**4.10 Synthetic analogues of other minerals from the supergroup of apatite**

These minerals were usually prepared in order to elucidate the structure of naturally occur‐ ring minerals or due to its potential applications in immobilization of nuclear and toxic waste (**Chapter 6**) and electrical properties (ionic conductivity). Synthetic analogues of ellestadite and britholite weren't included because they are described in **Chapter 5** and **6**, respectively. Rare earth apatites are described separately in next **Chapter 5**.

#### **4.10.1 Cesanite**

Synthetic cesanite as an analogue of mineral with the composition Na3Ca2(SO4)3(OH) (**Section 2.1.7**) shows typical features of the apatite structure, as shown in **Fig. 34**. The symmetry reduction from the centrosymmetric space group P63/M to the non-centrosymmet‐ ric space group P6 ¯leads to a doubling of the number of crystallographically independent sites. The origin of the unit cell is shifted by Z + ¼ relating to the origin in the space group P63/M. Alternating pairs of isolated tetrahedral anions (the sulfate-groups) form ribbons running parallel to the *c*-axis. As the sulfur atoms are located in special Wyckoff positions 3*j* and 3*k*, the tetrahedra have the point group symmetry *m* [205].

**Fig. 34.** The projection of the crystal structure of synthetic cesanite parallel to (001) (1) and the arrangement of cations and sulfate tetrahedra around the 63 and the 6̅ axes, respectively (2): phosphate apatite (a) and synthetic cesanite (b) [205].

Small spread in the S-O distances and O-S-O angles indicates only minor deviations from ideal tetrahedral symmetry. The sub-structure of the array of sulfate tetrahedra shows a distinct

pseudo-symmetry, closely mimicking P63/M. Maximal deviations from this symmetry occur at O(4) atom, which is shifted by 0.16 Å (synthetic) and 0.02 (natural) from its position in P63/M. Na and Ca cations are distributed either by six O atoms and one hydroxyl ion or water molecule (M(1) and M(2)) or by nine O atoms (M(3) and M(4) [205].

Synthetic analogues of minerals cesanite Halide sulfates have general formula [206]:

( ) ( )

 n n n

 n n n

(58)

( ) ( )

6 22 3 2 12 3 4 234

 n

respectively. Rare earth apatites are described separately in next **Chapter 5**.

 n

 n

G = ++ + + ++

*PO A E <sup>g</sup> <sup>g</sup>*

+ ++ + + ++

6 22 3 12 3 4 1 234 <sup>4</sup>

 n

**4.10 Synthetic analogues of other minerals from the supergroup of apatite**

These minerals were usually prepared in order to elucidate the structure of naturally occur‐ ring minerals or due to its potential applications in immobilization of nuclear and toxic waste (**Chapter 6**) and electrical properties (ionic conductivity). Synthetic analogues of ellestadite and britholite weren't included because they are described in **Chapter 5** and **6**,

Synthetic cesanite as an analogue of mineral with the composition Na3Ca2(SO4)3(OH) (**Section 2.1.7**) shows typical features of the apatite structure, as shown in **Fig. 34**. The symmetry reduction from the centrosymmetric space group P63/M to the non-centrosymmet‐

The origin of the unit cell is shifted by Z + ¼ relating to the origin in the space group P63/M. Alternating pairs of isolated tetrahedral anions (the sulfate-groups) form ribbons running parallel to the *c*-axis. As the sulfur atoms are located in special Wyckoff positions 3*j* and 3*k*,

**Fig. 34.** The projection of the crystal structure of synthetic cesanite parallel to (001) (1) and the arrangement of cations and sulfate tetrahedra around the 63 and the 6̅ axes, respectively (2): phosphate apatite (a) and synthetic cesanite (b)

Small spread in the S-O distances and O-S-O angles indicates only minor deviations from ideal tetrahedral symmetry. The sub-structure of the array of sulfate tetrahedra shows a distinct

¯leads to a doubling of the number of crystallographically independent sites.

( )

 n

*E A g u*

 n

nn

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

6 22 112 3 4

nn

nn

+ ++ +

the tetrahedra have the point group symmetry *m* [205].

*E u*

**4.10.1 Cesanite**

ric space group P6

[205].

$$\text{M}\_3^\* \text{M}\_2^{2\*} \text{(SO}\_4\text{)}\_\text{j} Z \tag{59}$$

where Z = OH, F and Cl. KLEMENT [207] synthesized sodium-calcium sulfatapatite, Na6Ca4(SO4)6F2, by full substitution of S6+ for P5+ through the substitution scheme [208]:

$$\text{Ca}^{6+} + \text{M}^{+} \left(\text{e.g.}, \text{Na}\right) \Longleftrightarrow \text{P}^{5+} + \text{Ca}^{2+} \tag{60}$$

where the hydroxyl equivalent is the equivalent to mineral cesanite, Na6Ca4(SO4)6(OH)2. KREIDLER and HUMMEL [209] also synthesized Na6Ca4(SO4)6F2 and Na6Pb4(SO4)6F2 apatite-like phases. KNYAZEV et al [206] prepared the compounds of the composition of Na3Ca2(SO4)3F, Na3Cd2(SO4)3Cl, and Na3Pb2(SO4)3Cl with the structure of apatite via the solid-state reactions:

$$\text{Na}\_2\text{SO}\_4 + 2\text{ CaSO}\_4 + \text{NaF} \rightarrow \text{Na}\_3\text{Ca}\_2\text{(SO}\_4\text{)}\_3\text{F} \tag{61}$$

$$\text{Na}\_2\text{SO}\_4 + 2\text{ CdSO}\_4 + \text{NaCl} \rightarrow \text{Na}\_3\text{Cd}\_2\text{(SO}\_4\text{)}\_3\text{Cl} \tag{62}$$

$$\text{Na}\_2\text{SO}\_4 + 2\text{ PbSO}\_4 + \text{NaCl} \rightarrow \text{Na}\_3\text{Pb}\_2\text{(SO}\_4\text{)}\_3\text{Cl} \tag{63}$$

from the stoichiometric reaction mixture in a porcelain crucible. The mixtures of compo‐ nents were calcined in several steps at the temperatures of 570, 770 and 1020 K for 10 h, with intermediate grindings in agate mortar every 2 h [206].

The Na3Ca2(SO4)3F:Ce3+ phosphor was prepared by NIKHARE et al [210] via the solid-state method according to the following reaction:

$$\begin{aligned} \text{2 NaNO}\_3 + \text{CaNO}\_3 + \text{NaF} + \text{3 (NH}\_4\text{)}\_2\text{SO}\_4 + \text{Ce} \text{(NO}\_3\text{)}\_3 &\rightarrow \\ \text{Na}\_3\text{Ca}\_2\text{(SO}\_4\text{)}\_3\text{F:}\text{Ce} + 4\text{ NO} + 8\text{ H}\_2\text{O} \end{aligned} \tag{64}$$

The pigment shows a single high intensity emission peak at 307 nm when excited by UV light of the wavelength of 278 nm

The compound having the formula: K3Ca2(SO4)3F, was identified in coatings of heat recovery cyclones of Portland clinker kiln. The structure of this phase (noncentrosymmetric, space group PN21A, *a* = 13.415, *b* = 10.943 and *c* = 9.127 Å, *V* = 1284.75 Å3 , *Z* = 4 and *ρ* = 2.61 g·cm−3) was reported as very distorted analogue of apatite where fluoride atoms are oriented along the pseudo-screw *a*-axis [211],[212].

**Fig. 35.** The structure of K3Ca2(SO4)3F according to FAYOS et al [212] phase in the perspective view along the c-axis.

The activation by Eu or Ce leads to the phosphor: K3Ca2(SO4)3F:Ce, Eu, which was prepared by PODDAR et al [213] via the precipitation method. The K3Ca2(SO4)3F:Ce luminescent pig‐ ment shows the emission at 334 nm when excited at 278 nm due to the 5*d*→4*f* transition of Ce3+ ions. The phases K3Ca2(SO4)3F:Eu2+ and K3Ca2(SO4)3F:Eu3+ show the emissions at 440 nm, and 596 and 615 nm via the transitions of 5 D0 → <sup>7</sup> F1 and 5 D0 → <sup>7</sup> F2 of Eu3+ ion, which are in blue and red region of the visible spectrum, respectively.
