Spectroscopic studies and Rietveld refinement of strontium-britholites

JOURNAL OF RARE EARTHS, Vol. 26, No. 4, Aug. 2008, p. 483

Spectroscopic studies and Rietveld refinement of strontium-britholites Khaled Boughzala1, Ezzedine Ben Salem1, Fethi Kooli2, Pierre Gravereau3, Khaled Bouzouita1 (1. U. R. Matériaux Inorganiques, Institut Préparatoire aux Etudes d’Ingénieurs, Rue Ibn ElJazzr, 5019 Monastir, Tunisia ; 2. Department of Chemistry, Taibah University, P.O.Box 30002. Almadinah Almunawwarah, Saudi Arabia; 3. Institut de Chimie de la Matière Condensée, Université de Bordeaux ,CNRS [UPR9048], 87 Avenue du Dr Schweitzer 33608 Pessac Cedex, France) Received 22 November 2007; revised 21 April 2008

Abstract: Strontium-britholites whose chemical formula was Sr10–xLax(PO4)6–x(SiO4)xF2, where x=0, 1, 2, and 4 were prepared by solid state reaction. The structural refinement carried out using the Rietveld method indicated that La3+ ions were located into the two sites with a strong preference for metal (2) sites especially for low contents. A progressive shift of the F- position along the c-axis outside the centre of the triangle formed by metal (2)-atoms was observed with the increase of x. The infrared and Raman spectra exhibited the characteristic vibration modes of PO4 and SiO4 groups confirming the incorporation of this last group into the apatite structure. The 29Si MAS-NMR spectra exhibited one resonance peak confirming the data obtained by X-ray diffraction, indicating that P and Si were located in the same crystallographic site. Keywords: strontium; britholites; Rietveld refinement; infrared spectroscopy; nuclear magnetic resonance (NMR); Raman spectroscopy; rare earths

Thanks to their chemical and thermal stability and their ability to accommodate a large variety of substitutes, apatites have attracted a considerable attention that they are, nowadays, used in a large variety of applications[1-4]. They constitute a family of compounds with the general chemical formula of M10(XO4)6Y2, where M represents a divalent cation (Ca2+, Sr2+, Ba2+, ...), XO4 a trivalent anion (PO3–4, VO3–4,…), and Y a monovalent anion (F–, Cl–, OH–…)[1]. They crystallize mainly in the hexagonal system (space group P63/m)[5], where XO3–4 groups build the framework. The M cations are distributed between two nonequivalent crystallographic sites. Four cations per unit cell occupy the M(1), i.e., (4f) sites are located along the three-fold axis and surrounded by nine oxygen atoms. Six other cations occupy the M(2), i.e., (6 h) sites are positioned in the summit of two alternated equilateral triangles centered on a helical six-fold axis of the structure (Fig.1). They are coordinated into six oxygens and one Y ion. The two sites can accommodate a wide variety of cations (mono-, bi-, or trivalent), and the distribution of a given substituent in the sites depended closely on the size and on some other properties such as the polarizability and electronegativity[6]. For example, when M2+ is substituted by rare earth elements, the resulting charge can be compensated by the substitution of a tetravalent anion for the trivalent one. Because of their potential use for nuclear waste storage, many studies have focused on the synthesis and characterization of

fluorapatites-called britholites-obtained by the simultaneous substitution of M2+ and PO3–4 by Ln3+ and SiO4–4 ions[7,8]. In fact, the discovery of the Oklo site demonstrated the ability of these materials to retain actinide elements and their thermal stability and chemical durability under radiation conditions[9,10]. The strontium (Sr) is one of the most important elements that are incorporated into apatite structures. In nature, its incorporation into apatites occurs in low contents[11]; whereas its total substitution for Ca is obtained only through a synthesized process. Thus, it is thought worthwhile to synthesize and characterize strontium-lanthanum fluorapatite solid solutions[12]. In the present work, we will deal with the structural refinement using the Rietveld method of compounds with the formula of Sr10–xLax(PO4)6–x(SiO4)xF2, where x=0, 1, 2, and 4. The FTIR and Raman spectroscopies and the 29Si MAS RMN investigations of these compounds will also be reported.

1 Experimental 1.1 Synthesis Strontium-britholites with the chemical formula of Sr10–xLax(PO4)6–x(SiO4)xF2, where x=0, 1, 2, and 4 were prepared by solid state reaction from strontium carbonate [SrCO3], strontium fluoride [SrF2], lanthanum oxide [La2O3],

Foundation item: Project supported by the Ministry of Higher Education, Scientific Research and Technology (Tunisian) Corresponding author: Khaled Bouzouita (E-mail: [email protected]; Tel.: +216-73-307960)

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silica [SiO2], and strontium diphosphate [Sr2P2O7]. To obtain a (Sr+La)/(P+Si) ratios of 1.67, appropriate amounts of reagent were mixed and ground in an agate mortar. The obtained mixtures were uniaxially pressed to make 30-mm diameter pellets, which were heated in an argon flow at 900 °C for 12 h. After that the reaction products were ground, pressed, and then reheated according to the SiO2 amount used at a temperature ranging between 1200–1400 °C for 12 h. This operation was repeated several times so that a pure phase was obtained. In the following sections, the samples will be assigned as SrLa0B, SrLa1B, SrLa2B, and SrLa4B, where B indicates britholite, and the digits indicate the value of x in the formula Sr10–xLax(PO4)6–x(SiO4)xF2.

man spectra were recorded at room temperature in the spectral range of 350–1100 cm–1 using an InVia Reflex Renishaw Raman microscope equipped with a deep-depleted thermoelectrically cooled CCD array detector and a high grade Leica microscope (objective 50×). Excitation was carried out at 514.5 nm with an argon ion laser. The MAS-NMR analyses were conducted on 29Si nuclei at a resonance frequency of 59.62 MHz using a Bruker MSL 300 spectrometer. Spinnig rate of the sample at the magic angle was 8 kHz. The reference material for the chemical shift was tetramethylsilane (TMS). The powder X-ray diffraction (XRD) analysis was carried out using a Philips PW 1800 diffractometer operating in Bragg-Brentano geometry with Cu Kα radiation. X-ray diffraction data were collected over 2θ range of 5°–85° with a step size of 0.02° and a counting time of 12 s per step. Further experimental details of the data are listed in Table 1. Rietveld refinements[14] of the samples were carried out with FULLPROF program[15] using the fluorapatite data as starting parameters[16]. For the refinements, the background was estimated by a fifth-degree polynom, and the peak shapes were modelled using a Pseudo-Voigt function.

1.2 Characterization The strontium and lanthanum contents were estimated using Inductive Coupling Plasma Atomic Emission Spectroscopy (ICP-AES) with Shimadzu ICPQ/V-1014S spectrometer. The silicon content was determined using atomic absorption spectroscopy (Perkin-Elmer 3110). However, the analysis of Phosphorus was carried out by colorimetry using the Gee and Deitz method[13] with a Janway 6400 spectrometer. Meanwhile, the fluoride concentration was determined by potentiometry using an ionometer equipped with a fluoride specific electrode. The Fourrier transformed infrared (FTIR) spectra were obtained by a Perkin Elmer 1283 spectrometer in the range of 1400–350 cm–1, using the KBr pellet technique. The Ra-

2 Results and discussion The results of the chemical analysis summarized in Table 2 are comparable with the expected values, whereas the samples’ atomic ratios (Sr+La)/(P+Si) are close to that of the stoichiometric apatite (1.667).

Table 1 Unit cell parameters and details of Rietveld refinement of Sr10–xLax(PO4)6–x(SiO4)xF2 samples Formula

SrLa0

SrLa1

SrLa2

SrLa4

Formula weight/g Space group symmetry Formula units per cell Z units cell dimensions a/nm c/nm units cell volume V/nm3 Density calculated/(g/cm3) Zero point 2θ/(°) Number of parameter refined Rp Rwp RB RF

1484.03 hexagonal P63/m 1

1532.41 hexagonal P63/m 1

1580.8 hexagonal P63/m 1

1677.59 hexagonal P63/m 1

0.9728(2) 0.7289(2) 0.59737(3) 4.124 0.0988(7) 35 8.92 11.9 4.19 3.49

0.9733(1) 0.7283(2) 0.59764(3) 4.257 0.0210(1) 35 5.40 7.40 5.73 5.49

0.9735(2) 0.7281(2) 0.59766 (3) 4.391 0.0261(2) 35 4.58 6.24 4.63 4.28

0.9741(2) 0.7273(2) 0.59794(3) 4.658 -0.0210(2) 35 5.17 4.49 4.59 4.49

Table 2 Chemical composition of Sr10–xLax(PO4)6–x(SiO4)xF2 samples (atoms per unit cell) Theorical compositions

Sr

La

P

Si

F

Sr+La/P+Si

Sr10(PO4)6F2

10.01

-

6.01

-

1.99

1.665

Sr9La(PO4)5(SiO4)F2

8.96

1.03

4.98

1.02

2.01

1.666

Sr8La2(PO4)4(SiO4)2F2

8.01

1.98

4.00

2.00

1.96

1.665

Sr6La4(PO4)2(SiO4)4F2

5.99

4.00

1.99

4.01

1.97

1.665

Khaled Boughzala et al., Spectroscopic studies and Rietveld refinement of strontium-britholites

Fig.1 Perspective view of the fluoroapatite structure

The FTIR spectra of the prepared samples are shown in Fig.2. The spectrum of the nonsubstituted sample is in agreement with those previously reported in the literature[12,17,18]. For the substituted samples, the spectra exhibited vibrational modes of SiO4 groups, in addition to those of PO4 ones in an apatitic environment[12]. The typical bands of the SiO4 groups were observed at 926–960 (υ3), 848–870 (υ1), around 550 (υ4) and 396–498 cm–1 (υ2). The intensities of these bands increased with the increase of the La content used. The Raman spectra of the prepared samples are presented in Fig.3. The bands of PO43– and Si44– ions were identified by comparison with the pure apatites spectra reported in the literature[17–19]. For the nonsubstituted sample, all the bands were assigned to the vibration modes of PO4 groups. The bands at ~1052–999 and 946 cm–1 correspond to the antisymmetric υ3 and symmetric stretching υ1 modes, respectively. However, the bands associated to the antisymmetric bending υ4 modes were observed at 610, 594, 579, and 570 cm–1. The two other bands at about 444 and 422 cm–1 were attributed to the symmetric bending υ2 modes. The spectra

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of the substituted samples (x=1, 2, and 4) showed additional bands at about 866–845 (υ1), 520 (υ4), and 380 (υ2) cm–1, assigned to the vibration modes of the SiO4 groups. Note that there is a covering between υ3 (SiO4) and υ1 (PO4) bands. As in the FTIR spectra, the intensities of the PO4 bands decreased with the increase of the SiO2 amount used. This clearly shows that the pair (Sr2+, PO43–) was substituted by (La3+, Si44–) in the fluorapatite structure. The 29Si MAS NMR spectra of substituted samples exhibited a single resonance peak at –74.80, –75.33, and –75.95 ppm for SrLa1, SrLa2, and SrLa4, respectively (Fig.4). These chemical shift values correspond to Q0 type silicon species[20]. 31P MAS-NMR analysis carried out on the same compounds revealed the presence of one tetrahedral P site[12]. We can deduce that P and Si occupied equivalent crystallographic sites in the britholite structure. The X-ray diffraction patterns of the samples are shown in Fig.5. All peaks of each pattern were indexed in the hexagonal system (space group P63/m) based on the fluorapatite pattern (JCPDS card No. 50-1744). All the samples were a single crystallographic phase.

Fig.3 Raman spectra of Sr10–xLax (PO4)6–x(SiO4)xF2 samples

Fig.2 FTIR spectra of Sr10–xLax(PO4)6–x(SiO4)xF2 samples

Fig.4 29Si MAS NMR spectra of Sr10–xLax(PO4)6–x(SiO4)xF2 samples

486

Fig.5 Experimental and calculated X-ray diffraction patterns, and their difference of Sr10(PO4)6F2, Sr9La(PO4)5(SiO4)F2, Sr8La2(PO4)4 (SiO4)2F2 and Sr6La4(PO4)2(SiO4)4F2

Structural refined parameters were as follows: the scale factor, the zero point of the diffraction pattern, the lattice parameters, the atomic coordinates, the site-occupancies, and the thermal parameters. The refinements of the fluorine atom coordinates were carried out taking into account its delocalization along z from the ideal 2a (0 0 1/4) position. It induced that the new 4e (0 0 z) position was statistically occupied with an occupancy factor 1/2. For the nonsubstituted phase (SrLa0B), deviation from the ideal position was relatively weak: z=0.236, but this shift increased progressively with the substitution, and the obtained refined z values were 0.204, 0.195, and 0.190 for SrLa1B, SrLa2B, and SrLa4B, respectively. This phenomenon is similar to that reported for numerous substituted fluoroapatites[21]. Several investigations had analyzed the occupancy of lanthanide ions in the phosphate apatites structure, and the obtained results indicated that these ions occupy preferentially the (6 h) sites[22,23]. However, other studies reported that the incorporation of the SiO44– ions into the apatite structure induced the distribution of the lanthanide ions over the two sites but with a strong preference for (6 h) one[24]. Then, the refinements of the sites occupation were carried out with a distribution of La atoms over the two sites with the unique constraint of the chemical composition.

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The refined data: atomic coordinates, thermal parameters, and occupancy factors are listed in Table 3. Observed, calculated, and their difference X-ray diffraction profiles for the samples are shown in Fig.5. The calculated patterns fit very well with the observed ones. As observed, the distribution of La atoms between both sites is in connection with the amount used (Table 3). For the small rates, La occupied M(2) sites preferentially, but this tendency decreased with the increase of x. This was confirmed when compared with the refined ratio “La atoms in M(1)/La atoms in M(2)” with the theoretic statistical one (2/3): 0.01, 0.18, and 0.36 for SrLa1B, SrLa2B, and SrLa4B, respectively. These results agree with the study on the natural britholites[25]. The distribution of a given substituent and especially of lanthanide ions was ascribed to a control by an electronegativity and bond valence[23,26]. Moreover, the nature of the anion located in the tunnel manifestly plays a significant role[23,27]. In fact, a study on the calcium hydroxyl-, fluor-, and chlor-britholites demonstrated that the La occupancy factors in the M(2) sites depended on the anion nature located in the tunnel[27]. For the nonsubstituted sample, the calculated values of a=9.728(2) and c=7.289(2) parameters are in agreement with those reported in the literature[21,28]. With the substitution of the pair (Sr2+, PO43–) by (La3+, Si44–), a slight reverse evolution of the lattice constants occurred: a increased while c decreased, and in general, the volume of the unit-cell increased very slightly following the rise of x. This small increase could be explained by the two antagonist variations: the replacement of Sr2+ (coord. 7: r=0.135 nm) by the smaller La3+ ion (coord. 7:r=0.124 nm) and of the phosphate group by the larger silicate group. In fact, the Si-O bond lengths (0.162 nm) are longer than those of P-O (0.151 nm)[29]. Moreover, the distribution of the substituent cation between the two sites affects the lattice parameters. If M(2) sites and the tetrahedrons have a larger effect on a-axis than on c-axis, M(1) sites will have more influence on c-axis than on a-axis. In the case of a parameter, it appears that the effect of the silicate group size on this axis is preponderant. Selective interatomic distances and angles for the different compositions are gathered in Table 4. As observed, the incorporation of Lanthanum and silicate did not affect the bond lengths significantly. The P/Si-O mean distance increased (0.1522→0.1554 nm) in agreement with the difference of the Si-O and P-O distances in SiO4–4 and PO3–4, respectively. Furthermore, the values of the mean distances of Sr/La(1)-O and Sr/La(2)-O decreased slightly as (0.2700→ 0.2680 nm) and (0.2587→0.2595 nm), respectively, when the (La) amount of lanthanum in the britholites increased. The Sr/La(2)-F was the distance that was most affected by the substitution; this variation would be related to the shift of

Khaled Boughzala et al., Spectroscopic studies and Rietveld refinement of strontium-britholites

487

Table 3 Positional, occupancy and thermal parameters with their standard deviations after Rietveld refinement for Sr10–xLax(PO4)6–x(SiO4)xF2 samples Apatite

Atom

Wyckoff site

x

y

z

Site occupancy factor

Beq[nm2]

Sr10(PO4)6F2

Sr(I)

4f

0.3333

0.6667

-0.0012(7)

1

0.65(8)

Sr(II)

6h

0.2420(3)

-0.0139(4)

¼

1

0.65(8)

Sr9La(PO4)5(SiO4)F2

Sr8La2(PO4)4(SiO4)2F2

Sr6La4(PO4)2(SiO4)4F2

P

6h

0.4001(1)

0.3670(2)

¼

1

0.37(2)

O1

6h

0.3291(2)

0.4782(1)

¼

1

1.01(4)

O2

6h

0.5863(2)

0.4631(2)

¼

1

1.01(4)

O3

12i

0.3390(1)

0.2584(1)

0.077(15)

1

1.01(4)

F

4e

0.0000

0.0000

0.2361(7)

0.5

1.98(8)

Sr(I)

4f

0.3333

0.6667

0.0012(5)

0.998

1.21(3)

La(I)

4f

0.3333

0.6667

0.0012(5)

0.002

1.21(3)

Sr(II)

6h

0.2401(2)

-0.0143(3)

¼

0.835

1.21(3)

La(II)

6h

0.2401(2)

-0.0143(3)

¼

0.165

1.21(3)

P/Si

6h

0.3997(7)

0.3668(7)

¼

1

0.83(4)

O1

6h

0.3297(5)

0.4784(2)

¼

1

2.77(17)

O2

6h

0.5715(3)

0.4527(8)

¼

1

2.77(17)

O3

12i

0.3419(4)

0.2604(7)

0.0680(5)

1

2.77(17)

F

4e

0.0000

0.0000

0.2044(3)

0.5

1.79(5)

Sr(I)

4f

0.3333

0.6667

0.0024(4)

0.923

1.40(2)

La(I)

4f

0.3333

0.6667

0.0024(4)

0.077

1.40(2)

Sr(II)

6h

0.2404(1)

-0.0133(2)

¼

0.718

1.40(2)

La(II)

6h

0.2404(1)

-0.0133(2)

¼

0.282

1.40(2)

P/Si

6h

0.3999(4)

0.3671(6)

¼

1

0.71(4)

O1

6h

0.3303(9)

0.4779(10)

¼

1

2.60(12)

O2

6h

0.5725(10)

0.4516(11)

¼

1

2.60(12)

O3

12i

0.3439(6)

0.2594(6)

0.0681(8)

1

2.60(12)

F

4e

0.0000

0.0000

0.1955( 9)

0.5

3.00(51)

Sr(I)

4f

0.3333

0.6667

0.0026(2)

0.738

1.25(3)

La(I)

4f

0.3333

0.6667

0.0026(2)

0.262

1.25(3)

Sr(II)

6h

0.2412(2)

-0.0136(2)

¼

0.510

1.25(3)

La(II)

6h

0.2412(2)

-0.0136(2)

¼

0.492

1.25(3)

P/Si

6h

0.3989(5)

0.3666(2)

¼

1

0.12(1)

O1

6h

0.3310(5)

0.4760(7)

¼

1

2.03(6)

O2

6h

0.5724(5)

0.4497 (5)

¼

1

2.03(6)

O3

12i

0.3428(3)

0.2574(5)

0.0667(9)

1

2.03(6)

F

4e

0.0000

0.0000

0.1901(6)

0.5

2.50(6)

the fluorine atom outside the center of the triangle formed by M(2)-atoms. Previous studies had demonstrated that the formation enthalpy of the series of Ca-La-fluorbritholites[30] increased and that of the Ca-La-oxybritholites series[31] decreased when the incorporated amounts of lanthanum and silicate in their structure rose. The same phenomena were observed with Ca-Nd-britholites[30,32]. If we assumed that the same behavior occurred for the analog Sr-La-britholites, we would conclude that the decrease of the thermodynamic stability of

the first series might be due to the shift of the F- position outside the centre of the triangle formed by M(2)-atoms. Whereas the results of structural refinements carried out on Sr-La-oxybritholites, which will be presented in the forthcoming article, showed that in this case, the O2- position shifted towards the centre of the M(2)-triangle. Thus, this displacement might be responsible for the increase of the stability with the progressive substitution.

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Table 4 Inter atomic distances (nm) and angles (°) with their standard deviations for Sr10–xLax(PO4)6–x(SiO4)xF2 samples Sr10F2

Sr9LaF2

Sr8La2F2

Sr6La4F2

(P, Si)-(O1)

1.516(1)

1.506(3)

1.501(1)

1.508(1)

(P, Si)-(O2)

1.574(1)

1.438(1)

1.452(2)

1.464(1)

(P, Si)-(O3)(×2)

1.551(1)

1.573(3)

1.584(2)

1.622(1)

(P, Si)-O means

1.548

1.522

1.530

1.554

(O1)- (P, Si)-(O2)

112.122(1)

112.480(3)

113.171(2)

113.731(2)

(O1)-(P, Si)-(O3) (×2) 109.044(2)

108.639(1)

109.527(2)

109.869(1)

(O2)- (P, Si)-(O3)(×2) 109.625(2)

107.680(1)

106.720(1)

106.270(2)

(O3)- (P, Si)-(O3)

107.251(1)

111.767(2)

111.125(1)

110.747(2)



109.45

109.47

109.47

109.45

(Sr1, La1)-O(1)(×3)

2.590(1)

2.585(2)

2.585(3)

2.579(1)

(Sr1, La1)-O(2)(×3)

2.561(3)

2.583(1)

2.575(1)

2.559(1)

(Sr1, La1)-O(3)(×3)

2.949(1)

2.921(1)

2.901(2)

2.902(1)

(Sr1, La1)-O means

2.700

2.696

2.686

2.680

(Sr2, La2)-O(1)

2.727(1)

2.742(1)

2.751(1)

2.746(1)

(Sr2, La2)-O(2)

2.482(2)

2.598(1)

2.605(2)

2.614(1)

(Sr2, La2)-O(3)(×2)

2.642(2)

2.681(2)

2.662(1)

2.668(1)

(Sr2, La2)-O(3)(×2)

2.514(2)

2.471(2)

2.477(2)

2.437(1)

(Sr2, La2)-O means

2.587

2.607

2.605

2.595

(Sr2, La2)-F

2.419(1)

2.433(2)

2.440(2)

2.457(1)

DI ((P, Si)-O)

0.0046

0.0320

0.0371

0.0400

DI (O-(P, Si)-O)

0.0091

0.0161

0.0170

0.0194

3 Conclusion A strontium-britholites was prepared by solid state reaction. Structural refinements using the Rietveld method showed that the incorporation of lanthanum and silicate into the apatite structure affected the F– position significantly. This last ion shifted outside the centre of the triangle formed by M(2)-atoms. The results also showed that the La3+ ions were located into the two sites with a strong preference for M(2) site. This was particularly marked for low La contents. The FTIR and Raman spectroscopies confirmed that the substitution of PO4 by SiO4 group was successfully obtained. The 31P and 29Si MAS NMR studies showed that PO3–4 and SiO4–4 ions occupied equivalent crystallographic sites as indicated by X-ray diffraction data.

Acknowledgment We thank Mr. Ridha Ben Abdlhafidh from the Higher Institute of Technological Studies of Sousse for his help with English.

References: [1] Rey C. From bone mineral to biomaterials, a particular biomineral: apatite. L’Actualité Chimique, Société Française

de Chimie, Paris, France, 1995 (Dec.): 41. [2] DeLoach L D, Payne S A, Smith L K, Kway W L, Krupke W F. Laser and spectroscopic properties of Sr5(PO4)3F:Yb. J. Opt. Soc. Am., 1994, B11(2): 269. [3] Boulon G. Luminescence des ions activateurs dans les matériaux inorganiques et application. Rev. Phys. Appl., 1986, 21: 689. [4] El Ouenzerfi R, Panczer G, Goutaudier C, Cohen-Adad M T, Boulon G, Ayedi M T, Ariguib N K. Relationships between structural and luminescence properties of Eu3+ doped oxyphosphate-silicates apatite Ca2+xLa8–x(SiO4)6–x(PO4)xO2. J. Opt. Mater., 2001, 16(1–2): 301. [5] Sudarsanan K, Makie P E and Young R A. Comparaison of synthesis and mineral fluorapatite Ca5(PO4)3F, in crystallographie detail. Mater. Res. Bull., 1972, 7: 1331. [6] Zhu K, Yanagisawa K, Shimanouchi R, Onda A, Kajiyoshi K. Preferential occupancy of metal ions in the hydroxyapatite solid solutions synthesized by hydrothermal method. J. Eur. Ceram. Soc., 2006, 26: 509. [7] Meis C. Computational study of plutonium-neodymium fluoroapatite Ca9Nd0.5Pu0.5 (SiO4)(PO4)5F2 thermodynamic properties and threshold displacement energies. J. Nucl. Mater., 2001, 289: 167. [8] Meis C, Gale J D, Boyer L, Carpena J, Gosset D. Theoretical Study of Pu and Cs Incorporation in a Mono-silicate Neodymium Fluoroapatite Ca9Nd(SiO4)(PO4)5F2. J. Phys. Chem., 2000, 104: 5380. [9] Trocellier P. Immobilization of radionuclides in single-phase crystalline waste forms: A review on their intrinsic properties and long term behavior. Ann. Chim. Sci. Mater., 2000, 25: 321. [10] Bros R, Carpena J, Sere V, Beltritti A. Occurrence Of Pu And Fissiongenic REE in hydrothermal Apatites from the fossil nuclear reactor 16at oklo (Gabon). Radiochim. Acta, 1996, 74: 277. [11] Rakovan J F, Hughes J M. Strontium in the apatite structure: Strontian fluoroapatite and Belovite-(Ce). Can. Mineral., 2000, 38: 839. [12] Boughzala K, Ben Salem E, Ben Chrifa A, Gaudin E, Bouzouita K. Synthesis and characterization of strontium-lanthanum apatites. Mater. Res. Bull., 2007, 42: 1221. [13] Gee A, Deitz V R. Determination of phosphates by differential spectrometric. Anal. Chem., 1953, 25: 1320. [14] Rietveld H. M. Profile refinement method for nuclear and magnetic structures. J. Appl. Crystallogr., 1969, 2: 65. [15] Rodriguez-Carvajal J. FULLPROF: a program for Rietveld refinement and pattern matching analysis, In Collected Abstract of Powder Diffraction Meeting, vol. 127, Toulouse, France, 1990. [16] Leroy N, Bres E. Structure and substitution in fluoroapatite. European Cells and Materials, 2001, 2: 36. [17] Khorari S, Cahay R, Rulmont A, Tarte P. The coupled isomorphic substitution 2(PO4)3– (SO4)2–+(SiO4)4- in synthetic apatite Ca10(PO4)6F2: a study by X-ray diffraction and vibrational spectroscopy. Eur. J. Solid State Chem., 1994, 31: 921.

Khaled Boughzala et al., Spectroscopic studies and Rietveld refinement of strontium-britholites [18] Fowler B O. Infrared studies of apatites: vibrational assignments for calcium, strontium and barium hydroxyapatites utilizing isotopic substitutions. Inorg. Chem., 1974, 13(1): 194. [19] Neubauer J, Pollmann H. Solid solution series of silico-sulphatechlirode-apatites. Neues Jahrb. Mineral. Abh., 1995, 168(3): 237. [20] Engelhardt G, Michel D. High Resolution Solid-state NMR of. Silicates and Zeolites, John Wiley, Norwich, 1987. [21] Aissa A, Badraoui B, Thouvenot R, Debbabi M. Synthesis Xray structural analysis and spectroscopic investigation (IR and 31 P MAS NMR) of mixed barium/strontium fluoroapatites. Eur. J. Inorg. Chem., 2004. 3828. [22] Schroeder L W, Mathew M. Cation ordering in [Ca.sub.2] [La.sub.8] [([SiO.sub.4]).sub.6][O.sub.2]. J. Solid State Chem., 1978, 26: 383. [23] Fleet, M E, Pan, Y. Site preference of Nd in fluorapatite [Ca10(PO4)6F2]. Journal of Solid State Chemistry, 1994, 111: 78. [24] Carpena J, Boyer L, Fialin M, Kienast J R, Lacout J L. Ca2+, PO43–↔Ln3+, SiO44- coupled substitution in the apatitic structure: stability of the mono-silicated fluor-britholite. Comptes rendus de l’academie de sciences-Serie IIa: Sciences de la Terre et des plantes, 2001, 333(7): 373.

489

[25] Fleet M E, Liu X, Pan Y. Site preference of rare earth elements in hydroxyapatite [Ca10(PO4)6(OH)2]. J. Solid State Chem., 2000, 149: 391. [26] Hughes, J M, Cameron M, Mariano A N. Rare-earth-element ordering and structural variations in natural rare-earth-bearing apatites. American Mineralogist, 1991, 76: 1165. [27] Fleet M E, Liu X, Pan Y. Rare-earth elements in chlorapatite [Ca10( PO4)6Cl2]: Uptake, site preference and degradation of monoclinic structure. Am. Miner., 2000, 85: 1437. [28] Swafford S H, Holt E M. New synthetic approaches to monophosphate fluoride ceramics: synthesis and structural characterization of Na2Mg(PO4)F and Sr5(PO4)3F. Solid State Sciences, 2002, 4: 807. [29] Shannon R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Cryst., 1976, A32: 751. [30] Ardhaoui K. titre de thèse. Doctoral Thesis, Université de Tunis, Tunisie (2006). [31] Ardhaoui K, Rogez J, Ben Chérifa A, Rogez J, Jemal M, Satre P. Standard enthalpy of formation of lanthanium oxybritholites. J. Therm. Anal. Calorim, 2006, 86: 553. [32] Ardhaoui K, Coulet M V, Ben Chérifa A, Carpena J, Rogez J, Jemal M. Standard enthalpy of formation of neodymium fluorobritholites. Thermochim. Acta, 2006, 444: 190.