**5.3 Apatite-type borates**

GeO4

x = 0.67 x = 0.67 x = 0.34 x = 0.17 x = 0

La(1) + La(2) fully occupied along this line

La(2) O(4)

**(b)**

**Fig. 10.** The unit cell view with GeO4 and NdO7 (a) and GeO4 and NdO9 down the c-axis. NdO7 and NdO9 are dotted

La9.33+x-2y/3Sry(GeO4)6O2+1.5x solid solutions at 1100°C

GeO2 SrO

**Fig. 9.** Apatite solid solutions in the LaO1.5-GeO2-SrO ternary system [24]: pure phases are indicated by filled circles

y = 2.0

y = 1.0

LaO1.5 La2GeO5

Triclinic

LaO1.5

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

GeO2 SrO

Hexagonal

and the presence of secondary phases is shown by half-filled circles.

La2Ge2O7

y = 0

**Fig. 11.** The structure of sodium lanthanum germanate NaLa9Ge6O26 [66].

**(a)**

polyhedra, and GeO4 are lined polyhedra [65].

La(1) / Na

Two high terbium content apatites Tb5Si2BO13 (*a* = 9.2569 Å, *c* = 6.8297 Å, *V* = 506.83 Å3 and *Z* = 2) and Tb4.66Si3O13 (*a* = 9.493 Å, *c* = 6.852 Å, *V* = 534.70 Å3 and *Z* = 2) were prepared by CHEN and LI [67] via spontaneous crystallization and synthesized with high purities and excellent crystallinities by the sol-gel process. Both compounds are isostructural with P63/M space group and exhibit paramagnetic behavior down to 2 K. Owing to high Tb3+ ion concentrations and good transmittance in the range from 500 to 1500 nm, Tb5Si2BO13 and Tb4.66Si3O13 may be promising magneto-optical materials in the visible-near-IR range.

Both Tb5Si2BO13 and Tb4.66Si3O13 contain two distinct sites for Tb3+ cations, which are depicted in **Fig. 13**. Tb(2) is at the 4*f* site, which is on a threefold axis and coordinated by nine oxygen ions. However, for Tb4.66Si3O13, the Tb(2) site is not fully occupied but leaves one of six Tb(2) positions randomly vacant while fully occupied in Tb5Si2BO13. In contrast, Tb(1) at the 6*h* site is fully occupied and sevenfold coordinated in both Tb5Si2BO13 and Tb4.66Si3O13. Moreover, in Tb5Si2BO13, one third of Si is disorderly occupied by B, which gives rise to extra 1/3 Tb3+ ion for the charge balance. The Tb(2)O9 polyhedron consists of Tb(2) and nine oxygens along the c-

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

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].

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 othertwothirds of BO3 groups are distributed differently.

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.

**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 emphasized [BO2] − groups and the coordinating trigonal antiprism formed by Sr3. The comparison of the arrangement of [BO2] − 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].

The structure of single crystal of strontium phosphate orthoborate metaborate (Sr10[(PO4)5.5(BO4)0.5](BO2)) that was grown from the melt by CHEN et al [70] is shown in **Fig. 14**(a). The phase crystallizes in the space group P3 with the cell parameters *a* = 9.7973 Å, *c* = 7.3056 Å, *V* = 607.19 Å3 and *Z* = 1. The crystal structure is closely related to apatite and contains linear metaborate groups, [BO2] <sup>−</sup> (point group D∞h, B-O = 1.284 Å), taking positions within the channels running along the threefold inversion axis. Strontium sites are found to be fully occupied, while [PO4] 3− tetrahedra are partially replaced by[BO4] 5− groups.

The comparison of the nearest neighbors around [BO2]<sup>−</sup> and F<sup>−</sup> located within the channels is shown in **Fig. 14**(**b,c**). F<sup>−</sup> ions (0,0,1/4) are situated on the mirror plane in the center of Srtriangle. As a result, constant F…F distances of 3.64 Å (*a*/2) are observed along [001] (**b**). In Sr10[(PO4)5.5(BO4)0.5](BO2), the incorporation of boron atoms between two O atoms draws these atoms closer (d(O-B-O) = 2.57 Å) and at the same time increases the gaps between two neighboring [BO2] <sup>−</sup> units (d(O…O) = 4.73 Å), which results in alternating O…O distances along the c-axis (**a**) [70].

By replacing Mn in YCa3(MnO)3(BO3)4 with trivalent Al and Ga, two new borates with the compositions of YCa3(MO)3(BO3)4 (M = Al, Ga) were prepared via the solid-state reaction by YU et al [71]. The phases are isostructural to gaudefroyite with the hexagonal space group P63/ M. The cell parameters of *a* = 10.38775 Å, *c* = 5.69198Å for the Al-containing compound and *a* = 10:5167 Å, *c* = 5:8146 Å forthe Ga analogue were obtained from the refinements. The structure is constituted of AlO6 or GaO6 octahedral chains interconnected by BO3 groups in the *ab* plane to form a Kagomé-type lattice15 [72],[73],[74], leaving trigonal and apatite-like tunnels. It was found that most rare-earth and Cr, Mn ions can be substituted into the Y3+ and M3+ sites, respectively, and the preference of rare-earth ions to be located in the trigonal tunnel is correlated to the sizes of the M3+ ions.
