**5.2 Apatite-type lanthanium germanates**

Lanthanum germanate and silicate apatite-based materials, both undoped and with partial substitution of, for example, Al, B instead of Si and Sr in place of La, are promising oxide ion conductors with potential applications as high-temperature solid electrolytes. Considerable uncertainties remain over the stoichiometry, the defect structure and the conductivity variations within various apatite systems, partly caused by the fact that the La:(Ge, Si) ratio is variable, giving rise to the solid solutions in the undoped systems as well as to the solid solutions formed by partial replacement of La and/or (Si, Ge) together with, depending on the solid solution mechanism, variations in oxygen content [24].

The preparation of single crystal of apatite-type lanthanium germanate of the composition of La9.33Ge6O26 was reported by NAKAYAMA and SAKAMOTO [61]. The mixtures of La2O3 and GeO2 were well mixed in ethanol under an atomic ratio of La:Ge = 9.33:6 using a ball mill. The mixture was dried and then calcined in air at 1000°C for 2 h. The resulting La9.33Ge6O26 powders were further ball milled into finer powders. After the pre-sintering, the closely packed La9.33Ge6O26 powders were heated at 1300°C for 2 h in air, and the surface of the specimen was mirror polished. The polished surface of polycrystalline La9.33Ge6O26 ceramics was then bonded to a <001> face of La9.33Ge6O26 seed crystal prepared by the CZOCHRALSKY (**Section 4.2**) method. On heating of the bonded sample at 1525 – 1550°C, continuous grain growth of polycrystalline La9.33Ge6O26 occurred and the single crystal was gradually grown from the seed crystal into the polycrystalline region [61].

Apatite-type lanthanium germanate possesses hexagonal structure with the space group P63/ M and the lattice parameters: *a* = 9.9256 and *c* = 7.2900 Å, *V* = 621.97 Å3 and *Z* = 2. The calculat‐ ed density of the phase is 2.148 g·cm−3. Similar to apatite-type lanthanium silicate (La9.33Si6O26) described above, the structure of La9.33Ge6O26 (**Fig. 8**) contains two different sites for atoms of La. The La(1) and La(2) sites are located at 4*f* and 6*h*, respectively. While the La9.33Ge6O26 single crystal showed little anisotropy in conductivity, the conductivity of La9.33Si6O26 single crystal gave 100 times higher value parallel to the c-axis than that perpendicular to the c-axis at each temperature (**Section 5.1**) [61].

**Fig. 8.** Hexagonal structure proposed for the apatite-type phase of La9.33X6O26, where X = Si and Ge [61].

The selective doping of La9.33+x(GeO4)6O2+3x/2 with Y leads to the stabilization of hexagonal lattice, even at high oxygen contents. Furthermore, this has the effect of enhancing the lowtemperature conductivities [62]. Depending on the composition, the cell can be either hexagonal or triclinic, with the evidence of reduced low-temperature conductivities for the latter, attributed to increased defect trapping in this lower symmetry cell. In summary, it was shown that the series La8Y2(GeO4)6−x(GaO4)xO3−x/2 can be prepared for 0 ≤ *x* ≤ 2 with all samples showing the hexagonal symmetry, compared to the series without Y co-doping, La10(GeO4)6−x(GaO4)xO3−x/2, for which all compositions display the triclinic symmetry [24],[62], [63].

**5.2 Apatite-type lanthanium germanates**

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

solid solution mechanism, variations in oxygen content [24].

polycrystalline region [61].

temperature (**Section 5.1**) [61].

Lanthanum germanate and silicate apatite-based materials, both undoped and with partial substitution of, for example, Al, B instead of Si and Sr in place of La, are promising oxide ion conductors with potential applications as high-temperature solid electrolytes. Considerable uncertainties remain over the stoichiometry, the defect structure and the conductivity variations within various apatite systems, partly caused by the fact that the La:(Ge, Si) ratio is variable, giving rise to the solid solutions in the undoped systems as well as to the solid solutions formed by partial replacement of La and/or (Si, Ge) together with, depending on the

The preparation of single crystal of apatite-type lanthanium germanate of the composition of La9.33Ge6O26 was reported by NAKAYAMA and SAKAMOTO [61]. The mixtures of La2O3 and GeO2 were well mixed in ethanol under an atomic ratio of La:Ge = 9.33:6 using a ball mill. The mixture was dried and then calcined in air at 1000°C for 2 h. The resulting La9.33Ge6O26 powders were further ball milled into finer powders. After the pre-sintering, the closely packed La9.33Ge6O26 powders were heated at 1300°C for 2 h in air, and the surface of the specimen was mirror polished. The polished surface of polycrystalline La9.33Ge6O26 ceramics was then bonded to a <001> face of La9.33Ge6O26 seed crystal prepared by the CZOCHRALSKY (**Section 4.2**) method. On heating of the bonded sample at 1525 – 1550°C, continuous grain growth of polycrystalline La9.33Ge6O26 occurred and the single crystal was gradually grown from the seed crystal into the

Apatite-type lanthanium germanate possesses hexagonal structure with the space group P63/

ed density of the phase is 2.148 g·cm−3. Similar to apatite-type lanthanium silicate (La9.33Si6O26) described above, the structure of La9.33Ge6O26 (**Fig. 8**) contains two different sites for atoms of La. The La(1) and La(2) sites are located at 4*f* and 6*h*, respectively. While the La9.33Ge6O26 single crystal showed little anisotropy in conductivity, the conductivity of La9.33Si6O26 single crystal gave 100 times higher value parallel to the c-axis than that perpendicular to the c-axis at each

*6h* site (La) 2*a* site (O)

4*f* site (La)

XO4 (X=Si,Ge)

and *Z* = 2. The calculat‐

M and the lattice parameters: *a* = 9.9256 and *c* = 7.2900 Å, *V* = 621.97 Å3

<sup>a</sup> <sup>b</sup>

**Fig. 8.** Hexagonal structure proposed for the apatite-type phase of La9.33X6O26, where X = Si and Ge [61].

The effect of Ga doping of the oxygen stoichiometric series containing the cation vacancies, La7.33+y/3Y2(GeO4)6−y(GaO4)yO2 (0 ≤ *y* ≤ 2), single-phase samples was obtained for *y* ≥ 1.0, with small impurities observed at lower Ga contents. The conductivities were shown to increase with increasing cation vacancy content, reaching the values of ≈0.02 S·cm−1 at 800°C, which are similar to the oxygen excess series. These results are in agreement with previous reports on the apatite systems, which showed that the oxide ion conductivity was maximized in sam‐ ples containing the oxygen excess and/or the cation vacancies [24], [62],[63].

The series of apatite-type silicates/germanates of the composition of La8+xSr2−xSi6O26+x/2 (0 ≤ *x* ≤ 1) and La8+xSr2−xGe6O26+x/2 (0 ≤ *x* ≤ 2) were prepared from high-purity La2O3, SrCO3, SiO2 and GeO2 by ORERA et al [64] via the thermal treatment of these components mixed in the stoichio‐ metric ratio.

The extent of, and the structural changes within, the apatite domain in the LaO1.5-GeO2-SrO ternary system at 1100°C was studied and the single-phase samples were obtained for La9.33+x −2y/3Sry(GeO4)6O2+1.5x with *x* = 0.17 and 0.34. The hexagonal to triclinic transition is clearly associated with increasing oxygen content rather than with filling the La sites by the addi‐ tion/substitution of Sr into the structure. The limits of undoped solid solution are ~0.17 ≤ *x* ≤0.5 at 1100°C [24].

The hydrothermal synthesis of apatite-type compound NaRE9(GeO4)6O2 (RE = Nd, Pr) with the hexagonal structure of the space group of P63/M was described by EMIRDAG-EANES et al [65]. The structure is composed of REO7 and REO9 polyhedra as well as GeO4 tetrahedra (**Fig. 10**). The unit cell dimensions are: *a* = 9.782(1) Å, *c* = 7.083(1)Å (*T* = 293 K) and *V* = 587.0(2)Å3 for REE = Nd and *a* = 9.802(1) Å, *c* = 7.116(1)Å (*T* = 293 K) and *V* = 592.1(2)Å3 for REE = Pr.

The high-temperature flux method for the preparation of single crystal of hexagonal Na‐ La9Ge6O26 apatite-type germanate (space group P63/M, *a* = 9.883, *c* = 7.267 Å and *Z* = 1) was used by TAKAHASHI et al [66]. The crystal structure (**Fig. 11**) was found to be similar to that of silicate oxyapatite NaY9Si6O26. The 4*f* cation sites are occupied disorderedly by La and Na. On the other hand, the 6*h* cation sites are occupied by La only. This compound constitutes a new member of the oxyapatite-type structure family with the composition given by general formula: Ax Ln10−xB6O24O3−x.

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

**Fig. 9.** Apatite solid solutions in the LaO1.5-GeO2-SrO ternary system [24]: pure phases are indicated by filled circles and the presence of secondary phases is shown by half-filled circles.

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

**Fig. 10.** The unit cell view with GeO4 and NdO7 (a) and GeO4 and NdO9 down the c-axis. NdO7 and NdO9 are dotted polyhedra, and GeO4 are lined polyhedra [65].

**Fig. 12.** Coordination environment of La atoms [66].

The coordination environments of La atoms by O atoms are shown in **Fig. 12**. La(1) atom on the 4*f* site is coordinated by nine O atoms. It is linked to three O(1) atoms in a distance of about 0.2479 nm, to three O(2) atoms in a distance of about 0.2561 nm and to three O(3) atoms in a distance of about 0.2924 nm. Because the distance between La(1) and O(3) is relatively large, La(1) atom can be also regarded to be in sixfold coordination, the environment of which is fairly distorted from an ideal octahedron. On the other hand, La(2) atom, which occupies the 6h position, is coordinated by seven O atoms, that is, O(1), O(2), O(4) and O(3). The distances ofthose two types of bonds between La(2) and O(3) atoms are 0.2618 nm×2 and 0.2431 nm×2, respectively. Those between La(2) and O(1), O(2) and O(4) are 0.2732, 0.2521 and 0.23281 nm, respectively [66].
