**3.2 X-Ray diffraction analysis**

**Figure 2** showed the XRD patterns of all compositions. It is evident that the samples were single apatite phase. By comparaison to the JCPDS 17-0609 file data for the strontium fluorapatite, the samples are characteristic of the hexagonal symmetry and the P63/m space group. No additional diffraction lines relative to supplementary phases were detected in any of the patterns. However, the presence of very small quantities of impurities was not excluded. The XRD patterns of the **Figure 3** indicated that when the substitution level increased, the peaks slightly shift towards the high 2θ angles


### **Table 2.**

*Number of atoms per unit cell of Sr8La2*�*xNdx(PO4)4(SiO4)2F2 (0* ≤ *x* ≤ *2).*

**Figure 2.** *DRX spectra of strontium fluorbritholites Sr8La2Ndx(PO4)4(SiO4)2F2 with (0* ≤ *x* ≤ *2).*

**Figure 3.** *Radiation (300) of fluorbritholites Sr8La2*�*xNdx(PO4)4(SiO4)2F2 (0* ≤ *x* ≤ *2).*

indicating a contraction of the unit cell. This contraction, which agrees with the Nd3+ radius (VI**r**Nd 3+ = 0.983 Å) [38], that is smaller than that of La3+ ( VI**r**La 3+ = 1.032 Å) [39] confirms the incorporation of Nd3+ into the apatite structure.

As shown in **Figure 4** and **Table 3**, the cristallographic parameters calculated using the Fullprof program without any structural refinement of the all compositions depended on the substitution level. In fact, if Nd content rose, *a* and *c* decreased. The calculated cristallographic parameters were similar to that existing in the literature [40, 41]. Moreover the evolution of the lattice parameters was linear in accordance with the Vegard's law:

$$\mathbf{a} = -0.0092\mathbf{x} + 9.7346\ \mathbf{\hat{A}}; \sigma(\mathbf{a}) = 2.5 \times 10^{-3} \mathbf{\hat{A}}\ (\sigma : \text{standard deviation})$$

$$\mathbf{c} = -0.009\mathbf{x} + 7.2806\ \mathbf{\hat{A}}; \sigma(\mathbf{c}) = 2 \times 10^{-3} \ \mathbf{\hat{A}}\tag{4}$$

**Figure 4.** *Lattice parameters as a function neodymium level in the Sr8La2xNdx(PO4)4(SiO4)2F2 with (0* ≤ *x* ≤ *2).*


### **Table 3.**

*Crystallographic parameters of strontium fluorbritholites Sr8La2xNdx(PO4)4(SiO4)2F2 (0* ≤ *x* ≤ *2).*

indicates the existence of a continuous solid solution in the explored substitution domain.

The rational parameters that govern the site occupation are the nature, the electronegativities, the valences and the polarizabilities of the ions. The bibliography studies' results indicated that like those observed in natural phosphosilicate apatites, the substituted cations in the apatite structure had preferential occupation for Me(2) sites [42–45]. Thus, it could be concluded that La3+ and Nd3+ ions substituting Sr2+ with 0 ≤ x ≤ 2 in our studied samples were subsequently preferentially localized in Me(2) sites.

### **3.3 FTIR spectroscopy**

The FTIR spectra of the samples were given in **Figure 5**. The identification of all the bands was done by comparison with un- and substituted strontium fluorapatite the previously reported in the literature [40, 41]. The characteristic absorption bands of SiO4 and PO4 were observed [41].

**Figure 5.** *Infrared spectra of Sr8La2xNdx(PO4)4(SiO4)2F2 (0* ≤ *x* ≤ *2).*

The PO4characteristic bands observed at 1072–1024 cm <sup>1</sup> coincide with to the asymmetric stretching mode (*υ*3). The band at 943 cm<sup>1</sup> is attributed to the symmetrical stretching mode (*υ*1). The asymmetric bending mode (*υ*4) bands appeared in the 538–562 cm<sup>1</sup> range and the symmetric one (*υ*2) are shown at 455 cm<sup>1</sup> . The bands observed at 909–943 (*υ*3), 832–870 (*υ*1), about 538 (*υ*4) and 460–498 cm<sup>1</sup> (*υ*2) were assigned to SiO4. Moreover as the neodymium amount in the samples increased a shift of the PO4 and SiO4 absorption bands towards the low wave numbers was detected. This was attributed to a reduction in size of the lattice inducing an increase in anionanion repulsion (PO4 vs. SiO4) [45]. This observation was in good agreement with those obtained by diffraction of X-rays and confirms that the neodymium substitution reduced the lattice size.
