Over the last decades, apatites of general formula Me10(XO4)6Y2 with Me a divalent cation, XO4 a trivalent anion and Y a monovalent anion have acquired an exhaustive importance because of their distinctive characteristics. Indeed, these materials have a flexible and rigid structure able to admit a large number of substitutions both cationic and anionic total or limited with unchanged crystallographic structure    . In addition, these materials are endowed with chemical and thermal stability and low solubility, they also have interesting mechanical properties      . All of these specifications allow these materials to be used in various fields such as chemical industry where the apatite ores are the main source of phosphate fertilizers and they are also used in phosphoric acid and various phosphate derivatives industry  . A further use of apatites is in optics   , in pharmaceuticals (excipient) and in chromatography (column)   or as materials for laser  .
Currently, calcium phosphate apatites are frequently used in orthopedic and dentistry surgery as a substitute for failing bones and teeth     . These compounds have shown interesting properties of bioactivity and osteoconduction as well as a chemical composition and a crystal structure similar to that of the mineral part of calcified tissue    . Apatites are particularly studied as electrolyte for solid oxide fuel cells (SOFC)  . Recently, new applications in the field of the environment have appeared; apatitic derivatives with coupled substitutions of divalent cations by rare earth elements (lanthanides: Ln3+ or actinides: Ac3+) and the silicates partially substitute XO4 groups. Such a family, called britholites, acted as a matrix for conditioning certain radionuclides and heavy metals    . However, it has been shown that the natural nuclear reactors Alko of Gabon contain britholites which immobilize radionuclide like cesium as well as minor actinides   . Consequently, several studies have been conducted in order to better valorize the retention capacity of these elements by this family of apatites.
The apatitic structure crystallizing in the hexagonal system, space group P63/m   , is built on a skeleton formed of parallel layers of XO4 ions arranged in a hexagonal way and giving rise to two types of tunnels where the metal ions Me are localized. As shown in Figure 1, these ions are distributed between two non-equivalent crystallographic sites. The Me (1) sites, also called 4f, four in number, are aligned in columns along the ternary axis c and surrounded by nine oxygen atoms. The six other Me (2) sites, called 6 h, six in number, were organized at the top of two equilateral triangles centered on the helicoidally axis (Figure 1). Me (2) sites are coordinated with six oxygen atoms
Figure 1. Perspective view of the fluorapatite structure  .
and one Y ion. Thus, calcium can be substituted by many mono, di or trivalent ions. Thereby, these cationic substitutions strongly depend on the difference in ionic radii, the valence, the electronegativity as well as the polarizability of the ions  .
An earlier spectroscopic and structural study of Ca10−xLax(PO4)6−x(SiO4)xF2 showed that the composition x = 2 is stable and the coupled substitutions of lanthanum by calcium and silicates by phosphates have occurred without structural modification  . The present work consists in studying the simultaneous substitution of lanthanum by neodymium in the designated composition x = 2. A series of compounds Ca8La2−xNdx(PO4)4(SiO4)2F2 with 0 ≤ x ≤ 2 should be investigated.
2. Experimental Process
2.1. Preparation and Synthesis of Powders
Britholitic powders Ca8La2−xNdx(PO4)4(SiO4)2F2 with 0 ≤ x ≤ 2 were prepared by solid state reaction using 1.8 × 10−3 moles of calcium fluoride CaF2 (99.99% Merck), 4.54 × 10−3 moles of calcium carbonate CaCO3 (≥99.0% Fluka), 3 × 10−3 moles of silica SiO2 (Prolabo) and Calcium diphosphate Ca2P2O7, lanthanum oxides La2O3 (99.999% Merck) and neodymium oxide Nd2O3 (99.99% Merck) whose molar quantities are given in Table 1. The formation reaction is as follow:
The preparation of Ca2P2O7 is similar to that of Sr2P2O7 strontium diphosphate previously described  . All the reagents were mixed in stoichiometric amounts to obtain 1.5 × 10−3 moles of each final product. The solid mixture of reagents was homogenized by prolonged grinding in an agate mortar and then cold pelletized at 100 MPa. The pellets were firstly treated at 900˚C for 12 hours then thrice milled, homogenized and repelletized for heat treatments during 12hours in the range 1450˚C - 1250˚C with a decreased temperature of 50˚C for each x value. All specimens will be afterwards abbreviated according to the index that bears the La3+ and Nd3+ ions as CaLa2F, CaLa1.5Nd0.5F, CaLa1Nd1F, CaLa0.5Nd1.5F and CaNd2F.
2.2. Characterization of Powders
Several techniques were employed for the characterization of synthesized powders; X-ray diffraction were performed using a PRO PANALYTICAL X’pert PRO
Table 1. Number of moles of lanthanum and neodymium oxides used in the synthesis of fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 with 0 ≤ x ≤ 2.
apparatus using Kα copper Cu radiation (λ= 1.5406Å). The collections are recorded in 2θ mode in the angular range 20˚ - 80˚ with a counting speed of 0.02˚ s−1. The crystalline phases have been identified by comparison with JCPDS files (Joint Committed Powder Diffraction Standard). The lattices parameters a and c of each sample were calculated by the Rietveld method using the Fullprof program. The FTIR spectra were recorded from 400 to 4000 cm−1 on a Perkin Elmer spectrometer and recorded data were in ATR (Attenuated Total Reflectance) mode based on material reflectance. 31P NMR-MAS spectra were recorded on a BRUKER MSL 300 spectrometer operating at Larmor frequency of 121.44 MHz for phosphorus. Spinning rate of the sample at the magic angle was 8 KHz. Chemical shifts were measured against tetramethylsilane (TMS) as a reference material. The amounts of chemical elements present in each synthesized sample were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a Shimadzu 9800 series apparatus while that of fluoride was measured potentiometrically by a specific ion-selective electrode.
3. Results and Discussion
The identification of crystalline phases was determined by examination of obtained diffractograms of synthesized powders given in Figure 2. Hence, their indexation was made with reference to the Ca10(PO4)6F2 phase (JCPDS card 01-076-0558), shows that the different synthesized samples are characteristic of a single-phase apatite with hexagonal symmetry spatial group P63/m. Therefore, due to the limited sensitivity of the DRX technique, no secondary phase has been detected. However, small amounts (less than 2%) of impurities may be present.
Figure 2. DRX diffractograms of fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 (0 ≤ x ≤ 2).
Despite the size of lanthanum ( ) slightly higher than that of neodymium ( )  , the solid solution doped with lanthanum and neodymium was formed and the substitution was in whole range. The slight displacement of the radiations (300) towards the higher diffraction angles confirm that the substitution of lanthanum by neodymium occurred (Figure 3) and that all the neodymium introduced at the starting solutions substituted lanthanum. The lines of the composition x = 1 showed a diffraction lines doubling which is probably related to the slight phase transition and slightly affected the symmetry of the crystalline structure. It may be caused either by the cationic substitution or by the thermal treatments during synthesis.
The calculated lattice parameters a and c presented in Figure 4 showed particularly a significant decrease of these parameters as the level of substituted Nd3+ increased. All obtained values given in Table 2 are similar to that of calcium britholites  . These parameters as well as the volume of the lattice decreased when the level of introduced neodymium increased in agreement with its small size compared to that of lanthanum. As shown in Figure 4, the lattice parameters a and c vary linearly with the substitution level x according to Végard’s law and satisfy the following equations:
(σ: standard deviation)
Consequently, the volume presented a linear decrease (Figure 4) according to the following equation:
Figure 3. Radiation (300) of fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 (0 ≤ x ≤ 2).
Figure 4. Variation of lattices parameters as a function neodymium level in the Ca8La2−xNdx(PO4)4(SiO4)2F2 (0 ≤ x ≤ 2).
Table 2. Lattices parameters (Å) and volume of fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 (0 ≤ x ≤ 2).
Many studies were interested to the repartition of the lanthanide ions between the two cationic sites of the calcium britholites structure. It was confirmed that the occupancy factors relative to substituted lanthanides depends on many factors particularly on the tunnel anions nature ( , , ...), the substitution level as well as the Y− nature (F−, Cl−, Br−, ...)      . Briefly, when La3+ substituted calcium in lower amounts (~2 atom/unit cell), the ions were located into the two sites with strong preference to occupy Me (2) sites. Whereas with higher substituted amounts the repartition became statistical between the two sites. In case of double substitution of calcium by lanthanum and neodymium with a rate inferior or equal to two and due to similarity of the substituting ions they should occupy the Me (2) sites. The preferential occupation of the two ions in Me (2) sites is coherent with the ionic radius of Ca2+ ( ) slightly superior to that of lanthanide ions meanwhile with inferior ionic radius like strontium Sr2+ the preference of localization in Me (2) also exist. Hence, the parameters other than the ionic sizes such as electronegativity, valence and polarizability are responsible for the two sites occupations. When the electronegativities of the ions were 1.00, 1.10 and 1.14 for Ca2+, La3+ and Nd3+, respectively. These values considered once again as similar lead to think that the valence and polarizabilities were the remaining relevant parameter for the preferential occupation of Me (2) sites. This was verified with the ions polarizabilities values, expressed in Å3, found significantly different (Ca2+:1.91; La3+:4.45; Nd3+:4.20) as well as the valence difference between calcium and the lanthanides. Therefore, a non-radioactive analogues to minor actinides were conditioned in calcium-fluorobritholite matrix.
In order to determine the content of each element in the synthesized powders, to verify the stoichiometry and to establish the formulas of the prepared phases, all samples were chemically analyzed. The results summarized in Table 3 are comparable with the expected values showing that the amounts of neodymium rise whereas those of lanthanum decrease verifying once again that all of the Nd3+ added to the starting solution completely reacted and substituted those of La3+. The stoichiometry of the powders verified by comparison with the theoretical
ratio equal to 1.67 was determined by molar ratios. The
values shown in Table 3 were slightly lower than the stoichiometric value. Experimental errors may be responsible for this small difference in stoichiometry values. This should indicate eventually, that the substitution of lanthanum by neodymium in the apatite structure slightly affected the powders stoichiometry. Finally, the amount of fluoride contained in powders was very close to that of the starting solutions highlighting electroneutrality of chemical formulas.
The infrared absorption spectra of fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 with (0 ≤ x ≤ 2) are shown in Figure 5. Their attributions were made by comparison with the spectra of the Sr8La2(PO4)4(SiO4)2F2 phase  . The spectra show that apart from the PO4 vibrational modes, those of the SiO4 and this was in an apatitic environment  . The characteristic bands associated to PO 4 as well as their vibration mode are as follows; The band range 1056 - 1028 cm −1 corresponding to the asymmetric stretching mode (υ3), the band at 954 cm−1 is relative to the symmetrical stretching mode (υ1). Bands at 544 - 585 cm−1 range and at 454 cm−1 were respectively assigned to asymmetric bending mode (υ4)
Table 3. Chemical composition in mmol∙g−1 of the as prepared powders of fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 with (0 ≤ x ≤ 2).
and symmetric one (υ2).. The typical SiO4 bands were observed at 926 - 960 (υ3), 848 - 870 (υ1), about 550 (υ4) and 460 - 498 cm−1 (υ2). However, as shown in the spectra (Figure 3), with the increased neodymium level a shifting toward lower absorption bands of both PO4 and SiO4 occured. This could be related to the reduced size of the lattice inducing an increase in anion-anion repulsion (PO4 vs SiO4)  . This result is consistent with those obtained by diffraction of X-rays and confirms that the doping with neodymium reduced lattice size.
31P NMR-MAS spectroscopy analysis of various samples is shown in Figure 6.
Figure 5. FTIR-ATR Spectra of fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 with (0 ≤ x ≤ 2).
Figure 6. 31P NMR-MAS spectra of fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 with (0 ≤ x ≤ 2).
Table 4. Chemical shifts δiso and FWHM (±0.01 ppm) of PO4 signals.
The spectra indicated the existence of a single isotropic peak revealing the presence of a one phosphorus type in the apatitic environment. This is coherent with the results obtained by DRX and IR showing a single apatite phase. Besides, as shown in Table 4, the isotropic signal was shifted and expanded with increasing neodymium content. This can be accounted for the heterogeneity of the PO4 environment related to the incorporation of neodymium into the apatite structure. The weak enlargement of 31P NMR signal is also related to neodymium substitution.
Compounds of the fluorbritholites Ca8La2−xNdx(PO4)4(SiO4)2F2 with (0 ≤ x ≤ 2) were prepared by solid state reaction. The chemical molar ratios
of each composition close to the 1.67 prove that all the powders were stoichiometric. Characterization by X-ray diffraction, IR and 31P NMR-MAS spectroscopy revealed that all the powders are of apatite character. The shifting in XRD radiation, in IR bands and 31P NMR peaks confirmed that lanthanum-neodymium substitution occurred in whole range of composition. The lattice parameters a and c as well as the lattice of the unit cell decreased as the Nd3+ increased coherently to the size of the two trivalent ions. The lanthanide ions preferentially occupy Me (2) sites. Hence, the ability of these materials to confine radionuclides was evident, remains to determine precisely the rate of each element that the britholitic matrix can store. Such a study needed to be deeply investigated in coming works.
The authors gratefully acknowledge Mrs Mounira Khelifi for her help with English.
Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.