommodates sevenfold oxygen-coordinated praseodymium (Pr1), attached to the inside of the tubes that are aligned parallel to the c-axis. Oxide ions O4, located on the longitudinal axis of the channels, exhibit anomalously high atomic displacement parameters along the c-

Single crystals of apatite-type Nd9.33(SiO4)6O2, Pr9.33(SiO4)6O2 and Sm9.33(SiO4)6O2 were descri‐ bed in **Section 4.2.2**. The structure of samarium orthosilicate oxyapatite (Sm5(SiO4)3O, **Fig. 16**) was resolved by MORGAN et al [76]. The phase crystallizes in hexagonal system with the space group P63/M and the cell parameters: *a* = 9.4959 Å, *c* = 7.0361 Å, *c*:*a* = 0.7410 and *V* = 549.46 Å3

The preparation and the structure of single crystal of strontium tetrapraseodymium tris(silicate) oxide (SrPr4(SiO4)3O), which was grown by the self-flux method using SrCl2, was

.

and the SiO4 tetrahedra are marked in gray, and the unit cell is outlined [75].

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

direction [75].

described by SAKAKURA et al [77].

**Fig. 16.** The structure of Sm5(SiO4)3O viewed along the c-axis [76].


Double REE silicate Gd4.33Ho4.33(SiO4)6(OH)2 with the hydroxylapatite structure was synthe‐ sized by WANG et al [81] using the piston-cylinder high-pressure apparatus at the pressure of 2.0 GPa and the temperature of 1450°C. Since they have nearly identical chemical character, two REE cations (Ho and Gd) are distributed randomly among the M(1) and M(2) sites, and the charge balance is maintained by the cation vacancies in M(1). The presence of two different REE cations in the same compound might promote better understanding of the cooperative effects of ions under the solid-state conditions (**Fig. 17**).

**Fig. 17.** Bond distances (Å) and anisotropic displacement in Gd4.33Ho4.33(SiO4)6(OH)2: note that the exaggerated aniso‐ tropic displacement of O(3) is attributable to the high proportion of vacancies in REE(1) and near-equatorial distribu‐ tion of strong bonds to Si and REE(2) [81].

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 channel [81].

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

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

$$\begin{aligned} \text{13 SrCO}\_3 + \text{(6-x)}/\text{2 Sr}\_2\text{P}\_2\text{O}\_7 + \text{SrF}\_2 + \text{x SiO}\_2 + \text{x La} \text{(OH)}\_3 &\rightarrow\\ \text{Sr}\_{\text{l0-x}}\text{La}\_{\text{x}}\text{(PO}\_4\text{)}\_{6-\text{x}}\text{(SiO}\_4\text{)}\_{\text{x}}\text{F}\_2 + \text{3 CO}\_2 + 3\text{x}/\text{2 H}\_2\text{O} \end{aligned} \tag{9}$$

and

$$\begin{aligned} \text{4 SrCO}\_3 + \text{(6-x)} / \text{2 Sr}\_2\text{P}\_2\text{O}\_7 + \text{x SiO}\_2 + \text{x La} \text{(OH)}\_3 &\rightarrow\\ \text{Sr}\_{\text{l0-x}}\text{La}\_x\text{(PO}\_4\text{)}\_{6-x}\text{(SiO}\_4\text{)}\_x\text{O} + \text{4 CO}\_2 + \text{3x} / \text{2 H}\_2\text{O} \end{aligned} \tag{10}$$

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

$$2\text{ SrCO}\_3 + 2\text{ (NH}\_4\text{)}\_2\text{HPO}\_4 \rightarrow \text{Sr}\_2\text{PO}\_7 + 2\text{ CO}\_2 + 3\text{ H}\_2\text{O} + 4\text{ NH}\_3\tag{11}$$

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 at the temperature of 1200 and 1400°C (depending on the content of SiO2) for 12 h. The samples were heated and cooled with the rate of 10°C·min−1. The incorporation of La3+ and SiO4 4− ions into the apatite structures, i.e. the substitution of the pair La3+ and SiO4 4− for Sr2+ and PO4 3−, induced an increase of parameter *a* and decrease of parameter *c* (**Fig. 18**) [84].

The formation of nanocrystalline Ce-Yb mixed silicate-type oxyapatite of the composition of YbyCe9.33−y(SiO4)6O2 via the solid-state synthesis was described by MAŁECKA and KĘPIŃSKI [85]. The phase was identified as an intermediate formed during the synthesis of Ce-Yb silicates.

**Fig. 18.** The variation of *a* and *c* parameters of Sr10−xLax(PO4)6−x(SiO4)xF2 and Sr10−xLax(PO4)6−x(SiO4)xO phases with the value of *x* [84].

Different compositions of apatite-type La10Si6−xWxO27+δ ceramics were prepared successfully by XIANG et al [86] via the high-temperature solid-state reaction route. Doping with W6+ is beneficial to the removal of La2SiO5 impurity phase. When the doping content of W6+ is more than 0.1, the rod-like grains of La10Si6−xWxO27+δ ceramics are replaced gradually by equiaxed apatite-type grains, and randomly shaped convex La6W2O15 particles appear at the grain boundaries. While doping with Nb5+ leads to the hexagonal-phase La9.5Ge5.5Nb0.5O26.5, the addition of MO6+ leads to the compound La9.5Ge5.5Mo0.5O26.75 with triclinic symmetry [87].
