**6.1.10. Actinides-bearing apatites**

Thorium and uranium (actinides [45],[46]<sup>3</sup> )-bearing apatites were synthesized by LUO et al [47] from doped phosphate-halide-rich melts. The structure refinements (**Fig. 7**) of U-doped fluorapatites indicate that U substitutes almost exclusively into Ca(2) site with the site occupancy ratios UCa(2)/UCa(1), which range from 5.00 to 9.33. Similarly, the structure refinements of Th-doped fluorapatites indicate that Th substitutes dominantly into Ca(2) site with ThCa(2)/ThCa(1) values, which range from 4.33 to 8.67.

<sup>3</sup> The actinides occupy the second row of the f-block in the periodic table. The actinide group or actinoids (An) include 14 elements with atomic numbers from 90 (Th) to 103 (Lr) [43]. The elements with atomic numbers greater than 92 (U) are termed as transuranes. The elements with atomic numbers greater than 100 are named as the super-heavy elements (SHE). There is also the concept of the periodic table developed by G.T. SEABORG predicting a new inner transition series of 32 elements (from 122 to 153 element), called the superactinite series [45],[46]. Only actinium, thorium, uranium and (in trace quantities) protactinium and plutonium are primordial, while the elements from neptunium onwards are present on Earth solely through artificial generation [46].

HUGHES et al [38], reasoning that La → Pr should preferentially substitute into Ca(2), whereas

The isomorphic substitutions of neodymium for strontium in the structure of synthetic Sr5(VO4)3OH apatite structure type (P63/M) were reported by GET'MAN et al [44]. The synthe‐ sis of apatite specimen was performed via the solution thermolysis on the assumption of the

3 2 23 4 3

where x = 0, 0.02, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18 and 0.20. The substitution scheme can be

The solutions for the thermolysis were prepared by dissolving Sr(NO3)2 in water; Nd2O3 was dissolved in water with nitric acid added; NH4VO<sup>3</sup> was dissolved in water with hydrogen peroxide added. Dry residues after concentrating the solutions were pestled in an agate mortar and calcined with the temperature steadily raised from 600 to 800°C and intermittent grind‐

from doped phosphate-halide-rich melts. The structure refinements (**Fig. 7**) of U-doped fluorapatites indicate that U substitutes almost exclusively into Ca(2) site with the site occupancy ratios UCa(2)/UCa(1), which range from 5.00 to 9.33. Similarly, the structure refinements of Th-doped fluorapatites indicate that Th substitutes dominantly into Ca(2) site

<sup>3</sup> The actinides occupy the second row of the f-block in the periodic table. The actinide group or actinoids (An) include 14 elements with atomic numbers from 90 (Th) to 103 (Lr) [43]. The elements with atomic numbers greater than 92 (U) are termed as transuranes. The elements with atomic numbers greater than 100 are named as the super-heavy elements (SHE). There is also the concept of the periodic table developed by G.T. SEABORG predicting a new inner transition series of 32 elements (from 122 to 153 element), called the superactinite series [45],[46]. Only actinium, thorium, uranium and (in trace quantities) protactinium and plutonium are primordial, while the elements from neptunium onwards are present

+¼

<sup>2</sup> 3 2 Sr OH Nd O + - +- + ÜÞ + (9)

)-bearing apatites were synthesized by LUO et al [47]

(8)

Pm → Sm should selectively substitute into Ca(1).

( ) ( )

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

( ) ( )

<sup>x</sup> 5 x Sr NO Nd O 3 NH VO 2


5x x 4 3 1x x

Sr Nd VO OH O - -

following reaction:

expressed as:

The procedure includes three stages:

**c.** Treatment of the dry residue.

**6.1.10. Actinides-bearing apatites**

Thorium and uranium (actinides [45],[46]<sup>3</sup>

on Earth solely through artificial generation [46].

with ThCa(2)/ThCa(1) values, which range from 4.33 to 8.67.

**a.** Preparation of solution;

**b.** Thermolysis;

ings [44].

**Fig. 7.** The structure of UFAP (a), ThFAP (b), UClAP (c), ThClAP (d), ThSrFAP (e) and ThSrClAP (f) [47] viewed along the *c*-axis.

The structure refinements of U-doped chlorapatites show that U is essentially distributed equally between the two Ca sites with UCa(2)/UCa(1) values, which range from 0.89 to 1.17. The results of Th-doped chlorapatites show that Th substitutes into both Ca(1) and Ca(2) sites with ThCa(2)/ThCa(1) values, which range from 0.61 to 0.67. In Th-doped strontium apatites with F and Cl end-members, Th is incorporated into both Ca(1) and Ca(2) sites. The range of ThCa(2)/ThCa(1) values is 0.56 to 1.00 for the F end-member and 0.39 to 0.94 for the Cl endmember. U-doped samples indicate that U in fluorapatite is tetravalent, whereas, in chlora‐ patite, it is heterovalent but dominantly hexavalent [47].

Based on the chemical analyses of U-, Th-doped fluor-, chlor- and strontiumapatite speci‐ mens in this study, local charge compensation may be maintained by the following coupled substitutions (M represents U or Th and [] represents the vacancy) [47]:

$$\begin{array}{c} \mathbf{M}^{4\*} + \left[ \begin{array}{c} \end{array} \right] \rightarrow \mathbf{2} \; \mathbf{Ca}^{2\*} \end{array} \tag{10}$$

$$\text{Ca}^{6\*} + 2\left[\begin{array}{c} \end{array}\right] \rightarrow 3\text{ Ca}^{2\*} \tag{11}$$

$$\text{M}^{4\*} + 2\text{ Na}^\* \rightarrow \text{3 Ca}^{2\*} \tag{12}$$

$$\text{M}^{6\*} + 4\text{ Na}^\* \rightarrow \text{5 Ca}^{2\*} \tag{13}$$

$$\text{Ca}^{4+} + \text{2 Si}^{4+} \rightarrow \text{Ca}^{2+} + \text{2 P}^{5+} \tag{14}$$

$$\text{M}^{6\*} + 2\text{ Si}^{4\*} \rightarrow \text{Ca}^{2\*} + 4\text{ P}^{5\*} \tag{15}$$

The incorporation of U and Th into fluorapatite results in a decrease of the size of both Ca polyhedra, but the incorporation of U and Th into chlorapatite gives rise to an increase in the volume of both Ca polyhedra. The decrease of both Ca polyhedral volumes in fluorapatite caused by the substitution of U and Th can be explained by the decrease of ionic radius from Ca to U and Th. However, the increase in the volume of both Ca polyhedra in chlorapatite is hard to understand. Because of the effect on Ca(2) polyhedron caused by the replacement of F− by Cl<sup>−</sup> , it can be explained by the structural distortion of Ca(2) polyhedron [47].

Uranium-doped oxy-silicophosphates (britholites) of the composition of CaxLay(SiO4)6−u (PO4)uOt :U4+ were synthesized by RIADH et al [48] via the high-temperature solid-state reaction. The uranium solubility limit was found to be comprised between 4.6 and 4.8 mol.%. The investigation of uranium heated to 1200°C led to the uranium diffusion coefficient of 2.14·10−14 m2 ·s−1. The synthesis and the characterization of uranium- (Ca9Nd1−xUx(PO4)5−x(SiO4)1+xF2) and thorium-bearing britholites (Ca9Nd1−xThx(PO4)5−x(SiO4)1+xF2) were also reported by TERRA et al [49],[50]:

$$\begin{aligned} &\frac{1}{4}\text{Nd}\_2\text{O}\_3 + \frac{9}{4}\text{Ca}\_2\text{P}\_2\text{O}\_7 + \frac{7}{2}\text{CaCO}\_3 + \text{CaF}\_2 + \frac{3}{2}\text{SiO}\_2 + \frac{1}{2}\text{An}^{4\*}\text{O}\_2 \rightarrow\\ &\text{Ca}\_2\text{Nd}\_{0.5}\text{An}\_{0.5}^{4\*}\text{(PO}\_4\text{)}\_{4\lesssim 5}\text{(SiO}\_4\text{)}\_{1.5}\text{F}\_2 + \frac{7}{2}\text{CO}\_2\text{(g)}\end{aligned} \tag{16}$$

or

$$\begin{aligned} &\frac{1}{4}Nd\_2O\_3 + \frac{7}{4}Ca\_2P\_2O\_7 + \frac{9}{2}CaCO\_3 + CaF\_2 + \frac{3}{2}SiO\_2 + \frac{1}{2}An^{4+}P\_2O\_7(a) \rightarrow \\ &Ca\_9Nd\_{0.5}An\_{0.5}^{4+} \text{(}\_{0.5}^{O\_0}\text{)}\_{4,5} \text{(}\_{0.5}^{O\_0}\text{)}\_{1,.5}F\_2 + \frac{9}{2}CO\_2(\text{g}) \end{aligned} \tag{17}$$

where An4+ substitutes for tetravalent U4+ and Th4+.

The incorporation of thorium in the structure is probably possible due to small differences of ionic radius between calcium (1.06 Å), neodymium (1.05 Å) and thorium (1.00 Å). In order to ensure the quantitative incorporation of thorium, it appeared necessary to consider the coupled substitution [50].

$$\rm{^1Nd^{3+}} + \rm{PO\_4^{3-}} \Leftrightarrow \rm{Th^{4+}} + \rm{SiO\_4^{4-}} \tag{18}$$

instead of the substitution scheme:

[ ] 6 2 M 2 3 Ca + + + ® (11)

4 2 M 2 Na 3 Ca ++ + + ® (12)

6 2 M 4 Na 5 Ca ++ + + ® (13)

4 4 25 M 2 Si Ca 2 P + + ++ + ®+ (14)

6 4 25 M 2 Si Ca 4 P + + ++ + ®+ (15)

4

+

( )

a

(16)

(17)

The incorporation of U and Th into fluorapatite results in a decrease of the size of both Ca polyhedra, but the incorporation of U and Th into chlorapatite gives rise to an increase in the volume of both Ca polyhedra. The decrease of both Ca polyhedral volumes in fluorapatite caused by the substitution of U and Th can be explained by the decrease of ionic radius from Ca to U and Th. However, the increase in the volume of both Ca polyhedra in chlorapatite is hard to understand. Because of the effect on Ca(2) polyhedron caused by the replacement of

, it can be explained by the structural distortion of Ca(2) polyhedron [47].

Uranium-doped oxy-silicophosphates (britholites) of the composition of CaxLay(SiO4)6−u

The uranium solubility limit was found to be comprised between 4.6 and 4.8 mol.%. The investigation of uranium heated to 1200°C led to the uranium diffusion coefficient of 2.14·10−14

·s−1. The synthesis and the characterization of uranium- (Ca9Nd1−xUx(PO4)5−x(SiO4)1+xF2) and thorium-bearing britholites (Ca9Nd1−xThx(PO4)5−x(SiO4)1+xF2) were also reported by TERRA et al

( ) ( ) ( )

( ) ( ) ( )

*Nd O Ca P O CaCO CaF SiO An P O*

1 9 7 31 Nd O Ca P O CaCO CaF SiO An O 4 4 2 22

9 0.5 0.5 4 4.5 1.5 4 2 2

1 7 9 31 4 4 4 2 22 2 3 22 7 3 2 2 2 7

4 9 9 0.5 0.5 4 4 2 2 4.5 1.5 2

<sup>+</sup> <sup>+</sup>

*Ca Nd An PO SiO F CO g*

<sup>7</sup> Ca Nd An PO SiO F CO g <sup>2</sup>

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

4

+

2 3 22 7 32 2 2

+

+ + ++ + ®

<sup>+</sup> + + ++ + ®

:U4+ were synthesized by RIADH et al [48] via the high-temperature solid-state reaction.

F− by Cl<sup>−</sup>

m2

or

(PO4)uOt

[49],[50]:

$$\text{Nd}^{3\*} + \text{F}^- \Leftrightarrow \text{Th}^{4\*} + \text{O}^{2-} \tag{19}$$

Indeed, in the first way, homogeneous and single-phase solid solutions were prepared from Ca9Nd(PO4)5(SiO4)F2 to Ca9Th(PO4)4(SiO4)2F2 leading to full neodymium substitution. Associated small increase of the unit cell parameters results from the simultaneous replace‐ ment of phosphate groups by bigger silicate. It was accompanied by a significant change in the grain morphology. These results contrast with those obtained when the coupled substitu‐ tion according to **Eq. 19** was performed, which confirmed the limitation of about 10 wt.% in the Th substitution [50]. Good resistance of the materials to influence of aqueous solutions enables their utilization for the immobilization of tetravalent actinides in phosphate ceram‐ ics [49].
